FROM THE LIBRARY OF WILLIAM A. SETCHELL,i864-i943 PROFESSOR OF BOTANY BULLETIN ILLINOIS STATE LABORATORY NATURAL HISTORY URBANA, ILLINOIS, U. S. A. VOL. VIII. MAY, 1908 ARTICLE I. THE PLANKTON OF THE ILLINOIS RIVER, 1894-1899, WITH INTRODUCTORY NOTES UPON THE HYDROGRAPHY OF THE ILLINOIS RIVER AND ITS BASIN. PART II. CONSTITUENT ORGANISMS AND THEIR SEASONAL DISTRIBUTION. BY C. A. KOFOID, PH.D. < -2- B1OLOSY LIBRAIY BULLETIN ILLINOIS STATE LABORATORY NATURAL HISTORY URBANA, ILLINOIS, U. S. A. VOL. VIII. ARTICLE I. THE PLANKTON OF f THE ILLINOIS RIVER, 1894-1899, WITH INTRODUCTORY NOTES UPON THE HYDROGRAPHY OF THE ILLINOIS RIVER AXD ITS BASIN. PART II. CONSTITUENT ORGANISMS AND THEIR SEASONAL DISTRIBUTION. BY C. A. KOFOID, PH.D. THE ILLINOIS PRINTING COMPANY DANVILLE, ILLINOIS BIOLOGY LIBRARY CONTENTS. PAGE. Introduction i_i 7 Distribution of Collections by Months (Table) 4 Methods 5 Acknowledgments 7 Definitions 7 The Composition of the Plankton 10 Comparison of Fresh-water and Marine Plankton 12 Organisms of the Plankton 13 Constituent Groups of the Annual Plankton of the Illinois River (Table) 16 Discussion of the Statistical Data of the Species composing the Plankton of the Illinois River in 1894-1899 18-290 Cryptogamia ; 18-62 Bacteriaceae 18 Schizophyceae 19 Discussion of Species 20 Chlorophyceae 22 Discussion of Species 24 Bacillariaceae 34 Factors controlling Diatom Production 35-43 Diagram showing the seasonal distribution of diatoms, total plankton, nitrates, and thermograph and hydrograph of Illi- nois River at Havana for 1898 37 Discussion of Species 43 Conjugatae 59 Discussion of Species 60 Phanerogamia . 62 Protozoa 62 Mastigophora 63 Location and Amplitude of Pulses of Chlorophyll-bearing Organ- isms in the Illinois River (Table) 64 Discussion of Species 68 Rhizopoda 92 Discussion of Species 95 Heliozoa 113 Discussion of Species 113 Sporozoa 114 Ciliata 115 Discussion of Species 118 Suctoria 131 Discussion of Species 131 Porifera.. 132 SZ246526 PAGE Coelenterata 132 Platyhelminthes 133 Turbellaria 133 Trematoda 134 Cestoda 135 Nemertini 135 Nematelminthes 135 Nematoda 135 Acanthocephala 136 Annulata 136 Oligochaeta 136 Naididag 137 ^Eolosomatidag 137 Rotifera 138 Rhizota 139 Discussion of Species 140 Bdelloida 141 Discussion of Species 141 Ploima 144 Pulses of Ploima (Table) 146 Discussion of Species 147 Tabular statistical data : Pulses of Anurcea cochlearis ISO, 153 Pulses of A nurasa hypelasma 155 Pulses of Asplanchna brightwellii 156 Evidence of polycyclic character of seasonal distribution of above . 158 Evidence of same for Asplanchna priodonta 161 Pulses of Brachionus angularis 165 Collection data for Brachionus bakeri and varieties 169, 170 Pulses of Brachionus budapestinensis 176 Pulses of Brachionus militaris 179 Pulses of Brachionus mollis 181 Brachionus pala and varieties. Average number per collection . 182 Pulses of Brachionus pala and varieties 184 Brachionus pala, type form. Sexual cycle 188 Seasonal distribution of B. pala and its var. amphiceros 189 Seasonal limits of B. pala var. dorcas 191 Pulses of Brachionus urceolaris 193 Brachionus urceolaris and varieties and B. variabilis. Average number per collection 196 Pulses of Polyarthra platyptera 204 Pulses of Synchceta pectinata 211 Pulses of Synch&ta stylata 213 Pulses of Triarthra longiseta 218 Scirtopoda 219 Seasonal fluctuations of Pedalion mirum (Table) 220 Gastrotricha . . 221 PAGE Entomostraca 221 Branchiopoda 222 Cladocera 223 Cladocera and Hydrographic Fluctuations (Table) 223 Discussion of Species 225 Tabular statistical data: Bosmina and hydrographic fluctuations . r- 228 Seasonal distribution of Chydorus. Average number per m. 3 . . . 235 Effect of temperature on numbers of Chydorus 238 A typical pulse of Daphnia cucullata 242 Diaphanosoma population, with data of temperature and river level 248 Moina and hydrographic changes 254 Ostracoda 257 Discussion of Species 258' Copepoda 258 Copepoda and Hydrographic Changes (Table) 260 Discussion of Species 261 Cyclops viridis var. insectus and Hydrographic Changes (Table). . 274, 275 Relative numbers of adult, young, and larval Cyclopidae (Table). . 278 Amphipoda 283 Arachnida 283 Acarina 283 Tardigrada 284 Hexapoda 284 Ephemerida 285 Hemiptera 285 Diptera 285 Mollusca 287 Gastropoda ' 287 Lamellibranchiata 287 Bryozoa 288 Discussion of Species 288 The Periodicity in the Multiplication of .the Organisms of the Plankton. . . 291-311 Comparison of Plankton Pulses and Lunar Cycle (Table) 296-299 Gaseous Contents of Pond Water (Table) 306 Relation of Pulses of Chlorophyll-bearing Organisms to Lunar Cycle (Table) 308 General Considerations on Seasonal Changes 312 Lake versus River Plankton 312 Organisms per Cubic Meter in Plankton of Illinois River in 1898 (Table) . . 314-340 Bibliography 341-354 Explanation of Plates 355 Diagrams of Seasonal Distribution 356360 Errata and Addenda. . 361 ARTICLE I. Plankton Studies. V. 1 The Plankton of the Illi- nois River, 1894-1899. Part II. Constituent Organisms and their Seasonal Distribution. BY C. A. KOFOID. INTRODUCTION. This paper gives the results of a statistical study of a series of quantitative plankton collections made in the channel of the Illinois River near Havana, 111., at the Illinois Biological Station, in 1894- 1899. The environmental conditions and the volumetric results of this investigation have been given in Part I. (Kofoid, '03), published in Volume VI. of this Bulletin. Of the 235 collections made in channel waters and used in the quantitative study, only 182 were subjected to numerical and qualitative analysis. The omitted collections were intercalated at brief intervals of one to several days between those enumerated, principally in the summer of 1895 and during the winter flood of 1896 and the summer of the same year. The collections chosen for this study, whenever possible, represent a weekly interval, and a full list of all collections, with environmental data, may be found in Table III. of Part I. The chronological distribution of the collec- tions studied by the statistical method is given in the table on the following page. The work of enumeration and the primary tabulation was com- pleted at Urbana December 31, 1900, when my formal connection with the State Laboratory ceased. The manuscript has been 1 The four preceding numbers of this series, all by the present writer, have been published as articles of the Bulletin of the Illinois State Laboratory of Natural History, as follows: Article I., Vol. V. Plankton Studies. I. Methods and Apparatus in Use in Plankton Investigations at the Biological Experiment Station of the University of Illinois. Article V., Vol. V. Plankton Studies. II. On Pleodomna illinoisensis , a New Species from the Plankton of the Illinois River. Article IX., Vol. V. Plankton Studies. III. On Platydorina, a New Genus of the Family Volvocidce, from the Plankton of the Illinois River. Article II., Vol. VI. Plankton Studies. IV. The Plankton of the Illinois River, 1894-1899, with Introductory Notes upon the Hydrography of the Illinois River and its Basin. Part I. Quantitative Investigations and General Results. prepared at Berkeley, being completed in May, 1904, after my connection with the University of California was begun. My separation from the collections and the library of the State Labora- tory has rendered impossible some verifications, comparisons of specimens with more recent literature, especially among the algae, DISTRIBUTION OP COLLECTIONS BY MONTHS. '94. 91 '96. '97. '98. '99. I 4 3 5 II 1 4 2 4 4 Ill 5 1 5 4 IV 2 4 1 4 V 4 1 5 VI 2 1 5 1 4 VII 2 4 5 3 4 VIII 1 5 6 4 5 IX.. 2 4 2 4 4 X 1 5 1 4 4 XI 1 4 1 5 5 XII 1 5 2 4 5 Total 10 31 43 30 52 13 some desirable amplifications from omitted intermediate collec- tions, and the elimination of a few minor errors in the statistics. It should be understood that the data of this paper are derived from channel collections, and the conclusions apply only to that region. Conditions of plankton development in the adjacent back- waters, as shown in Part I., differ greatly in volumetric character and seasonal distribution. The composition of the plankton and the seasonal distribution of its constituent organisms also exhibit there many points of difference from those here described for channel waters. METHODS. The collections were preserved in bottles of uniform capacity (60 cm. 3 ), in alcohol-formalin mixture (2 per cent, formalin in 70 per cent, alcohol), and after measurement by the centrifuge were released from the compressed condition in the measuring tubes and returned to the containers. The counting was done by a modified Sedgwick-Rafter method (see Kofoid, '97), in which 1 cm. 3 of a suitably diluted plankton is distributed evenly in a cell 20 X 50 mm. The plankton was diluted or condensed (from 60 cm. 3 of fluid) according to the quantity of plankton and the amount and nature of the silt. Larger organisms such as the Entomostraca were counted in the whole catch, or in larger collections in Yio to 1 / 50 of the total catch ; and the smaller organisms in l l^ to V^o- The filter-paper catches which supple- mented those of the plankton net from August 3, 1896, to the end of the series, March 28, 1899, were often subjected to considerable dilution on account of the great amount of fine silt in the collections, from Vio "to Yioo being the limits of dilution as a rule. The even distribution of the organisms in the Rafter cell was secured by shaking the collection in a mixing cylinder gently till the sediment was thoroughly distributed, and taking the sample immediately with a long 1 cm. 3 pipette, inserted to the bottom of the jar and raised to the surface during the filling process, and by discharging the contents immediately into the cell at one corner, the cover having been previously displaced at a slight obliquity to admit the end of the pipette. With the filling of the cell the cover automatically moves into place, and practice soon enables one to fill the cell without inclusion of air bubbles. With the exception of the heavier rhizopods, all of the organisms are as a rule very evenly distributed by this method. The identification and enumeration of the contents of the cell were carried on with the help of a mechanical stage and a f Bausch & Lomb objective, with a Zeiss C for higher magnification when needed for the detection of fine details or for counting the smaller organisms in the filter-paper catches. After considerable experimenting, the following method was established in the work of enumeration. Four sheets, each with numbers 1 to 76 at the left, were fastened temporarily to accom- panying key sheets, each number on each sheet standing for one of the more common species. One sheet was assigned to algae, diatoms, and miscellaneous organisms; and one each to Protozoa, Rotifer a, and Entomostraca. As the plankton sample was examined under the microscope the identifications were called off, and entered on the sheets by a clerical assistant. Six of the most abundant species were recorded by the observer himself on six tallying machines registering 1,000, and conveniently arranged in a box at his right. By adjusting the springs to give different sounds when registry was made, and by modifying the surfaces pressed by the fingers so as to differentiate the several machines without looking at them, it w r as possible to use these without raising the eye from the microscope, and thus to avoid the fatigue arising from the repeated muscular readjustment of the eyes necessary when the observer makes his own entries in a written record. Common species not recorded by the tallying machines were generally abbre- viated or designated by easily-called tokens. When once fairly familiar with the species it was possible by means of these labor- saving devices to make identifications and enumerations of several heavy planktons per day. By a number of tests I found that when the enumerations of a species in a given collection reached 1,000, little was gained by carrying it to higher numbers. A limit of error of 5 per cent. can be thus obtained if the species in question is distributed evenly in the cell and all precautions are observed to secure accuracy. Enumerations were often carried beyond this point, but rarely beyond 3,000. The accessions numbers of the collections from our catalog of collections served to designate each sheet of data and all note slips bearing on the collection or its constituent organisms. When the enumeration was completed, the factors of collection, dilution, and enumeration were entered on the sheets, and the number of individuals of all species represented was computed and carried to the right of the sheet. The totals of the various groups for ex- ample, diatoms or Cladocera were then added up and entered on the sheets in differential colors. By the use of the key sheets the number perm. 3 of water of any given species could be quickly ascer- tained. Species not in the key were entered by name on the sheets. When the enumeration of all collections was completed, the numbers per m. 3 giving the seasonal distribution of the various 7 species and groups through the collections of 1894-1899 were drawn up on uniform folio sheets, and the annual totals and averages com- puted therefrom. With the data in these forms it is possible to turn at once to the statistics of the plankton of a given day, or to the seasonal distribution of any desired species. ACKNOWLEDGMENTS. I am indebted to Prof. S. A. Forbes, Messrs. E. B. Forbes, F. W. Schacht, and R. W. Sharpe for many suggestions concerning the Entomostraca; to Prof. Frank Smith for assistance with the Oligo- ch&ta of the plankton ; and to Mr. A. Hempel for my introduction to the Rotifera. The identification of the cosmopolitan species of the fresh-water plankton of the Illinois River was greatly facilitated by the most excellent library of the Illinois State Laboratory of Natu- ral History, the accumulation of many years' careful selection by its director, Prof. S. A. Forbes. The literature of fresh- water fauna, and to a large extent of its flora also, is very fully represented therein. The excellent Laboratory collection of identified Entomostraca from European specialists was also of great service. I am indebted to Mr. R. E. Richardson for valuable services as clerical assistant, and for substantial help in organizing the great mass of data resulting from the enumerations. ' Except as noted in the discussion in subsequent pages, I hold myself responsible for all of the identifications of the species re- corded. The enumeration is also all my own work, with the excep- tion of that of the nauplii, of two species oi^ttifflugia, and of Pedi- astrum in about one third of the collections, in which I had the assistance of Mr. R. J. DeMotte, and that of the commoner Rotifera in a few of the collections, which were counted by Mr. Richardson. DEFINITIONS. The term "plankton" was used by Hensen ('87) to designate "Alles was im Wasser treibt." It was applied by him only to that assemblage of marine organisms which float passively in the open sea, without active recourse to shore or bottom, and unable by their own efforts materially to change their location. The term has since been extended also to assemblages of organisms in fresh water which bear a similar relation to open water. This fresh-water plankton has been designated in turn " limnoplankton " by Haeckel ('90), a word which in a restricted sense is retained for the plankton of lakes, while that of rivers has been distinguished by Zacharias ('98a) as "potamoplankton," and that of ponds ('98) as "heleo- plankton." These distinctions are based upon the nature of the environing body of water, and the terms are convenient, though the separation of these types everywhere in nature is difficult, if not impossible. Owing to the smaller size of fresh-water basins as compared with those of marine character, the shore and bottom be- come more important as factors in the environment of the plankton. Within the fresh-water environment we also find degrees of impor- tance of the shore and bottom which in ascending scale dominate in the lake, river, pond, and marsh. Although each of these repre- sents distinct conceptions, in nature we find them imperceptibly intergrading, and neither these conceptions, geographical nomen- clature, nor local parlance give us any final criterion which will enable us to use the terms with the precision which a scientific terminology would demand. The distinctions between these forms of fresh-water plankton must lie in the plankton itself, if anywhere. As I shall attempt to show later, these distinctions, though appar- ent, in some cases at least, are nevertheless of minor importance, and depend very largely upon the relative predominance of the adventitious littoral fauna and flora rather than upon distinctive assemblages of eulimnetic species. The striking similarity of this eulimnetic plankton in all these types of environment and in widely separated continents is a biological phenomenon of far more sig- nificance than these minor differences. These distinctions between the different types of fresh-water plankton are thus more a matter of terminology than of biological import. Among the organisms found in open water there are varying degrees of dependence upon the shore and bottom. Some, as Cyclops and many of the lower algas, have life cycles in which no encysted or quiescent resting stage has been found, and actively or passively their whole existence is passed in the open water. They are at all times components of the plankton; that is, are continuous planktonts. Others, as Dinobryon, many of the Rotifera and Cladoc- era, and, in fact, the greater part of the eulimnetic organisms, have an encysted stage which as a winter egg or a cyst descends to the bottom and remains there for a season. Such organisms only periodically, wholly or in part, leave the open water for a littoral or benthal existence. They are periodic planktonts. Some organ- isms, such as many of the rhizopods and diatoms and Hydra, appear in the plankton under certain conditions of temperature and food. They temporarily adopt the limnetic mode of life as a result either of a change in their specific gravity due to internal changes, such as an increase of the gaseous or fatty contents of their protoplasm, or to changes in the buoyancy of the water due to changes in temperature or in substances in solution in the water, or because of the abundance of food in the open- water. They become under these conditions actively adventitious planktonts. Still other organisms are released from their usual contact with or attach- ment to the substratum, or from their association with debris or vegetation of shore or bottom, by movements or disturbances in the water, and are swept into the open water only to return again to their customary habitat when conditions favor. Practically all of the smaller organisms inhabiting the shore and bottom and the debris and vegetation found thereon are liable thus to enter the open water, and to be found in forced and temporary association with the eulimnetic fauna and flora. They are passively adventi- tious planktonts. Another class of organisms which occur in the plankton are those which either as internal or external parasites find in plankton organ- isms either a host or a substratum for attachment. These are in a certain sense passive planktonts, and they may be distinguished from other passive planktonts as attached or parasitic planktonts. Sharp lines between these various classes of organisms found in open water can not be drawn upon distinctions based upon their degree of dependence upon the bottom and shore. An equally vague line separates the organisms of the plankton from those more active forms which by virtue of their powers of locomotion are to a con- siderable degree independent of waves and current, and are able freely to maintain their position in their preferred habitat. Among the organisms commonly included in the plankton, the flagellates, rotifers, and Entomostraca exhibit some degree of activity, such as is seen in their limited vertical migrations, while larger organisms, such as Leptodora hyalina and the larvae of Corethra, are capable of movement sufficient to give them considerable independence in the matter of their position in the water. We thus find degrees of inde- 10 pendence which approach closely that found in young fish and the large insect larvae organisms not always regarded as planktonts. The plankton is thus a composite assemblage of organisms whose association depends in varying degrees upon their relation to their common habitat, the open water. In actual practice, all the organ- isms found in the open water are regarded as within the scope of plankton investigations, and justly so, for by virtue of their pres- ence they become more or less involved in the complex interrela- tions which pertain to the flux of matter, the succession of species, and the food relations which exist through the changing seasons in the aquatic environment. In our own investigations it has been our purpose to include all the organisms found in our collections ; that is, all which our meth- ods of examination give us a sufficient means of investigating. Naturally, the bacteria are to large extent excluded from our con- sideration, though they properly belong to the plankton, and in the processes of nitrification and denitrification play an exceedingly im- portant part in the economy of aquatic life. THE COMPOSITION OF THE PLANKTON. The composite character of the plankton is especially marked in streams , as, for example , in the Illinois River, owing to the mingling of organisms from a great variety of tributary sources backwaters, lakes, ponds, pools, marshes, swamps, brooks, rivers, canals, sewers, drains, and industrial wastes. Few lakes possess so varied a supply, and in none can the proportional effect of these contributions exceed that of the stream. Added to this contributed assemblage, and in some seasons predominating over it, is the indigenous or autono- mous plankton of the stream itself. The component organisms of the plankton of the Illinois River number 528 forms, including only those which have been identified from collections made in the main stream and including both species and well-defined forms or varieties. Species found thus far only in the backwaters are not included, though there is little doubt that they occur also in the main stream. No effort has been made to build up merely a long list of species, but only to identify, so far as possible, the common and recurring forms. Neither has any attempt been made to establish new species or revise those already 11 described, though a magnificent opportunity awaits the naturalist who has the fortitude to analyze the exceedingly variable forms which compose the plankton, and to determine by modern methods which of these variants are entitled to specific rank. It has seemed to the writer that the only satisfactory basis upon which species, and pre-eminently those of the fresh-water plankton^ can rest, lies in a careful determination of the limits of seasonal and local variation within the area of distribution. This means breeding under con- trol, and the study of variation by modern statistical methods. Both of these lines of inquiry lie beyond the purpose of the present paper, and plainly beyond the possibilities of accomplishment by any one investigator, when the great number of species and the pres- ent state of the literature of the subject is considered. It is becom- ing constantly more evident that the species of the plankton are in the main cosmopolites, and the world literature of the subject must be taken into consideration in any thorough attempt to handle the systematic side of the subject. During the progress of this work, which was begun in 1894, every effort was made to secure all perti- nent literature bearing on the genera of plants and animals repre- sented in the plankton, and so far as possible in the enumeration of the collections the individuals were referred to "species" already described, or, in default of this, recorded as "unidentified." In some groups notably the desmids, diatoms, and unicellular algas it was not possible under the conditions of plankton enumer- ation to apply to all the individuals enumerated the fine distinc- tions which specialists in these groups have made. They have been thrown under certain of the better-defined species, which thus stand in our records as representatives of closely related variants as well as of the types of the species named. Examples of this appear in Closterium, where two species only were listed. Probably a num- ber of so-called species among the scores described in this genus will be found among the individuals in our plankton here referred to the two species C. acerosum and C. lunula. So, also, in the case of Melosira; two principal types were listed, M. varians and M. granulata, though even these two seem at times to intergrade. Other described species will be found among the individuals thus distributed. In the case of Difflugia globulosa and D. lobostoma a large number of intergrading and variable forms are included. It would be possible to find among these, representatives of many 12 recently described species. In these instances the difficulty lies not so much in finding representatives of these closely related species, but, rather, in drawing the lines between them and placing every individual enumerated in the proper pigeon-hole. To avoid this difficulty, the separation was not attempted in every case. With the hope that the results would throw some light on the ques- tion of seasonal variation, this separation was attempted in the genus Brachionus, where the species characters are confined to prominent structural features. So far as it was feasible, specific distinctions w r ere accepted as found, and utilized whenever possible. In the lists and discussions which follow, the inclusion of a species does not necessarily carry with it the inference that it is regarded by the writer as valid or well founded. It merely represents in our enumerations a more or less continuous succession of organisms which conform approxi- mately to the descriptions and figures of the species designated by the name in question. Inferences regarding the rank or validity of the species reported will be given whenever the statistical data or my observations on the variability of the organism seem to afford data bearing on the standing of the species. While not a few of the species reported may justly be regarded as synonyms, an effort has been made to use only names which represent valid species or at least a variety or a seasonal form. COMPARISON OF FRESH-WATER AND MARINE PLANKTON. The plankton of fresh water is very generally composed of an assemblage of organisms, of plants and animals, principally crypto- gams and invertebrates. Not all orders are represented, and those that do occur vary greatly in the number of their representatives. The fresh-water plankton differs from that of the sea in the almost universal absence of larval forms, in the smaller number of inverte- brate groups represented, and in the smaller size of its component organisms. Fresh-water plankton has almost no limnetic coelen- terates, Hydra fusca being the only representative as yet discovered in our locality. The absence of the larger Crustacea, of limnetic mollusks and worms, and of tunicates and Radiolaria robs limnetic life of the diversity found among pelagic organisms of the sea. The only larval stages found in our locality are the glochidia of the 13 Unionidae, whose limnetic sojourn is at the best but brief, and the larvas of certain dipterous insects, such as Chironomus and Corethra. The limnetic habit of these larvas is hardly established as yet. The small size of fresh-water planktonts as contrasted with those of the sea is very striking. Representatives of the same group for exam- ple, the Dinoflagellata and the Entomostraca in the two habitats exhibit this contrast. The largest entomostracan of fresh water is less than a centimeter in length, and there is nothing to compare with the pelagic ccelenterates, Mollusca, or such tunicates as Salpa and Pyrosoma. The smaller size of fresh- water planktonts may be due to the lower specific gravity of the environing medium, and perhaps also to the effect of smaller quantities of dissolved salts upon the metabolic processes of limnetic animals. Notwithstanding this absence of large individuals in the plank- ton of fresh water, the total quantitative production of plankton per cubic meter is greater here than in the sea. For example, the average production in the Illinois River is 2.71 cm. 3 , and the average amount in adjacent backwaters rises as high as 22.55 cm. 3 (in Phelps Lake) . These measurements were made by the centrifuge, and the results of the "Plankton Expedition" of Hensen reduced to this basis of measurement by Kramer ('97) show that the Atlantic Ocean at the time of this expedition had in the upper strata exam- ined but 0.12 to 0.48 cm. 3 of plankton per cubic meter ORGANISMS OF THE PLANKTON. The groups of plants represented in the plankton of the Illinois River are principally algas, of which the Bacteriacea are but partially retained in the collections and are usually omitted in plankton investigations. The Sckizophyce&, or blue-green algae, furnish a few important representatives and a number of adventitious .species. The ChlorophycecB, or green algas, on the other hand, abound both in species and individuals, and afford an element of great impor- tance in the primal food supply. The Bacillariacece are exceedingly abundant, and are represented by a number of eulimnetic, as well as many adventitious, species. They also constitute one of the primal sources of food for the zooplankton. The Conjugate furnish but few species and individuals -principally desmids to the phytoplankton. The phanerogams afford a few species which 14 are often taken with the plankton by virtue of their semi-limnetic habit, but do not in the living state enter the food cycle of the plankton nor affect its economy except as competitors. The zooplankton includes representatives of a considerable range of groups, though both in species and individuals the Proto- zoa, Rotifer a, and Entomostraca predominate among the animals. Representatives of other groups are in the main adventitious. Among the Protozoa, the Rhizopoda are constantly represented by many individuals and a considerable number of species, many of which may be adventitious, but most of which are wont to adopt the limnetic habit during the warmer months. The Heliozoa are few both in species and individuals. The Mastigophora (which in our discussions include all green and brown flagellates often clas- sified with the Chlorophyce and Ph&ophycece] vie with the Chloro- phycecs and Bacillariacecz for the first place as converters of the inorganic (and perhaps also the dissolved organic) matter into food for the zooplankton. They are exceedingly numerous in our plank- ton both in species and individuals, and form quantitatively a con- siderable part of the plankton during the summer months. The usual method of plankton collection by silk bolting-cloth per- mits a large proportion of these organisms to escape. The Ciliata furnish a few constant members of the plankton, and numerous adventitious and parasitic species. During the low water of autumn, when bacterial contamination is at its height, these organ- isms form a large part of the plankton. The small size of some of the ciliates, combined with their motility and flexibility, renders the loss by their escape through the silk net considerable. The Suctoria furnish but few species and individuals mainly adventi- tious or attached to other planktonts. The Rotifera constitute, both in species and .individuals, the most important single group of analytic organisms, that is those of distinctly animal metabolism, occurring in our plankton. This may in part be due to our shallow warm waters and to the abundance of Chlorophyceoz and Mastigophora, which enter largely into their food. This abundance of the Rotifer a may prove to be character- istic of the plankton of rivers ( potamoplankton) as contrasted with that of lakes (limnoplankton) . While many rotifers are eulimnetic, the plankton also contains numerous adventitious species. 15 The Entomostraca include the largest fresh-water planktonts, and in every respect constitute an important element of our river plankton. They form the final link in the food cycle which con- nects the nutrients in solution in the water and in decaying detritus with the fish and other aquatic vertebrates. They include numer- ous species, some of which are adventitious. All of the Ostracoda belong to this latter class. The Cladocera furnish some of the most important eulimnetic species and a large number of adventi- tious forms, while the Copepoda are almost wholly eulimnetic. In addition to these groups, the Turbellaria, Oligoch&ta, Hexap- oda, Hydrachnida, Gastrotr-icha, and Bryozoa furnish a few species and individuals of a semi-limnetic or adventitious character to the plankton. In the table which follows, these various groups are listed, and the number of forms occurring in each is noted. In order to give some idea of the proportionate representation of these groups in our plankton, the table includes the sum of the number of indi- viduals per m. 3 of water in the weekly collections for the year 1898. This was a year of no marked departure from the normal regimen of hydrographic conditions (Part I., PI. XII.). The summer and autumn flushes tend to lower the population somewhat below that of more stable seasons, but beyond this feature there is nothing to suggest that the plankton of this year may not represent a fair average of that recurring each year in the Illinois River. The fig- ures given, in all cases refer to the number of individuals per cubic meter (excepting only such cases as Synura and Uroglena, where the colony rather than the individual becomes the unit). The algae and Protozoa include many species enumerated in filter-paper col- lections, which accounts for the large numbers in some of the totals. The "number of forms" listed refers to the total number found in the waters of the river during the period of our operations. Some species not noted in 1898 are therefore included. Unidentified forms are not included in the list of number of species, though the groups here listed to which they belong were known. Some forms referred to genera but not determined as to species are, however, included. This table throws some light upon the ecological relations of the groups composing the plankton, since it gives some clue to their relative numbers, and these condition in a general way the food 16 relations existing between the different groups. The plants are more abundant (and generally smaller) than the animals, outnum- bering them nearly 5 to 1 . Computation shows that for each one of the Cladocera there are 7 Copepoda, the predominance of the latter CONSTITUENT GROUPS OF THE ANNUAL PLANKTON OF THE ILLINOIS RIVER. AVERAGE OF 52 WEEKLY COLLECTIONS IN 1898 NUMBER PER M 3 . Number of forms recorded. Number of individuals. Algae: Bacteriaceae 3 (57,142,822)* Schizophyceae 9 85,909,985 Chlorophyceae 33 53,175,105 Bacillariacece 29 396,192,716 Conjugatas 7 48,459 Phanerogamia 2 9 Total phytoplanktonts 83 535,326,274 Protozoa total (185) (111,731,000) Mastigophora 68 95,856,449 Rhizopoda 59 55,364 Heliozoa 5 4,871 Sporozoa 3 1,638 Ciliata 45 15,812,346 Suctoria 5 332 Rotifera 104 592,416 Entomostraca total (43) (47,041) Cladocera 26 6,242 Ostracoda 4 191 Copepoda 13 40 , 608 Miscellaneous . . 114 9,393 Total zooplanktonts 446 112,379,850 Total planktonts enumerated. 529 647 706 124 Synthetic (chlorophyll-bearing) 613 017,986 Analytic (non-chlorophyll-bearing) 34 687 781 being accounted for in part by the fact that their larval stages are free-s Dimming and appear in the enumerations, while the young of the Clidocera are not set free until nearer maturity. About 10 to 20 per cent, of the Copepoda are adults. The relative numbers of * Represents fragments of filaments, and is not included in totals. 17 the two groups are not so disproportionate as the figures might seem to indicate. For each one of the Cladocera there are 95 roti- fers and almost 18,000 Protozoa. The latter are distributed as fol- lows: There are 9 rhizopods, almost 2,400 ciliates, and over 15,000 flagellates for each one of the Cladocera. There are also about 86,000 plants for each of these Cladocera. Of these plants, 64,000 are diatoms, 14,000 are Schizophycecz, 9,000 Chlorophycece, while but 8 are desmids. The great abundance of diatoms, of green and blue-green algae, and of chloryphyll-bearing flagellates affords, it would seem, an abundant food supply for the zooplankton. If of the Mastigophora the colorless flagellates only be retained in the zooplankton, and the remainder which are predominantly synthetic forms be included with the phytoplankton, we find the latter outnumbering the analytic organisms (zooplankton) 18 to 1. Quantitative values in the matter of food relationships are not readily determined except by a combination of the chemical and experimental method. These results by the statistical method express, with more or less error, the equilibrium of the biological components in terms of the individual organisms. DISCUSSION OF THE STATISTICAL DATA OF THE SPECIES COMPOSING THE PLANKTON OF THE ILLINOIS RIVER IN 1894-1899. In the following pages the organisms occurring in the plankton of the Illinois River will be recorded, and from the statistical data accumulated by the enumeration method, facts pertaining to their relative abundance, seasonal distribution, and periods of max- imum occurrence will be cited. The average number per cubic meter for the year 1898 will be given, based upon the averages of 52 collections distributed regularly throughout the year (Part I., Table III.). This year is chosen because of the regularity of the times of collection and the absence of any considerable irregularity in the hydrograph. Statements concerning seasonal distribution, etc., are based upon the records for all the years 1894-1899. All figures pertaining to species or groups marked with an asterisk, and starred figures elsewhere, are based upon filter-paper catches; all others, upon those of the silk net. Temperatures are in Fahren- heit, and are of surface waters at time of collection.. The margin of error in statistical work of this sort is confessedly large. The complex character of the data with which I am deal- ing, and especially the extreme range in numbers, have made it necessary that I should adopt some consistent method of treating the computations. I have therefore chosen to carry out the num- bers to units, as the most feasible method of avoiding confusion in the handling of the data. The use of round numbers would have been just as accurate. Computation to units is therefore to be understood as a matter of convenience, and not as an effort to exhibit a false and unattainable accuracy. CRYPTOGAMIA. BACTERIACE^;.* Records were kept of the masses of the larger members of this group which occurred in our plankton catches. They were princi- pally the dichotomously branched brownish fragments of Creno- thrix, filaments of Beggiatoa, and colonies of Micro coccus. The average number recorded for this year was 57,142,822, and they occur throughout the year in every collection, rarely falling below 18 19 10,000,000 per m. 3 , and reach their maximum development (over 600,000,000) in winter months (December to February), especially during low water and more stable conditions, as in January, Feb- ruary, and December, 1898 (Pt. I., PL XII.). At such times the temperature is at or near 32. With flood conditions and rise in temperature the numbers fall below 100,000,000, Tunning from 10,000,000 to 50,000,000 during most of the summer. The decline is due in part to the dilution by flood waters, and largely to the retreat up the stream of the crest of the wave of bacterial activity caused by the Peoria pulse of sewage. As noted in the discussion of the chemical conditions, in Part I., this wave lies considerably above Havana during the warmer months. Summer floods, as in June and September, 1897, are wont to wash into the river large quantities of these organisms, bringing the numbers up to 300,- 000,000 at times. The figures above cited give but a feeble repre- sentation of the real conditions in the river during this period of maximum. Many of these organisms become attached to objects along shore, and accumulate in great quantity in quieter waters along the channel. They form a serious menace to the fishing industry, since they accumulate in a day or two upon the fyke-nets in quantity so great that their- weight and resistance to the current are sufficient to break down the nets. Their effect upon the consti- tution of the plankton is seen in the marked increase in certain ciliates which accompanies the maximum of these organisms. SCHIZOPHYCEyE. Nine forms were recorded, though a number of others which occurred but rarely in the plankton remained unidentified. The average number (combined silk and filter-paper records, but omit- ting the former when the latter are available) is 85,909,985 per m. 3 This group contributes to the plankton throughout the year, and though numerically abundant is quantitatively less important, owing to the small size of its most abundant member, Microcystis. This species and Oscillatoria constitute quantitatively the greater part of the blue-green algas of the plankton. In contrast with the plankton of Lake Michigan, there is a noticeable decrease in the proportion of AnabcBua and Clathrocystis. Rivularia, Gloiotrichia, and Aphanizomenon flos-aquce, often reported in fresh- water plank- (3) 20 ton, were not found in our fluviatile environment. This group contributes to the water-bloom, contains a number of adventitious planktonts, and is one of the primal sources of the food supply. In our waters it seems to be quantitatively much less important than either the Chlorophycea, the BacillariacecE, or the synthetic Mastigophora. DISCUSSION OF SPECIES OF SCHIZOPHYCE^E. Anabozna spiroides Klebahn.* Average number, 637,692 (silk 15,431). In the water-bloom from the last of June till the end of October. Not noted in 1898, but not infrequent in 1897 a low- water year. Temperature range, 60-89. Data insufficient to determine maximum. Largest number recorded, 7,200,000, June 28. Clathrocystis ozruginosa (Kutz.) Henfr. Average number of colonies or masses, 83. More abundant in the previous low- water year. From May till the end of November in the water-bloom. Predominantly a midsummer species. Maximum in August and September (108,000) . Confined principally to the low water of mid- summer, appearing when the water reaches a temperature of 70, and reaching its maximum development in temperatures above this point, declining at once to small numbers (less than 1,000) when the temperature falls below 60, but lingering till the water approaches the freezing point late in November. Merismopedia glauca (Ehrbg.) Nag. Average number of col- onies, 93. In 1897, 889,412.* In the water-bloom. Recorded from July till the end of October, and also singly in January and February. It was more abundant in 1897 than in 1898, and the maximum number (15,840,000*) appeared on August 31. Microcystis ichthyoblabe Kutz.* Average number, 83,059,615. Recorded in all collections throughout the year, except in some flood waters of February and March, when the silt probably ob- scures it. Minimum numbers (less than 50,000,000) prevail during cold months, November to April, when the temperature ranges from 32 to 50. A well-sustained pulse exceeding 200,000,000 appears with the volumetric plankton maximum of April-May (Pt. I., PI. XII.) and declines to the previous minimum with the falling off in the plankton. The maximum pulse appears later, in August and September in 1898, in September and October in 1897, 21 averaging about 200,000,000, and reaching 1,697,000,000 August 9, 1898. The temperatures during these pulses are above 60, and the period of the maximum comes toward the close of that of max- imum summer temperatures, and sometimes in the autumn decline (Pt. I., PL XI. and XII.), when low and often stable jriver-levels usually prevail. A vernal and an early autumnal pulse are thus both present in the distribution of this species. It is not improb- able that other species than the one named have been included in the enumeration along with it on account of the small size and lack of striking characteristics. There are suggestions of recurrent pulses at intervals of 2-6 weeks in the records (Table I.). Oscillatoria spp. Average number, 15,431 (filter-paper, 637,692). The probable inclusion of several species in the sums under this heading may account in part for the irregularity of the seasonal curve. Oscillatoria has appeared in every month of the year, though the occurrences were most frequent in the period from July till the first of October. The numbers are exceedingly irregular and variable, and the pulses of numbers seem to attend the initial stage of floods following stable conditions. Thus, while these organisms occurred but singly or sparingly in the plankton during the autumn of 1897, they rose to 277,200 with the flood of January 11, 1898, doubtless torn loose by the current from the bottom- their normal habitat. They are thus usually adventitious addi- tions to the plankton. Their frequent irruption into the plankton during midsummer and early autumn, and to some extent at other times, is due in part to the evolution of marsh gas in the detritus on the bottom. This breaks up the mats of Oscillatoria which coat the bottom and distributes them through the upper levels, where they remain in suspension for some time. This phenomenon is more prevalent in the marshy backwaters than it is in the river. Flood invasion in midsummer into the backwaters, such as Quiver Lake, is wont to cause there stagnation and great increase in Oscil- latoria, which to some extent enters the river with the run-off of the flood. Movements in the water and the evolution of marsh gas are thus principally responsible for the presence of Oscillatoria in the plankton. It still remains possible that its flotation during periods of optimum conditions of growth may be due to internal physio- logical conditions which lower the specific gravity of the organism. Its great abundance at times in upper levels in the backwaters sug- 22 gests the action of this factor, and if this be true, it becomes a tem- porary rather than an adventitious planktont. Temperatures seem to bear little relation to the occurrence of Oscillatoria in the plankton. Tetrapedia emarginata Schrod.* Average number, 242,308. From the first of August till the end of October in numbers from 1,000,000 to 3,500,000 per m. 3 , appearing later and in larger num- bers in October in 1897 than in 1898. At temperatures above 65. Tetrapedia gothica Reinsch, Glceocapsa polydermatica Kutz., and Gloeocapsa sp. were recorded once or twice in the midsummer plank- ton in relatively small numbers. CHLOROPHYCEyE. (Plates I. and II.) Average number, 53,175,105, including, without duplication, species from both silk and filter-paper collections. In 1897 this was very much greater (139,739,850), owing to the prolonged low water and higher temperatures of the late autumn. Although abundant, these organisms are outnumbered by the diatoms six to one, and by the synthetic Mastigophora by about two to one. The ChlorophycecB of the plankton, with few exceptions, are minute, and generally escape through the silk net. Pediastrum and colonies of Botryococcus are about the only species of which the usual method of plankton collection in our waters affords a fair representation. The Chlorophycea appear in every collection examined through- out all the years of our operations, with the exception of eight in midwinter floods in 1895 and 1896. As a group they are adapted to the whole range of temperatures, and exhibit in 1897, on April 28, a well-defined vernal pulse of 367,200,000, and a series of autumnal pulses culminating September 21 at 216,000,000, October 19 at 367,200,200, and November 23 at 52,000,000. In this year the -midsummer pulses are of minor importance in comparison with those of spring and autumn. In 1898 the vernal pulse is also well defined, culminating May 3 at 212,406,400, and it is followed by a series of four midsummer pulses of considerable magnitude, w r hich culminate June 14 at 46,000,000, July 19 at 277,000,000, August 9 at 370,000,000, and August 30 at 189,000,000. The autumnal pulse appears September 27, attaining 70,526,400. The summer and autumn hydrographs of this year are much more disturbed than in 23 the previous year (cf. PL XI. and XII., Pt. I.), especially at the time of the autumnal pulse. This may account for the contrast in the two years. The Chlorophycea as a whole exhibit (PI. I. and II. and Table I.) the tendency to form a seasonal curve of recurrent pulses at approximately monthly intervals (three to six weeks)-which gen- erally coincide with those of other chlorophyll-bearing organisms. Thirty-three forms of Chlorophycecz were recorded, and closer inspection of the collections will undoubtedly yield a considerable additional number either of closely related, and therefore included, species, or of those which occur but occasionally or in small numbers in the plankton. Numerically the leading species in the order of their importance are Scenedesmus quadricauda, Crucigenia rectangularis, Actinastrum hantzschii, Raphidium polymorphism, Scenedesmus genuinus, S. obli- quus, Richteriella botryoides, Ophiocytium capitatum, Oocystis naegelii, Ccelastrum cambricum, Oocystis solitaria, and Schroederia setigera. With the exception of Botryococcus braunii and the species of Pedias- trum, the remaining forms are both quantitatively and numerically of minor importance. The species just named were enumerated only in the silk-net collections, and ccenobia rather than individual cells were listed. If allowance is made for the loss of small individuals through the silk, and for the increase that would follow if individ- uals rather than ccenobia were the basis of representation, Pedi- astrum would occupy a place in the front rank of importance in the ChlorophycecB of the plankton numerically as well as quantitatively. As quantitative factors in the ecology of the plankton, Pediastrum, Scenedesmus, Ccdastrum, and Botryococcus take precedence over the smaller, though more numerous, forms, such as Raphidium and Crucigenia. The group is thus well represented in our plankton both in species and individuals. The leading planktonts of the group reported in European and other waters in lakes and rivers are here represented almost without exception by identical or closely related species. Botryococcus alone seems to be less abundant than in lakes at least, according to my own observations, it is. much more abundant in the summer plankton of Lake Michigan than in that of the Illinois River. The maximum numbers of Pediastrum reported by Apstein ('96) for Dobersdorfer See in July, when reduced to number per m. 3 , are frequently equaled or surpassed in our waters. 24 Data for comparisons in the case of the more minute organisms which escape the silk are lacking, since results of supplementary methods have not, up to the present, been published elsewhere. It seems probable, however, that the Chlorophycecs will be found to be somewhat more characteristic of the plankton of rivers than of lakes, and to be more prevalent wherever the shore with its decay- ing vegetation forms a large factor in the environment or where sewage contamination affords the requisite food for their develop- ment. DISCUSSION OF SPECIES OF CHLOROPHYCE^E. Actinastrum hantzschii Lagerh.* Average number, 199,038 (silk net, 338). From May until the middle of November, with maximum of 21,600,000 on August 30, 1898, and of 122,000,000 on September 21, 1897. There are also indications of a vernal pulse, which on May 25, 1897, attained 90,000,000. The major pulse occurs late in the summer, in August and September, while dimin- ished numbers continue until the first of November. Three single occurrences were noted in January, 1898, following the unusual prevalence of 1897, but aside from these the species occurs in the plankton at temperatures above 45, and both pulses lie in temper- atures above 65. As in many other species, a greater development was attained in 1897, in stable low water, than in 1898 in disturbed hydrographic conditions. This species occurs in the water-bloom, is favored by stable conditions, and finds its optimum temperature between 65 and 80. Botryococcus braunii Kiitz. Average number of colonies, 75. In previous years it was much more abundant, averaging 3,300 in 1897. It occurs from the first of April well into October, though in 1897 it continued until the middle of December. It may thus appear throughout the whole range of temperatures, 32 to 90, but as a rule occurs above 60. There is a suggestion of a minor pulse in June, 1896, but not in other years. The major pulse attains 57,200 on August 15, 1896, and 42,000 on September 14, 1897, and appears, with smaller numbers, in August of preceding years. The species occurred but sparingly in 1898. It is found in the w T ater- bloom, and is more abundant in the backwaters than in the main stream. Ccelastrum cambricum W. Archer.* Average number of cceno- bia, 640,384 (silk, 477). Occurs from the latter part of March till 25 towards the end of November, but principally from May through October. There are but slight indications of a vernal pulse, which on May 25, 1897, culminates at 3,600,000. The major pulse cul- minates at 10,800,000 on August 9, 1898. In the low water and prolonged high temperatures of 1897 the major pulse continues through September, culminating on the 21st at 32,000,000. The average number in this year was about four times as great as in 1898. The temperature limit is 43, though occurrences are few and numbers small below 65. The maximum development appears within the period of maximum heat, and towards its close. It is characteristic of the plankton of late summer and early autumn. Crucigenia rectangularis Nag.* Average number of colonies, 7,153,846. Recorded in all months but March and April, but spar- ingly from November till May. In 1897 pulses appeared in August, September, and October, attaining 32,400,000, 57,600,000, and 118,800,000, respectively. In 1898 there was but a single pulse in August, of 158,400,000. It was more abundant in the former year. It is present continuously in large numbers from July to October, though in 1897 the impetus of the unusual development was manifested by the continuance of the species even into Janu- ary. The optimum temperatures lie above 70, in the latter part of the period of maximum heat, though the species has been found in the plankton throughout the whole range of temperatures. The abrupt decline in numbers occurs between 65 and 40. It is char- acteristic of the plankton of late summer and early autumn. Golenkinia radiata Chodat. Average number of colonies, 519,231. It appears most abundantly during the April-May plank- ton pulse (7,200,000) and again, in increased numbers, at the end of August, thus suggesting a vernal and a late summer maximum. It seems to be most abundant at about 60, a temperature somewhat below the optimum for the two preceding species. Two occur- rences in December, 1896, and large numbers in August indicate its adaptability to the full range of temperatures. Oocystis naegelii A. Br.* Average number, 207,692. In 1897, much more abundant (average, 4,243,235). Present in numbers (over 5,000,000) from the end of May till the end of September. In 1897, pulses of 10,800,000, 46,800,000, and 24,750,000 appear in May, July, and September respectively. Both numbers and oc- currences are much less in 1898. The optimum conditions thus lie 26 above 70, though isolated occurrences in March and December in- dicate its presence throughout the whole range of temperatures. It appears to be a summer planktont without the marked prefer- ence for the close of the period of maximum heat noted in some other ChlorophycecE. Oocystis solitaria Wittr.* Average number, 121,153. In 1897 much more abundant, averaging 2,170,588. In this year it occurs in numbers above 1,000,000 from the end of July till the end of October, reaching a maximum of 36,000,000 on September 21, 1897. Its optimum conditions occur during the latter part of the period of maximum heat, at temperatures approaching 80. It disappears at 60, save for isolated appearances in December, at 33- - a fact which suggests its persistence in small numbers thoughout the year. It is characteristic of the plankton of late summer, that is, of low water, high temperatures, and stable con- ditions. Ophiocytium capitatum Wolle*. Average number, 1,465,385. More abundant in 1897, averaging 2,858,823. Present from the last of April until the beginning of November. There is some indi- cation of a vernal pulse, which on May 25, 1897, attains 3,600,000, and on April 26, 1898, 10,800,000. The major pulse appears in late summer or early autumn, attaining 57,600,000 on September 21, 1897, and 28,800,000 on August 9, 1898. The two pulses are separated by an interval in which occurrences are less frequent and numbers smaller. This planktont thus exhibits the tendency towards seasonal maxima near the average temperature. The greater development in 1897 is followed by a prolongation of the occurrences into November. The optimum temperature appears to be about 60 or above, the vernal pulse appearing at that tem- perature, and the major one at 71. No records occur below 46. Pediastrum boryanum (Turp.) Menegh. Average number, 4,510. This alga was found in every month of the year, though not in every collection examined. The numbers present fluctuate greatly and are usually much less than those of P. pertusum, with which.it is associated, and with which it fluctuates, often with remarkable coincidence. I have included under this head those individuals in which the ccenobium is a plate with no intercellular spaces or only insignificant ones. Individuals are not lacking which serve to connect this species with P. pertusum, and, indeed, with others 27 which have been described in this genus. This genus includes the most abundant of the larger algae in the plankton of fresh waters, and it affords an attractive field for the study of variation by statis- tical methods and for the determination by the experimental method of the effect of environmental changes upon structure. The two groups of individuals included here under P. boryanum and P. pertusum give typical curves of seasonal distribution which are so similar that their combination in a single series would not greatly modify the resultant seasonal curve. In the sum total of all collections P. boryanum (1,034,000) includes about one tenth of the number referred to P. pertusum (10,830,117). A few scattering individuals, generally less than 1,000 per m 3 ., appear at irregular intervals, during the colder months, from the first of December until the end of March. The number increases as the temperature rises, and the species appears in all collections un- til November, when it again becomes irregular in its occurrence in the plankton. The fluctuations in numbers during this period are very marked, the pulses of frequency being set off by intervals in which the numbers are small. A slight pulse of 2,120 appears on November 17, 1894. In 1895 the vernal pulse attains the very unusual number of 572,824 in the unusually low water of that year, and the autumnal pulse of September 5 is but 10,600, and is followed by a secondary one on November 27 of 4,081, perhaps as a result of the stable conditions and the abnormally high temperatures (above 45) which then prevailed (Pt. I., PI. IX). In 1896 the vernal pulse culminates May 18 at 31,164, while the autumnal pulse is scarcely visible and the numbers throughout the summer are small, as a result, it may be, of the repeated floods of that year (Pt. L, PI. X). In 1897, with few vernal data, the vernal pulse does not appear, though a rise to 8,000 occurs on July 21. The major autumnal pulse culminates on September 14 at 14,400, and another one on October 12 at 6,000, attending the late autumn of that year. In 1898 there are vernal pulses on May 10 of 6,400 and on June 14 of 32,000. The autumnal pulse on September 27 reaches the considerable number of 65,600. In the winter of 1898-99 Pediastrum was seemingly absent from the plankton. The pulses are thus somewhat irregular, though there is in this species a suggestion of vernal and autumnal pulses at corresponding 28 temperatures. The optimum conditions seem to lie above 60 and the maximum numbers to occur at or near 70. Pediastrum pertusum Kiitz. Average number of coenobia, 44,372. This species appears in the plankton in all months of the year and in almost all of our collections. It is the most abund- ant representative of the Cklorophyceaz which is retained by the silk of the plankton net, and is quantitatively an important factor in the ecology of the plankton. The numbers during the colder months, from November to April, when the water is from 32 to 40, are few, and the sequence of their appearance is fre- quently interrupted. As the temperature rises in April the num- bers increase, and the vernal pulse culminates in a maximum in May or June. There is no indication of the vernal pulse in the scattered collections of 1894. In 1895 the pulse is extreme, reaching 5,264,860 on June 19, in a period of exceptionally low water. In 1896 a pre- liminary vernal pulse culminates May 8 at 23,580 and is followed on June 17 by one of 107,200. In 1897 the few spring collections do not reveal any vernal pulse, while in 1898 a minor one on May 17 reaches 5,600, declines to 600 at the end of the month, and rises again to 56,000 by June 21. These vernal maxima all occur or at least pass through their period of development before the water reaches its midsummer temperature of approximately 80. They develop during the transition from 60to 80 (Pt. I., PI. IX. to XL). Autumnal pulses during the decline from 80 to 60 appear on Sep- tember 5, 1895, (105,996), on September 30, 1896 (9,200), on Octo- ber 12, 1897 (231,200), and on September 27, 1898 (259,200). In addition to these pulses there are others at irregular intervals during the summer: on July 30, 1894 (154,548), on July 2, 1896 (68,400), on August 15, 1896 (22,000), on July 14 (289,600) and on August 31, 1897 (442,000), and on August 2 (295,200) and 30 (326,400), 1898. The optimum conditions of development thus lie above 60, and pulses are more frequent in spring and late summer or early autumn near 70, though they appear somewhat less frequently during the summer in our maximum temperatures near 80. The cause of these pulses is not conclusively demonstrable from the data at hand, owing in part to the interval between examinations. Daily examinations of the plankton and chemical analyses seem to be desirable for such demonstration. There are indications, how- 29 ever, that certain conditions in the environment increase the amplitude of the pulses by hastening the rapidity of reproduction of these organisms. Of the fifteen well-defined pulses appearing in our records of six years, all but three minor ones occur in stable con- ditions, such as pertain to sustained low water. The ""greater part of these pulses, however, occur in declining floods, when contribu- tions from backwaters are considerable. It may seem ill-advised to refer to the conditions of falling river-levels as "stable"; neverthe- less, they are relatively much more stable than those which attend the in-rush of silt-laden flood- waters, and involve fewer changes in factors of the environment. Save in the matter of the relative con- tributions of backwaters and of sewage dilution they resemble those of sustained low water. These Pediastrum pulses are also re- lated to the nitrate pulses (Pt. I., PI. XLIII.-XLV. and Table X.), but the relation is not uniform. In the majority of instances the pulses of 1896-1898 (during which time chemical anaylses are available) coincide approximately with the crest or decline of in- crease in nitrates. For example, the pulse noted on July 17, 1896, of 107,200 from a previous level of 1,210 on June 1, follows a wave of nitrates 'progressing for three weeks and culminating on June 9 at 3.25 parts per million arise from 1.5 (Pt. I., PI. XLIIL). On June 16 the nitrates have fallen again to 2.2, and on the 23d to 2.0, but rise on the 30th to 2.8. Pediastrum responds to these changes by dropping from 107,200 on the 17th to 15,000 on the 27th, and by rising again on July 2 to 68,400. Not all of the fluctuations in the two are concomitant. Some of the most marked pulses of Pedi- astrum appear at the lowest levels of the nitrates. For example, that of August 30, 1898, of 326,400, follows no nitrate wave, though it coincides with a reduction in nitrates to the minimum of .05. On the other hand, the nitrites had just passed on August 23, an un- usual pulse, to .42, falling again on August 30 to .22 and on Septem- ber 6 to .05 with the passing of the Pediastrum pulse. Pulses of Pediastrum are thus apparently not dependent for their develop- ment upon an abundance of nitrates above the levels shown in the analyses, though a decline in these sources of food or in other forms of nitrogen usually attends these pulses. Pediastrum is but one of many factors among the planktonts, and in the environment, biological and chemical, concerned in these changes, and con- clusive demonstration of its ecological relations must be obtained 30 by the experimental method. The data here cited are suggestive only; not conclusive. The relation of Pediastrum to the volumetric pulses of the plank- ton is not a constant one, though there is some correspondence in their fluctuations. The extreme maximum (3,264,800) of June 19, 1895, is coincident with a plankton pulse of 30.42 cm. 3 , but the num- ber of collections is insufficient to show the relative fluctuations of the plankton and Pediastrum at that season. In May and June, 1897, and in October, 1898, the Pediastrum pulses culminate shortly after the volumetric pulses. In July and September, 1897, arid in August, 1898, they coincide. Polyedrium trigonum Nag.* Average number, 432,692. Ap- pears from June through September, disappearing when falling temperatures reach 60. In 1897 it continues through October with the higher temperatures (averaging 65) of that year. There are slight indications of a September pulse. Polyedrium trigonum forma minus Reinsch and var. tetragonum (Nag.) Rabh., P. bifurcatum Wille, and P. gracile Reinsch, were also recorded in a few collections during the period of occurrence of P. trigonum. They are all evidently summer planktonts. Raphidium polymorphum Fresen.* Average number, 2 1 ,450,000. Occurs in every month of the year and in a majority of the collec- tions. In 1897 a vernal maximum of 201,600,000 occurs on April 27 and an autumnal one of 28,800,000 on September 21. In 1898 a vernal pulse culminates May 3 at 24,000,000, and thereafter throughout the summer at intervals of three to six weeks there occur five other pulses, the greatest of which culminates July 19 at 75 ,600,000. A pulse of 90,000,000 on a declining flood in February, 1899, indicates an adaptation on the part of this organism to the whole range of temperatures. A pulse of 25,200,000 December 3, 1896, further illustrates this adaptability. Records in 1897 and 1898, however, suggest that the optimum lies above 60. It is thus a perennial planktont. Raphidium longissimum B. Schroder. Appeared sparingly in February, August, October, and December, suggesting that it has also a perennial distribution. Richteriella botryoides (Schmidle) Lemm.* Average num- ber, 6,399,705 (in 1897). From May to November, with a vernal pulse of 25,200,000 on May 25, and an autumnal one of 100,800,000 31 on September 21. Optimum temperature about 70, and disappear- ing from our records below 60. Scenedesmus bijugatus (Turp.) Kiitz.* Average number, 155,769. Sparingly from May till the close of September, with slight traces of vernal and autumnal pulses. Scenedesmus denticulatus Lagerh.* Average number, 86,538. A few occurrences in late summer and early autumn. Scenedesmus genuimts Kirchner.* Average number, 778,846. From May till the first of October, but continued through this month in 1897. Vernal pulse not observed, though the autumnal pulse attains 28,800,000 on September 21 and October 26, 1897. Midsummer pulses appear in 1897 on July 14 (16,200,000), August 17 (14,400,000), and in 1898 on August 9 (19,800,000). Optimum temperatures lie above 60, though an occurrence in December in- dicates the adaptability of this organism to lower temperatures. Scenedesmus obliquus (Turp.) Kiitz.* Average number, 1 ,505 ,769 (silk, 673). This form appears in our records from the last of April until the middle of November. Traces of vernal and autumnal pulses appear in both 1897 and 1898, with intervening midsummer fluctuations of even greater magnitude. In 1897 the vernal pulse on May 25 reaches 3,600,000; a midsummer one on August 10, 5,400,000; and the autumnal one appears twice, once on September 21 at 28,800,000, and again on October 19 at 25,200,000. In 1898 the vernal pulse appears May 10 at 1,800,000; midsummer ones, on July 19 at 10,800,000, and August 9 at 36,000,000 ; and the autumnal on September 9 at 8,100,000. As in some other organisms, these pulses are separated by intervals of three to six weeks. The optimum temperatures lie above 60, though development begins before that temperature is reached, and the impetus of the autumnal pulse, or acclimatization to lower temperatures, carries the species beyond this limit into temperatures of 45. There is a marked absence of pulses below 60. This seems to be a summer planktont with no marked preference for the lower temperatures of spring and autumn. Scenedesmus quadricauda (Turp.) Breb.* Average number, 9,276,923 (silk, 8,611). In this species, as in the case of others of the genus and of the Chlorophycecs generally, the numbers present in 1897 were much greater than in 1898 (32,492,647,* silk, 5,818). Prolonged low water and concentration of sewage afforded stable 32 conditions and food requisite for such development. This species appears in our collections in every month of the year, though in much smaller numbers and less frequently from November to April that is, below 50. Pulses of noticeable magnitude appear only above this temperature, and usually above 60. Slight traces of vernal and autumnal pulses appear in the col- lections of the silk net in 1894-1896. In the filter-paper collections of 1897-1898 they are well defined. The vernal pulse appears in 1897 on May 25 at 46,800,000, and in 1898 on May 10 at 70,200,000. The autumnal maximum in 1897 is remarkable both for its large numbers and its prolongation, culminating twice first on Septem- ber 21 at 151,200,000, and again on October 19 at 154,800,000. This remarkable development, combined with the stable conditions and higher temperatures (Pt. I., PI. XI.) of that low- water autumn, is responsible for the continuance of the species in our collections throughout the winter. In 1898 the species declined earlier, in November, and was but sparingly represented in collections of the winter of 1898-1899. As in other species of the genus and other Chlorophycece, midsummer pulses appear at intervals, often of four weeks, but ranging from three to six. In 1897 these occurred on July 14 at 55,800,000 and on August 31 at 21,600,000. In 1898 they appear on June 28 at 10,800,000, on July 19 at 79,200,000, on August 9 at 39,600,000, and on August 30 at 54,000,000. At inter- vals between the pulses the numbers decrease, and in the regular collections of 1898 the minima between the pulses do not in any case exceed 30 percent, of the adjacent maxima, and are usually very much less. The distribution of the pulses of this species coincides very closely with that of the other species of the genus, and also with that of other Chlorophycea. For example, Pediastrum pertusum, the most abundant of the larger algas, has seven of its thirteen pulses on the same dates with those of Scenedesmus quadricauda and three others on adjacent dates, leaving but three which are not practically coincident. The operation of some common and general factor in the environment is suggested by such phenomena. The wide seasonal range of this organism gives it a' claim to rank as a perennial planktont, though its quantitative distribution shows clearly that the optimum temperatures for its growth lie above 60. The largest number recorded in 1897 appears October 19 at a temperature of 65, and in 1898 on July 19 at 84. It is 33 thus predominant only during the warmer part of the year; and while autumnal and vernal pulses occur, there is no sustained mid- summer minimum intervening between them. The pulses in Scenedesmus as a rule follow the volumetric pulses as shown in silk- net catches (Pt. I., PI. XI. and XII.). Thus in 1897, on September 14 and 21, the plankton measures 19.8 and 3.0 cm. 3 per m. 3 , respec- tively, Scenedesmus quadricauda numbering 20,700,000 and 151,- 200,000; and, again, on October 5 and 19 the plankton measures 12.92 and 1.86 cm. 3 , and this alga numbers 93,600,000 and 154,- 800,000. Its share in the volumetric pulses is thus indirect to a large degree, and is perhaps modified by food relations. Schroederia setigera (Schroder) Lemm.* Average number, 21,450,000. In 1897, 69,040,912. It appears in all months of the year and in almost every collection. It has well-defined vernal and autumnal pulses separated by the summer period, in which only minor pulses occur. In 1898 midwinter numbers are as high as those of midsummer. Schroederia is thus truly a perennial plank- tont. The vernal pulse appears in 1897 on April 27 at 302,400,000, and in 1898 on May 3 at 150,000,000. The autumnal pulse in 1897 culminates on September 21 at 565,200,000, and is followed by sec- ondary culminations on October 26 at 136,800,000, and on Novem- ber 23 at 203,400,000. In 1898, when hydrographic conditions were less stable, the autumnal pulse reached only 50,400,000, on September 6. This is followed by minor pulses, declining to a mini- mum in the following February. It disappeared in the collections with the flood waters of March, 1899. The sequence of these sec- ondary pulses follows much the same course as has been described for other species, namely, maxima at intervals of approximately a month (two to six weeks) separated by more or less sharply defined minima. There are twelve such pulses (including the major ones) in 1898 and an interval of seven weeks in March- April in which none occurs. Six pulses appear in the last five months of 1897. The optimum temperatures as indicated by the position of the vernal (60 in both 1897 and 1898, as shown in Table III., Pt. I.) and autumnal (71 in 1897 and 79 in 1898) pulses lie between 60 and 80. This appearance of the vernal pulse at a lower temperature than the autumnal (usually about 10 lower) is not confined to this species but is a general phenomenon among other Chlorophycea. 34 It is apparently a phenomenon of seasonal acclimatization, by virtue of which the low temperatures of the winter lower the optimum for the vernal pulse, and the high temperatures of the summer raise it for the autumnal pulse. Selenastrum bibraianum Reinsch.* Average number 519,235. Recorded only from the beginning of August till the end of Novem- ber, and never in great abundance. Slight evidence of a September pulse. Some other Chlorophycecs have been included in the totals as "unidentified," and isolated occurrences of the following have been noted: Cerasterias longispina (Perty) Reinsch, C. raphidioides Reinsch, Dactylococcus infusionum Nag., Glceocystis gigas (Kiitz.) Lagerh., Staurogenia lauterborni Schmidle, and a few of the Con- fervacecz which are probably adventitious. These are a species of Conferva, of Prasiola, and of Ulothrix all of which appear sparingly in spring and autumn planktons, the first-named and the last as minute filaments in the filter-paper collections. A thorough analysis of the unidentified forms would greatly extend the list of species and varieties. BACILLARIACE^E. (Plates I. and II.) Average number, 396,192,716, including, without duplication, diatoms from both silk and filter-paper collections. They were almost twice as abundant in the more stable conditions in which the collections of 1897 were made. The Bacillariacece are more abun- dant than any other synthetic group of organisms in our plankton. They exceed (in 1898) the Schizophycecz five to one, the Chloro- phycecs seven to one, the desmids eight thousand to one, and the synthetic Mastigophora by more than four to one. Their numerical preponderance is, with the exception of the synthetic Mastigophora, equaled or exceeded by their relative quantitative significance in the ecology of the plankton. They appear without exception in every collection, and their seasonal distribution in its main features is repeated from year to year. There is a principal vernal pulse in April-May and a hiemal pulse in November-December. Minimum periods separate these pulses and are varied by other pulses, usually of minor importance, at intervals, in 1898, of three to five weeks. The winter minimum 35 is at a lower level than the summer one. In. 1894 the interval of collection is too great to follow the seasonal distribution, but there are hints of summer and autumnal pulses. In 1896 there were no May collections, and the largest number, 6,060,665, appears June 19, five minor pulses on July 18, August 21, September F2, October 11, and November 5 intervening before the hiemal pulse of 3,574,- 028 appears on November 27. Other pulses follow on December 18, January 6, February 4, March 4, and March 17, before the vernal pulse of 1896 culminates at 105,440,858 on April 24. This is fol- lowed by minor pulses on May 18, June ] 1, July 18, August 8, and September 16, and by the hiemal pulse of December 3 of 346,982,- 928*. The vernal pulse of 1897 appears April 27 at 6,207,473,520, but is surpassed by a pulse on July 14 principally of Melosira spinosa of 11,459,289,600, and minor pulses then follow on August 17, September 29, October 26, and December 7 and 21. The hie- mal pulse of this year is insignificant. In 1898 three minor pulses appear, January 21, February 15, and March 22, and the vernal pulse culminates May 10 at 3,865,257,360. Minor pulses follow on June 14, July 19, August 9, August 30, September 27, October 25, and November 22, and the hiemal pulse culminates December 15 at 436,535,790, followed in 1899 by minor ones on January 10, Febru- ary 14, and March 14. Some of the pulses here indicated are due to the development of single species, as that of Melosira on July 14, 1897. Most of them, however, are composite, including a number of species. This is especially true of the vernal pulse, w T hich in 1898 is due to the com- bined increase in Fragilaria virescens and F. crotonensis, Cydotella, Asterionella, Navicula spp., and Synedra acus. Asterionella culmi- nates early in the vernal pulse and the majority of the others towards its close. Melosira varians is among these, but M. spinosa contributes less' to this pulse than it does to later ones. Minor pulses are also composite, as, for example, that of August 9, 1898, which is due to Melosira spinosa, Cydotella, and Navicula. FACTORS CONTROLLING DIATOM PRODUCTION. The fact that many of these pulses represent the combined fluctuations of a number of species leads us to look for some factor * Filter-paper collections included in this and in following years. (4) 36 in the environment common to them all to which these pulses may be attributed. On the following page the seasonal distribution of the total diatoms has been plotted for 1898, along with that of the ni- trates and of the total plankton (volumetric), the thermograph, and the hydrograph. An examination of the changes in nitrates yields no marked evidences of correlation. The vernal pulse of diatoms follows the high nitrates of winter and spring, and the hiemal pulse in December appears after their autumnal rise, and in this particular year develops at the- time of an unusual drop in nitrates (Pt. I., PI. XLV.). The diatom pulses do not show any constant relation to the movement in nitrates either in amount or direction. Whipple ('94) has noted the importance of nitrates in the development of diatoms in reservoir waters. The fact that little correlation appears in our waters between the fluctuations of the nitrates and the growth of diatoms may be due to the presence here of nitrates owing to sewage contamination far in excess of the demands which the diatoms make, and the limitations placed by other elements in the environment are reached before that of the nitrate food- supply becomes operative. The distribution of these diatom pulses throughout the whole year, even in seasonal extremes, seems to pre- clude the factor of temperature as the immediate cause of the pulses except as it may affect the growth of individual species, which is sometimes apparently the case, as is shown in subsequent pages. The vernal pulse is attained each year about May 1, at which time the water passes the temperature of 60. The average of the recorded surface temperatures of 1898 in the river is about 58. Surface temperatures, except in winter months, are usually several degrees higher than bottom temperatures (Pt. I., Table III.). Our records are always of diurnal temperatures. The true average tem- perature, owing to colder water at lower levels and to the nocturnal decline, will lie several degrees below 58 probably about 55. The greatest development of diatoms thus takes place at a temper- ature a few degrees higher than the average temperature for the year. Owing to the somewhat greater abundance of diatoms dur- ing the warmer months, the average thermal exposure of the plank- ton diatoms will be somewhat higher than the average temperature of the year. There may be some significance in this phenomenon of the occurrence of the optimum temperature for development at 37 Fig. A. Diagram showing the seasonal distribution of diatoms, total plankton, nitrates, and thermograph and hydrograph of Illinois River at Havana for 1898. 38 approximately that of the average thermal exposure. The vernal pulse may, in part at least, be the result of a process of natural acclimatization. The fact that a similar development does not recur when this temperature is repassed in the autumnal decline militates, it is true, against the potency of this temperature as a factor in the vernal pulse. This temperature is passed in October (Pt. I., PL VIII. XIII. ), but October pulses are rarely so pronounced as those of adjacent months. Other factors more potent than tem- perature are operative at that season of the year. As will be seen in the diagram, the most pronounced and pro- longed minimum appears in January, February, and March. In these months but a single record in excess of 100,000,000 per m. 3 is found. This or at least the first two months of it is the period of the ice blockade (Pt. I., PI. IX.-XIIL), during which the aeration of the water by the wind is prevented, and the customary equilib- rium in gaseous contents may be disturbed. It is the time when stagnation most threatens disaster to the plankton. The earlier stages of this blockade in December do not seem to be deleterious to the growth of diatoms, since at such times the blockade is less complete, the exclusion of light by the ice less effective, and the accumulation of the products of decay less pronounced. The data at hand do not suffice to elucidate the matter further. The position of the diatom pulses with respect to the movement of the hydrograph is suggestive though not conclusive of a pos- sible correlation between the two phenomena. The double vernal pulse of April-May appears in the declining waters of the major spring flood. The diatom pulse of June 14 is found in the decline of the May- June flood. The pulse of August 9 is caught on the ris- ing waters of a slight flush of the river, and that of August 30 on its decline. That of September 27 appears after a series of slight rises, and those of both October and November attend rising water, but the well-developed pulse of December appears with its decline. There are, counting the double vernal pulse, ten pulses in 1898, from March to January. Of these, seven are found on declining floods, and .but three on rising water, and two of these three appear during the slow rise of October-November. Furthermore, the magnitude of the flood is correlated with that of the diatom pulse. The vernal pulses of 3,453,778,080 and 3,865,257,360 attend the major spring flood, culminating April 2 at 18 feet; the pulse next in 39 size, that on June 14 of 1,039,619,680, attends the decline of the flood next in importance that culminating May 25 at 13.9 feet; while the third pulse, that on December 15 of 436,535,790, attends the decline of the flood culminating November 25 at 8.7 feet. The hydrograph of 1897 (Pt. I., PI. XI.) is unlike tha^of 1S98 (Pt. I., PI. XII.) in the delay of the so-called "June" rise, which culminates July 5 at 7.5 feet. Its decline runs through the month into August. The diatom pulse attending the "June" rise of 1897 appears about a month later than it did with the earlier pulse of 1898, culminating July 14 at 1 1,459,289,600. A delay in the flood is thus attended by' a delay in the diatom pulse. In 1897 there is no December rise and no diatom pulse of noticeable magnitude, though in 1895, in similar absence of the flood, there is a well-defined diatom pulse. In 1896 there is a series of five floods, each involving the early stages of overflow (Pt. I., PI. X.), and on the decline of each occur one or more diatom pulses. It is but natural that the greater number of diatom pulses should fall on declining river-levels, since, as I have previously shown, these periods exceed in duration those of rising floods. They also predominate during the prevalence of seemingly favorable temper- atures, and are characterized by relatively more stable conditions in the environment. There is, however, it seems to me, another and more potent reason why diatom pulses appear at such times. It lies in the overflow of seed-beds in the margins of the permanent backwaters and the run-ofl of the plankton which develops there with the fall in levels. This is very apparent to one familiar with the locality. During the decline of the flood the channel current is often diverted in minor lateral channels, such, for example, as that (Pt. I., PI. II.) which courses through Thompson's Lake Slough into Thompson's Lake and out again into the river at its southern end by way of "the swale" and the "cut road." A similar current on the eastern bottoms, which enters partially by way of Mud Lake Slough, rejoins the river through Quiver Lake. These lateral currents are joined by the run-off from overflowed bottoms and adjacent marshes and swamps, all of which, as well as the permanent back- waters thus draining into the channel, breed at such times an abun- dant plankton including diatoms. The contributory function of the backwaters to the plankton of the river proper is thus at its maxi- mum during the decline of the flood. As the flood recedes, relict pools on the bottom-lands and along the margins of the permanent backwaters are formed, in which the conditions favoring sporulation or other means of providing for resuscitation are to be found. The emerging bottom-lands thus be- come the seed-bed for starting a new cycle of diatoms whenever flood conditions return. In the river, on the other hand, the conditions for sporulation are not so favorable, and the current tends to carry away such resting stages as may be formed. The observed facts regarding the distribution of diatoms and the examination of the conditions under which these pulses occur thus alike yield corrob- oration of the view that floods are potent factors in determining the occurrence of diatoms in fluviatile waters, especially where back waters are extensive. The nature of the action of floods is in some respects similar to that of the overturning of the water which occurs in lakes when the point of maximum density, 39.2, is passed in either direction. In lakes of some depth the vertical circulation of so large a volume of water results in a stirring up of the bottom deposits containing the resting stages of diatoms, so that they are brought again into increased light and to better aeration. Whipple ('94) has emphasized the importance of this overturning in starting the growth of diatoms. In our shallow waters this physical phenomenon is of less impor- tance than in the deeper waters of the lake or reservoir. The vol- ume in circulation is smaller, though some compensation for this may exist in the possibility of repeated over turnings with fluctua- tions in temperatures at the critical stage. The existence of cur- rents, the movements of fish, and the roiling effect of strong and long-continued winds upon our shallow backwaters, combined with the fact that much of the seed-bed area of overflow is dry land at the time of the autumnal overturning, all serve to minimize the effect of this overturning in our waters upon the growth of diatoms in the plankton. The spring overturning occurs early in March, and in 1896, 1898, and 1899 a slight pulse not exceeding an increase of 100 per cent, follows the overturning within an interval of a fortnight. The vernal pulse is about two months later than the overturning, and the relation of this to the overturning does not seem to be inti- mate. The autumnal overturning occurs towards the middle or end of November, and in 1895, 1896, and 1898 the hiemal pulse of December follows close upon it, within two, or at most three, weeks. 41 The relation is here more apparent, but the resulting pulse is no larger than those following upon floods during summer, and but little larger than the ones which precede it in the autumn. The effect of this overturning upon the plankton of the Illinois River may thus be detected, though it is here of less importance than in lakes and reservoirs since it is overshadowed or replaced by other and more potent factors. The relation of the seasonal distribution of the diatoms to that of the total plankton is not readily unraveled. The latter is the resultant of a most complex series of factors, whose number and relative potency are subject to constant change and readjustment in the unstable environment of the stream. It is the biological expression of the state of tension among these various factors which for the moment exists. Of these factors the diatoms are but one, though an important one, in the food cycle and ecology of the plankton. The volumetric determinations in. the diagram (p. 37) do not give" the true seasonal distribution of the total plankton owing to the escape of an unknown quantity through the meshes of the silk net. They represent more truly that of the animal plank- ton than that of the phytoplankton. A comparison of the seasonal distribution of the diatoms and total plankton may serve, in spite of the errors involved in the volumetric determinations and the disparity of individuals among the diatoms, to throw some light on the effect of the fluctuations of the latter upon the movement in the volume of plankton. A close comparison of the two seasonal curves reveals the fact that the diatom curve is not identical with the vol- umetric curve. It is true that the double vernal (April-May) pulse of diatoms coincides in location with the vernal volumetric pulse. This is also true of the pulses of June 14 and July 19. The crest of the volumetric vernal pulse is, however, lodged between the double apices of the diatom curve, and all the subsequent volumetric pulses from July on lie in depressions of the diatom curve, and vice versa. It is apparent at once on examination of our planktons that the catches of the silk net are from the volumetric standpoint largely, indeed overwhelmingly, of animal origin. These volu- metric pulses are as a rule largely pulses of the zooplankton. It is therefore to be expected that the diatoms would decrease at such times, since they form the food of many Entomostraca and not a few Rotifera. The appearance of the diatom pulses before or after the 42 volumetric (animal) pulse may therefore in a measure present the wavering tendency to establish an equilibrium between these two elements of the plankton. The presence of an abundant animal plankton may therefore be a cause of some of the minimum periods between diatom pulses. Other causes, such as decline of food ele- ments, may also arise, but in our waters the nitrates at least rarely ever reach a level where an unutilized margin capable of support- ing a large diatom population is not still present. Data concerning other food elements are not at hand, but their paucity in water derived from such varied sources and so liberally fertilized by organic wastes seems improbable. There is also the further possi- bility and, indeed, from the data in hand the probability of the existence among diatoms of reproductive cycles, interrupted by resting periods. The available data do not, however, throw any light upon the nature of this internal factor or the cause for the running down of the energy of reproduction, and but little upon the operation of environmental factors which stimulate anew the process of reproduction. The seasonal distribution of the diatoms as a whole, and that of individual species also, offer repeated instances of recurrent pulses at intervals approximating four weeks the lunar month. In 1898 thirteen such pulses can be detected. These often correspond roughly to minor flood intervals, but not always so, for occasionally two pulses occur on the decline of a single flood. Similar appear- ances may be traced in other years, when collections were frequent enough to exhibit minor pulses. They are, however, in all cases quite irregular, and exceptions are frequent. That cosmic factors may indirectly, through immediately environ- ing factors, affect the reproductive phenomena of pelagic organisms has been suggested by the work of Kramer ('97), Mayer ('00), and Friedlander ('01) in the case of the "Palolo" worm, a coral-reef annelid whose seasonal swarming for reproductive purposes occurs at somewhat definite lunar intervals. While the data concerning the seasonal distribution of diatoms in the Illinois River may serve to suggest the operation of an enig- matic cosmic factor, I wish distinctly to state that in my opinion they are wholly inadequate to establish either its presence or its potency. It is much more probable that we have to deal merely with some matter of food relations between the plants and animals 43 of the plankton, and perhaps with the result of increased photo- synthesis in periods of lunar illumination, which tends to establish the limits of the pulses. The number of forms of diatoms noted in our records in the plankton of the Illinois River is thirty-one. This number could be greatly increased by the inclusion of the many adventitious species which flood-waters bring into the plankton and by the addition of rarer limnetic species. Of these thirty-one at least twelve are eulimnetic, while the others are in the main adventitious. There are no species among them peculiar to the potamoplankton, and the dominant forms here are also abundant in the fresh-water plank- ton of our own Great Lakes and of European streams and lakes, barring a few mooted points of specific identity. The limnetic species are fourteen in number, viz. : Asterionella formosa, A. gracillima, Cydotella kuetzingiana, Diatoma elongatum var. tenue, Fragilaria crotonensis, F. virescens, Melosira . granulata var. spinosa, M. variant, Meridian circulare, Rhizosolenia eriensis, Stephanodiscus magaroz, Synedra acus, S. acus var. delicatissima, and Tabellaria fene strata. Of these limnetic forms the more impor- tant ones are Asterionella gracillima, Cydotella, Fragilaria virescens, Melosira granulata var. spinosa, and Synedra acus and its varieties. The absence or small number of certain limnetic species is notice- able. These are several species of Tabellaria and Attheya. On ac- count of the abundance of silt and the transparency of Attheya it may have been overlooked. It has hitherto been reported from waters much nearer the sea, and this coupled with its affinities to marine diatoms may explain its absence in our waters. The remainder of the forms are adventitious, or largely so, and with the exception of the species of Navicula they have little effect upon the ecology or quantity of the potamoplankton. DISCUSSION OF SPECIES OF BACILLARIACE^I. Asterionella formosa Hassall. Average number of individual cells, 960. Average size of colony, 4.8 cells. Recorded only in November, December, and from February through April, and never in large numbers. The greatest pulse attained at any time cul- minated on March 30, 1896, at 54,540. Aside from an isolated occurrence on June 27, 1896, no individuals were recorded at tem- peratures above 48, and three fourths of the occurrences are at 44 temperatures below 40. The data .are insufficient to trace the pulses satisfactorily. This species is distinguished with difficulty from A. gracillima, and may include only old, and in our planktons often heavily incrusted, individuals; or it may be only a low-tem- perature variety of the species above named, which in the grand total of all our collections outnumbers it ten thousand to one. Asterionella gracillima Heib. Average . number of individual cells, 28,860,160. In 1897 the species was only one third as abun- dant, a contrast which finds its explanation in the fact that the June rise of that year (Pt. I., PI. XI.) did not reach the stage of overflow, and a June pulse is absent in the collections of that year. The seasonal distribution of this organism is one of the best-defined and most striking of all the components of the river plankton. It is peculiar in the fact that it appears in numbers only during spring and the beginning of summer, and in the absence of any autumnal pulse upon the return of the temperatures in which the spring pulse ap- peared. This species was recorded in every month of the year but October, but always in small numbers after July 1. In 1894, collec- tions were not commenced until after the time of the spring pulse. In 1895 the spring collections were few, and at intervals so great as to preclude the detection of the full course of the spring pulse. The maximum number in the collections 'of that year appears April 9 at 1,203,100 and falls to 445,995 on April 29 which is approximately the time of the maximum of subsequent years. This was a year of unusually low water during the spring, and overflow stage was at no time reached (Pt. I., PL IX.), which may account for the apparent suppression of the spring pulse. The species does not reappear in the collections of that year until December, but it continues in small numbers (less than 5,000 per m. 3 ) until the end of March, 1896, when there is a rapid increase which culminates April 24 at 26,281,400. It disappears entirely from the records at the end of a fortnight, and save for a single entry in June and two in September it does not again appearin 1896. In 1897 the culmination of thespring pulse occurs April 27 at 324, 633, 600 three hundred-fold larger than in the previous year. There is a normal March flood (Pt. I., PI. XL), on the declining stages of which this pulse appears. With the close of June the species disappears from the records. The June rise does not reach the stage of overflow, and the scanty records show but this single pulse throughout the year. Beyond a single entry in August and in 45 November the species does not again appear in the records during the year. In 1898 there is an unusual midwinter pulse on January 11 of 146,280, followed by a decline and irregularities due to the ris- ing winter flood (Pt. I., PI. XII.). At the middle of March a rapid increase ensues, culminating April 26 at 891, 648,000 ^on-the declin- ing spring flood. A decline to 197,683,200 is found at the close of a week, and it is accelerated by the secondary spring flood, which attains the overflow stage of 15 feet in the closing days of May (Pt. I., PI. XII.). With the decline of this flood in June a second pulse appears, increasing from 15,080 on May 26 to 336,194,880 on June 14, and at the end of three weeks the species practically dis- appears from the plankton. A few scattered entries appear during the summer and fall, and a minor pulse of 10,500 appears on Decem- ber 20, followed by a decline in the next month. This species in our waters exhibits a well-defined vernal pulse towards the end of April at about 60, but no autumnal pulse appears when this temperature recurs. , There is a slight indica- tion of a minor midwinter pulse at the minimum temperatures of the year. This occurrence of a midwinter pulse was noted by Whipple and Jackson ('99) in the reservoirs of the Brooklyn water- works, and in the same paper its seasonal distribution in Fresh Pond, Lake Cochituate, and Wenham Lake, Massachusetts, is given for the years 1890-97, in the majority of which a mid- winter pulse commensurate in magnitude with the vernal pulse is to be found. Autumnal pulses are of infrequent occurrence, the vernal pulse being the most frequent but not constant. In European waters no such long-continued examination of the seasonal distribu- tion of this organism has as yet been reported. Apstein ('96) finds two pulses per year in Ploner See in May and the last of July ; and two in Dobersdorfer See, one in April and one in October, separated by midsummer and midwinter minima. Lauterborn ('93) finds that this species in the " Altwasser" of the Rhine attains its maximum in June and again increases in October. In the backwaters of the Elbe, Schorler ('00) reports Astenonella as abundant in April, June, July, and October, but refers the organisms to the preceding species. The existence of the vernal pulse only in our waters is thus somewhat unique, and the cause of the phenomenon probably lies in some environmental conditions, perhaps in our peculiar bacterial and sewage contamination of the autumn. Our vernal pulses appear on 46 declining floods about the end of April at about 60. It can not be temperature which limits the occurrence of the species, for this apparent optimum recurs again in October. This is the period of declining nitrates (Pt. I., PL XLIII.-XLV.), but they rise again in the autumn, and in our sewage-fed waters they contain even in the midsummer minimum a quantity adequate to support an abundant growth of Asterionella. Whipple and Jackson ('99) have found on analysis that Asterionella to the number of 10,000,000,000 per cubic meter yield but .079 parts per million of organic nitrogen. The nitrates in our waters rarely fall below. 25 parts permillion, which, with the other forms of nitrogen that may be available, would seem to afford nurture not only for Asterionella but also for competing organisms. These authors have also found that silica to the amount of 1 . 78 and manganic oxide to .03 per million are contained in Asterionella to the number per cubic meter above quoted. As was shown in Pt. I., p. 234, the silica is present in great excess (26 to 81 parts), and the manganic oxide, though not reported in the analyses of November waters, is present on June 15 to the amount of .07 parts per million more than double the amount required to support Astenonella to a maximum twelve times as great as any recorded in our plankton collections. This also occurs at a season when Asterionella is usually declining rapidly in numbers. Such chemical data as are available thus afford us no explanation of the limitation of Asterionella in our waters to the vernal pulse alone. Some evidence bearing on a factor which may be operative in producing this phenomenon is to be found in the hydrographic con- ditions attending the vernal pulse. As previously noted, this appears, each year with the decline of the spring flood. A repetition of the overflow in 1898 at the end of May brought with it a repetition of the vernal pulse of Asterionella in early June. With the decline of the flood the backwaters make their major contribution to the channel plankton, and it is during this period that Asterionella reaches its maximum and also declines. If the spring flood is sup- pressed, as in 1895 and 1896, the spring pulse of Asterionella is cor- respondingly feeble. The environmental conditions are thus more favorable in the impounded backwaters than in the main stream. Whipple and Jackson ('99) have noted in frustules of this diatom the appearance of structures which they interpret as spores. If these are spores, and if the sedimentation of spore-bearing frustules occurs 47 extensively in the relict pools of the emerging bottom-lands, a seed- bed for re-stocking the waters of overflow is formed with each declin- ing flood, and this seed-bed becomes potent only when floods return. The absence of an autumnal overflow and the minor part that the autumnal overturning plays in our shallow waters whm 39.2 is passed, may alike tend to suppress here the autumnal or midwinter pulses which occur elsewhere in deeper water. The occurrence of the vernal pulse of Asterionella in the last days of April brings it into close relation with the major volumetric pulse of the year (Pt. I., PL IX.-XIL). It is not only an important con- stituent of this spring maximum, but it is one of the most prominent primal sources of food of the Entomostraca Bosmina, Daphnia, Cyclops, and Diaptomus, all of which exhibit an increase in numbers at this period. It shares with Cyclotella the claim to the first place quantitatively among the synthetic organisms upon which the early spring plankton depends for its development. Our records are all based upon the catches of the silk net, through whose meshes the isolated cells of Asterionella readily escape. Filter- paper catches give much higher numbers except during the period of maximum, when the numbers by the two methods do not materially differ. This seems to be d.ue to the fact that isolated cells are rela- tively much more abundant after the maxima than they are be- fore them, and especially at the time of their appearance. These diatoms form arcs, circles, or whorls, of a varying number of cells. During the vernal pulses of 1898 the average number in these clus- ters in the middle of March was three or four, and at the time of the maximum on April 26 it rose to five or six, often reaching sixteen or more. A fortnight after this maximum the average fell to 1.4, rising again with the second pulse, on June 14, to 8.4, and declining in three weeks, with the fading out of the pulse, to 1.2. Asterionella is frequently infested with great numbers of a minute craspemonad flagellate protozoan which appears in thick-set rows upon the ray-like cells, a single cell sometimes bearing a score of these organisms. This diatom exhibits considerable variation in size and proportions. The longer and more slender cells appear at the times of the maxima. Cocconeis communis Heib.* Average number, 520,000, but more than three times as abundant in 1897. This diatom occurs some- what irregularly in the filter-paper collections, and has been recorded 48 in every month of the year. It is somewhat more prevalent in spring and autumn, and there are indications of a vernal pulse in May and an autumnal one in September, separated by prolonged midsummer and midwinter minima. Vernal pulses appear in 1897 on June 28 at 14,400,000, and in 1898 on May 17 at 7,200,000. Autum- nal pulses occur in 1896 on September 16 at 2,700,000; in 1897 on September 29 at 10,800,000; and in 1898 on September 13 at 5,400,000. The optimum temperatures lie between 60 and 75, the autumnal pulse appearing in higher temperatures than the vernal as a rule. This diatom is reported as often epiphytic upon algae, and it may be wholly adventitious in the plankton. There is nothing, however, in the curve of its distribution to corroborate this view. Cydotella kuetzingiana Thw.* Average number 243,659,615, but slightly more abundant in the preceding year. This is one of the smallest as well as one of the most abundant of all the diatoms of the river plankton. It readily escapes through the meshes of the silk net, and plankton collections made by this means give no adequate conception of its prevalence or importance in the ecology of the plankton. It appears in every month in the year and in practically all of our collections, and is thus a perennial planktont. There is a considerable variation in size among the individuals in the plankton, but the greater number lie near the smaller rather than the larger limits. It may be that several species have been combined in the enumeration. The fluctuations in the seasonal distribution of this diatom are considerable, and pulses occur at all seasons of the year. The vernal pulse is, however, preeminent, and is not approached in magnitude by those of any other season of the year. In 1897 this pulse culmi- nates at 5,724,000,000 on April 27, and in 1898 on April 26 at 2,880,- 000,000. Throughout the summer and autumn in both years there is a series of minor pulses at intervals of two to eight weeks. In 1897 an autumnal pulse of 223,200,000 appears on September 29, and though not of greater magnitude than two previous summer pulses, it does surpass anything prior to the pulse of the following spring. In 1898 there are seven pulses during the summer and fall, culmi- nating as follows: on May 10 at 2, 668, 000,000; on June 28 at 291,- 000,000; on July 19 at 561,600,000; on August 9 at 401,400,000; on August 23 at 122,400,000 ; on September 6 at 1 15,200,000 ; on Septem- 49 ber 27 at 57,600,000; on October 25 at 25,200,000; and in December a pulse well sustained throughout the month culminates on the 15th at 414,000,000. The temperature optimum appears to be about 60, though its return in the autumn does not induce a development comparable with that of the closing days of April. The midsummer pulses and that of December show that other causes than temperature are operative in regulating the occurrence of this organism. The appearance of the vernal pulse of Cyclotella at the time of the volumetric maximum (Pt. I., PL IX.-XII.) in April-May sug- gests its function as one of the primal sources of food for the animal components of that plankton. The plates are based on collections of the silk net, and Cyclotella constitutes an insignificant part of the volumetric total there graphically presented, since it is so small that it escapes readily through the silk. Cymatopleura solea (Breb.) W. Sm.* Average number, 2,115 (silk, 1,292), but slightly more abundant in 1897. Isolated occur- rences in small numbers appear during the colder months, generally below 60, though several individuals appear in summer records. This is apparently an adventitious planktont, whose presence is often due to flood waters. Diatoma elongation var. tenue Van Heurck.* Average number, 2,471,923. This is a perennial limnetic diatom occurring in every month of the year and in the majority of our collections. It is but sparingly present during midsummer. There are well-defined vernal pulses in 1897 on May 25 of 50,400,000, and in 1898 on May 3 of 18,000,000. A second large pulse appears on the approach of winter, in 1897, on November 15, culminating at 2,700,000, and in 1898, on November 22, at 9,000,000. In the silk collections of 1895 and 1896 pulses also appear in the last days of April and in Novem- ber or December. The records thus indicate a decided preference of the species for temperatures below 70 and the possibility of rapid development in midwinter as in 1895, during a fortnight of minimum temperatures (32 + ), culminating at 53,424 (silk) Decem- ber 18. The vernal pulses coincide approximately with the volu- metric maximum, and the December pulse of 1895 attends an unusual winter development of the plankton (Pt. I., PI. IX. and Table III.). 50 Diatoma vulgare Bory occurred sparingly at irregular intervals, and is apparently an adventitious species in the plankton. Encyonema prostratum (Berk.) Ralfs appears a few times during the summer months, and is evidently adventitious, as is also the still rarer Epithemia turgida Kiitz. Fragilaria crotonensis (Edw.) Kitton. Average number of cells, 2.1. This limnetic diatom is much less abundant in our waters than the following species. In 1898 it appeared in February, and increased from 19,200 on April 19, to 14,469,120 on May 10, dis- appearing entirely from the records after May 17. Such meteoric pulses were not detected in previous years, when only scattered entries in April, May, and December were recorded. The number of cells in the filaments is very much less than in F. virescens, aver- aging but 14 to its 108. Its optimum temperature lies about 60, and its vernal pulse occurs immediately after the volumetric maxi- mum (Pt. I., PI. XII.) and upon the same date with that of F. vi- rescens. It seems to be predominantly a vernal planktont in our waters. In German lakes Apstein ('96) finds maxima as late as June-July, but always, it seems, at temperatures below 70. Fragilaria virescens Ralfs. Average number, 73.1. Apparently ten times more abundant than in 1897, as a result possibly of the absence of collections during the period of the vernal maximum in that year. This is a perennial organism, with two well-defined pulses ; a vernal one in April-May and another in November- December. The uniformity with which these pulses appeared in 1895-1898 is very striking when one considers the unstable environ- ment in which the pulses occur. In 1894 the species is not present in numbers in any of the scattered collections of the year. In 1895 the vernal pulse is indicated in the collection of April 29 (2,754,675), after which the species disappears until September, increasing with a temporary backset by the December flood (Pt. I., PI. IX.) to a second culmination December 30 at 282, 225. After a minimum in January, 1896, the numbers increase, with minor fluctuations, to a vernal maximum of 76,224,000 on April 24, followed by a mini- mum period from May 18 to the following November. The winter pulse again appears in December, culminating on the 3d at 867,048. In 1897 the vernal pulse seems to culminate somewhat later than usual, though the interval of collection is too great to follow its full course. The maximum appears on May 25 at 3,549,600, after 51 which the species dwindles away and disappears in August to return early in November. The winter pulse culminates December 14 at 8,159,250, at a break in the ice blockade. In 1898 the winter mini- mum continues into April, and the vernal pulse appears May 10 at 253,960,000, rising with rocket-like suddenness from 390,000 of the previous week, and declining the week following to 4,110,400. The decline to the summer minimum is prolonged into July, and the species does not reappear until October. The winter pulse begins earlier than usual, on November 1, and is well sustained through the month, culminating on the 29th at 2,254,000. The winter mini- mum which follows, does not reach the low levels of that of summer. This species has thus a characteristic distribution, the analysis of which is by no means simple. The contrast between the summer and winter minimum may be due to the low nitrates of the summer and the larger amount in the winter (Pt. I., PI. XLIII.-XLV.), which favor a proportionate development of this diatom, though not every species shows this response. The two minima separate the seasonal occurrences of this species into two periods of growth ; a vernal, from March to June, and a hiemal, from October to Janu- ary, the limits and relative development of each being somewhat variable from year to year. The temperatures of the two periods differ. Both are times of rapid change, of rise and fall respectively, and the culminations of the periods of growth lie at widely sepa- rated temperatures. The vernal pulses in 1896 and 1898 in which years collections were frequent enough to locate them with some degree of accuracy appear at 72 (April. 24) and 61 (May 10) respectively, and in every year the vernal pulse appears during a period of rapid change. The hiemal pulse, on the other hand, cul- minates in each year after the winter minimum approaching 32 has been reached, and in two years during the ice blockade. Tempera- ture within these limits seems not to be a determining factor in the pulses of this organism. The nitrates (Pt. I., PI. XLIII.-XLV.) have been uniformly high (above 2 parts per million) whenever the pulses occurred. In 1898 they decline abruptly (Pt. I., PI. XLV) and remain at a low level throughout December, and in this month, when usually Fragilaria attains its hiemal maximum, we find it dropping to the unusual minimum of 20,000. The pulse which began in November is cut off apparently by this unusual decline in nitrates. Abundance in nitrates is not, however, in itself sufficient (5) 52 to cause a pulse of development of Fragilana, for nitrates are abundant when the diatom declines and is at its minimum. It does not seem possible to find in the unstable environment of this organism any external factor which shows a causal connection with its periods of growth. Apstein ('96) found that this diatom reached its major pulse in March and April in Dobersdorfer See, and a minor one in November. The cells of this diatom form long twisted bands, visible to the unaided eye. They reach a much greater length in this species than in the preceding one, and are longest during the height of the growing period, decreasing rapidly in length as it declines. The average number of cells in a ribbon at the time of the maximum lies between 150 and 200, and at other times is usually below 100 and often below 2 5 . The vernal pulse of this species coincides with that of F. croto- nensis, and appears either with or just after the volumetric pulse. The December pulses may in part serve as primal food sources for the fairly constant minor volumetric pulse of December. Gomphonema constrictuvn Ehrbg.* Average number, 501,923. This species appears irregularly, with a predominance of occur- rences in May and November, and is apparently adventitious. Melosira granulata (Ehrbg.) Ralfs var. spinosa Schroder. Average number of cells, 1,181,125 (filter-paper, 34,762,365). In 1897 it was more than five times as abundant. In the filter-paper collections as a whole it is about fifty times as abun- dant as in those of the. silk net. A much greater proportion of single cells and short filaments occurs in the latter collections, since the longer filaments are the more readily retained by the silk. In the discussion which follows, the data from the silk collections will be used, since they cover the whole period. The data from the filter-paper collections indicate very nearly the same seasonal routine, and the differences between the results by the two methods lie in the proportions of the numbers rather than in the direction of movement in the fluctuations. The pictures of the seasonal changes in occurrence of the diatom given by the two methods are essentially alike aside from greater irregularity during minimum periods, resulting from the larger margin of error in the filter-paper method as I used it. 53 This Melosira is a perennial planktont in that it occurs in every month of the year in the river. Its appearances from December to March are, however, irregular, and its numbers small. Its large pulses above 1,000,000 all lie between May 15 and October 1, with the single exception of the pulse of April 24, 1896, culminating at 2,056,400, in temperatures of 72, occurring fully a fortnight earlier than usual. The major pulse seems normally to occur in June; at least in 1896 and 1898, when collections were frequent at this season of the year, such pulses appear on the llth at 12,940,000 and on the 21st at 32,114,880. A June pulse also appears in 1895. September pulses appear in 1895, on the 12th, at 2,254,182, and in 1898, on the 27th, at 5,499,840. There is, however, no well-defined vernal and autumnal growth period, since large pulses occur through- out the whole summer. The greatest pulse on record (111,456,000) is on July 21, 1897, and in 1898 there are three minor pulses between those of June and September. Including the major pulses, there are in 1895 five, in 1896 six, in 1897 five, and in 1898 eight, pulses at intervals of two to six weeks between May and October, the ones at either end of the season being often but slightly developed, the remainder usually running from 1,000,000 to 5,000,000. This species is predominantly a summer planktont, and its optimum temperature lies above 70, the greatest number recorded appearing at 81. This is one of the most abundant diatoms of the potamoplankton, and in our waters it attains its greatest de- velopment during the season of the minimum occurrence of nitrates, in whose utilization it is quantitatively an important agent. It fills the gap between the vernal and autumnal or hiemal appear- ances of Asterionella and Fragilaria, thus providing a continuous source of food for the zooplankton with w r hich it is associated. It is, by virtue of its numbers, its size, and its seasonal distribution, quantitatively and ecologically the most important of all the diatoms of the plankton of the Illinois River. The only factor in the environment to which the limitation of the rapid growth of this species 'to the May-October period can be re- ferred is temperature. There are but three instances in the records of Melosira exceeding 100,000 per m. 3 at temperatures below 60, and one of these is but a few days prior to the attainment of that temper- ature. It cannot be food which deters its development below this point, since the nitrates at least are then most abundant (Pt. I., PL 54 XLIII.-L.). Other diatoms, as in the hiemal pulse of Fragilaria, develop in numbers at temperatures approaching 32, but not M. granulata var. spinosa. Whipple ('94) concludes from the records of examinations of potable waters in Massachusetts that temperature has possibly a slight influence on the growth of diatoms, but that it is of so little importance that it does not affect their seasonal distribu- tion ; and, on the other hand, that a sufficient supply of nitrates is one of the most important conditions for their growth. The seasonal dis- tribution of Melosira was not separately discussed in his paper though included in his general statements. In our waters the data at hand seem to show conclusively that abundance of nitrates is of no avail in the case of Melosira when the temperature falls below 60. There are times, therefore, in the case of this, our most important diatom, when temperature is more potent than food as a factor con- trolling its growth. Melosira does not appear in its maximum pulses at the time of the major volumetric pulse of the total plankton of April-May, nor do its fluctuations seem to bring about directly any considerable changes in the volume of the plankton. For example, the extreme pulse of 111 ,456,000 on July 21, 1897, occurs at the time of a sudden drop in the amount of plankton (Pt. L, PL XL). The amount of plankton on July 14, 21, and 30 is 8.16, 0.92, and 1.05 cm. 3 per m. 3 , and the corresponding numbers of Melosira are 66,528,000, 111,456,- 000, and 13,176,000. The diatoms here discussed are predominantly of the type designated as var. spinosa, marked by the spinous prolongations from the valves at the ends of the filaments. The cells of the forms in our plankton are proportionately much longer, as a rule, than those figured by Schroder ('97), usually attaining one and a half to two times the length without proportional increase in diameter. Not infrequently in the height of the growing season much elongated and curved cells and filaments are to be found. In one instance an unusual number of filaments approaching M. varians in form though still, of the spinous type were found. It is not improbable that several so-called species of Melosira have been included with this variable species in the enumeration. Melosira is the bearer of numerous passive planktonts, the most abundant of which is Bicosceca lacustris Clk. Associated with this, and often on the same filament, is the elegant little craspemonad 55 Salpingceca brunnea Stokes. Cells to which several of these flagel- lates are attached very frequently exhibit a breaking up of the cell contents into eight brownish masses, often of spore-like form, and it is not an uncommon thing to find such parasitized filaments with several empty cells. The eggs of the rotifer Diurella ligris are fre- quently found attached to the filaments of this diatom. The num- ber of cells in the filaments in the silk collections averages 6.4 in 1897, and 7 in 1898, while in the filter-paper collections it averages 3.5 in both years. The numbers per filament range from 1 to 40, and the filaments are wont to be somewhat longer during rapid growth than in periods of decline or minimum. Melosira varians Ag. Average number, 148,626 (filter-paper 3,455,538). The discussion is based upon silk catches. The species was about equally abundant in 1897 but much less so in previous years. This is a perennial species, reported in every month of the year and in most of the collections. It exhibits two well-defined pulses, a vernal one in April-May and an autumnal one in September- October. The reduction in the minimum intervals varies from sea- son to season and from year to year. It was most pronounced, al- most to suppression, in July and August in 1894, 1895, and 1896, and in December-February in 1896-97 and 1898-99. In other seasons the minimum falls to 1,000 to 15,000. The vernal pulse (146,916) appears in 1895 on April 29, in 1896 (229,235) on May 18, in 1897 (2,419,200) on May 25, and in 1898 (3,164,160) on May 5. The autumnal pulse (150,720) is found in 1895 on October 30; in 1896, on September 16 at 378,900; in 1897 there are two pulses, one on August 30 at 738,000, and the other on No- vember 15 at 458,800; and in 1898 one, on October 18 at 348,000. The autumnal pulses are thus much smaller than the vernal ones and exhibit a greater range in the time of their appearance. As in the case of many other organisms this diatom also exhibits the phenomenon of recurrent minor pulses at intervals of a few weeks. They range in height from 25,000 to almost 1,000,000, and are largest when found in the proximity of the major pulses. The records are not frequent enough to trace them in all seasons. They appear in January in 1896, 1898, and 1899; in February in 1898; twice in March in 1896; in April in 1896; twice in June in 1897 and again in 1 898 ; in July in 1897 and 1898 ; in August in 1897 and 1898 ; 56 in September in 1898; in November in 1896, 1897, and 1898; and in December in 1894. The optimum temperatures, omitting the pulse of August 30, 1897, at 80, all lie below 72, averaging 65 for the vernal pulse and 62 for the autumnal. But three pulses in all, exceeding 100,000, lie at temperatures above 70, and but three below 50. In the case of this species likewise temperatures seem to be potent factors in limiting its seasonal occurrence. The fluctuations in nitrates do not seem to bear any constant relation to its develop- ment. The midsummer minimum of the diatom may appear, as in 1896, during an abundance of nitrates (0.5 to 3.0 parts per mil- lion Pt. I., PI. XLIII.) unusual for the season. On the other hand, a minimum of nitrates (.1 to .35) in August and December, 1898, coincides with a suppression of this species in the plankton. Thus in the presence of food, temperature seems to be a determining factor in the seasonal distribution of this organism. Whipple ('94) expresses the opinion that the growth of diatoms occurs at those seasons of the year when the water is in vertical circulation ; that is, when it passes 39.2. In our waters this generally occurs early in March and late in November. In this species the only pulses which it seems might exhibit the effect of this phenomenon are those of December and March, and neither of them are in any way constant or prominent. Neither of the major pulses, vernal nor autumnal, can be attributed to it. The latter pulse occurs prior to the autumnal overturning of the water. The vernal pulse usually follows the spring volumetric maxi- mum, and the autumnal one generally appears during a volumetric minimum. No immediate quantitative effect of this species upon the plankton is apparent. In European waters this is a common planktont, and Apstein ('96) reports vernal maxima in March, April, and May, and an autumnal one of minor value in November. The number of cells in the filaments varies from one to sixty, and in filter-paper collections averages four, while in the silk catches it varies from seven to fifteen from year to year. The fila- ments average somewhat longer during the periods of maximum growth, reaching twelve to twenty-five. This species also occasion- ally bears the flagellates found upon M. granulata var. spinosa, but not in such abundance. It is quantitatively much less important 57 in our plankton than that species, though this does not seem to be the case in some European waters. Meridian circulare Ag. has appeared but four times in winter planktons, from December to March, and seems to_ be adventi- tious. Navicula iridis Ehrbg.* Average number, 297,307. Appears at irregular intervals, often with flood waters and in the colder months. It seems to be adventitious. Navicula spp.* Average number, 8,569,038. About twice as abundant in 1897. Under this head I have included a number of species of Navicula, and, possibly, even species of genera resembling Navicula. The individuals are all of small size, and are principally of the type of the smaller forms of N. brebissonii Kiitz. and N. gracilis Ehrbg. They are quite abundant in collections from Quiver Creek and Spoon River. Their greater abundance in 1898 as compared with 1897 may be caused by the greater movement in river levels in the former year (85.6 ft.) as compared with that of the latter (55.5 ft.). This feature of the distribution of these forms suggests that they are adventitious in the plankton. This view is further supported by the fact that some, though not all, of their apparent pulses appear with flood waters; for example, the pulse of 64,000,000 on May 17, 1898. There are indications, independent of floods, of pulses in April-May and November-December, which may, how- ever, be simply reflections of pulses in the normal habitat of these diatoms the shores and bottom of the river and its tributaries. They are represented in the plankton at all seasons, and the diver- gence in numbers is at no time so marked as it is in typical plank- ton diatoms, such as Asterionella. Nitzschia amphioxys (Ehrbg.) Kutz. appeared several times in winter collections, and N. sigmoidea (Nitzsch) W. Sm. is adventi- tious in small numbers in flood waters. Several species of Pleu- rosigma appear at irregular intervals throughout the year in both flood waters and stable conditions and are apparently adventitious, appearing in relatively small numbers. Rhizosolenia eriensis H. L. Smith was noted on a few occasions in winter planktons. Its exceeding transparency and the abun- dance of silt and debris at the times of its occurrence so obscure it that it may have escaped detection in many instances. 58 Stephanodiscus niagarce Ehrbg., a common planktont in the waters of the Great Lakes, appeared but once, in May, in our plank- ton, though the river had for years received, by way of the Chicago River, constant access of water from Lake Michigan. The turbid, sewage-laden, and warmer waters of the Illinois are evidently not favorable for its growth. Surirella ovalis Kiitz. var. minuta (Breb.) Kirchner.* Average number, 761,538. Present sparingly throughout the year, but principally during summer months. Vernal pulse in May. Surirella spiralis Kiitz. Average number, 1,612. Less abundant in the more stable conditions of 1897. This species is most abun- dant in Quiver Creek and Spoon River. Its fluctuations are slight, irregular, and often appear with flood waters, all of which phenom- ena indicate its adventitious character in the river plankton. Synedra acus Kiitz.* Average number, 36,558,462 (silk, 308,- 330). This species is a perennial planktont, appearing, for example, in 1898 in every collection. It has a highly developed and shifting vernal pulse, and an inconstant and but slightly developed autumnal or hiemal pulse. The vernal pulse appears in 1895 on April 9 at 209,880; in 1896 on April 24 at 366,828; in 1897 on May 25 at 2,620,800 (82,800,000*); and in 1898 on May 10 at 9,043,200 (813,600,000*). The second pulse appears in 1895 on November 14 at 99,360; in 1896 on December 3 at 44,464; in 1897 no pulse occurs; in 1898 it occurs on November 8 at 19,000. As in some other diatoms, there are minor pulses throughout the year, though in this case they are all feebly developed, exceeding 100,000 (silk) in but a single instance. The minor pulses of midwinter often exceed in prominence those of midsummer. The meteoric char- acter of the vernal pulse is very pronounced in this species both in the suddenness of its appearance and its disappearance and in the height which it attains. The variety delicatissima W. Sm. is included here with the type acus. During the autumn of 1898 a separate record was kept of the two, with the result that the variety appears to include about four fifths of the individuals at that season. The tw T o are not readily separated. The colorless form recently described by Pro- wazek ('00) as 5. hyalina is also included, and it is not uncommon when S. acus is abundant. Colorless forms of other diatoms of the plankton, as Asterionella, Melosira, and Fragilaria, also occur, but 59 it would seem from the intergradation with the normal condition that it is a phenomenon of physiological import rather than of specific significance. It would seem desirable that experimental breeding of diatoms should be employed as a test before specific diagnoses utilize this character. Synedra capitata Ehrbg. is occasionally adventitious in the plank- ton in spring months. Synedra ulna (Nitzsch) Ehrbg.* Average number, 302,308 (silk, 34,510). This appears somewhat irregularly in the plankton, with a vernal pulse on May 17 of 5,400,000 and an autumnal one November 15 of 1,800,000. It is abundant on the ooze of exposed springy shores after rapid decline of the river, and is probably adventitious in the plankton to some extent from this region. Tabellaria fene strata Kiitz., which is exceedingly abundant in the plankton of European lakes and in our own Great Lakes, was found but a single time in the w r aters of the Illinois. It can hardly be lack of food elements which prevents its development, and there are times when favorable thermal conditions would seem to be offered in spring and autumn, when the river temperatures do not exceed the summer temperatures of our Great Lakes. It may be that the chemical conditions attending sewage contamination exert a dele- terious influence upon this species and others of the genus, such as T. flocculosa, which abound in purer lake waters. CONJUGATE. This group of algae is represented in the plankton only by a few desmids, which neither in number, or quantity, play any important part in the ecology of the plankton. The filamentous algae are abundantly represented in spring in the backwaters of the Illinois River, where they form extensive littoral fringes of "blanket moss," which load down the emerging littoral flora. This fringe is fre- quently stranded by the retreat of flood waters. In some localities, as in Phelps Lake, it plays a very important part in the food cycle, since by its decay, as temperatures approach the summer maximum, it contributes immediately its store of organic nitrogen to the sup- port of the small algre and flagellates which develop in great num- bers on those waters at that season. Some species of Spirogyra and Zygnema have a habit of breaking up into short filaments, and 60 in this condition they have often been taken in some quantity in the plankton of the river, but they are so plainly adventitious and irregular that no notice has been taken of them in our enumeration work, and when possible they have been removed before measure- ment or deducted by estimation from the volumetric records. The desmids are few both in species and individuals. Seven species have been recognized, of which but four are of general occurrence in the plankton. These are three species of Closterium and Staurastrum gracile. The latter and Cosmodadium saoconicum are the only eulimnetic organisms among them. The center of dis- tribution of the other species is the shore and bottom. The stom- achs of fish such as the CatostomidcB, the carp, and Dorosoma cepe- dianum, which often feed upon the bottom ooze or slime about aquatic plants, usually contain many desmids, including the species here noted. Other species also are occasionally adventitious in the plankton, and the list might be considerably extended, though the absence of extensive peat bogs in the drainage basin of the river reduces the desmids to a position of much less importance than that which they occupy in more northerly waters. As a group they exhibit a well-defined seasonal distribution, with a vernal pulse at about the time of the volumetric maximum in April-May and an autumnal pulse of less regular occurrence, location, and size. The optimum temperature for their appear- ance in the plankton lies below 70, and in winter months they occur but rarely. DISCUSSION OF SPECIES OF CONJUGATE. Closterium acerosum Ehrbg. Average number, 348. More than three times as abundant in the previous year. This desmid is perennial in the plankton, having been found in every month of the year, but at irregular intervals, and never in large numbers. Its distribution is such as to suggest that it is at the most only semi- limnetic in habit. The numbers are too small to follow closely the seasonal distribution. There are pulses on May 3 (3,200), Septem- ber 6 (2,400), and November 1 (2,500) in 1898; and in 1897 a pulse on June 28 (2,000) and one on September 21 (24,000). In previ- ous years vernal pulses in April and occasional autumnal pulses are to be noted. In so far as the optimum temperature is indicated, it 61 seems not to lie near either extreme, and above rather than below the average for the year. Closterium gracile Breb.* Average number, 49,616 (silk, 305). This species was found in small numbers from March to December, and shows pulses on May 17 (1,600) and September ~27"(6,400) at temperatures of 64 and 73. The tenuity of the form of the frustule of this species suggests a limnetic habit. Closterium hmula Ehrbg. Average number, 556. This also is a perennial species, and is somewhat more abundant and constant than C. acerosum. It likewise has a vernal pulse, which in 1895 appears on April 29 (2,915) ; in 1896, on May 1 (5,364) ; in 1897, on May 25 (3,200); and in 1898, on May 24 (6,000). In both this species and C. acerosum there are slight indications of recurrent minor pulses which are often coincident in the two species. Nine such movements appear in 1898. The autumnal pulses are less regular in their appearance and size than the vernal, and appear from September to November. The optimum temperatures seem to lie between 45 and 70. This species is only semi-limnetic, and never attains the fluctuations which characterize most limnetic organisms. Doubtless other so-called species of Closterium have been included among the variable organisms referred here to C. lumila and C. acerosum. Cosmarium constrictum Delp. was found occasionally from March to September, and is probably adventitious. Cosmocladium saxonicum De By. A single isolated pulse of this minute limnetic desmid appeared in the filter collections of Septem- ber, 1897. It was first noted on August 31 and disappeared after September 29, and was never found at other times in the plankton. The pulse culminated September 9 at 13,500,000*. Gonatozygon brebissonii De By. The filaments of this desmid were noted in the plankton only in March, 1899, attaining a maxi- mum of 136,800 on the 14th. Staurastrum gracile Ralfs. Average number, 31. About two hundred times as abundant in the plankton of 1897. It occurs from March to January. No vernal pulse was detected, but an autumnal one of 14,000 appears September 29. It appears in much larger numbers in the filter-paper collections, and is probably a limnetic planktont in our waters. 62 Undetermined species of Penium, Arthrodesmus, and Docidium have been found in the plankton but always singly. They are doubtless adventitious. PHANEROGAMIA. The Lemnacecz are represented in our waters by several species of Lemna, by Spirodela, and by two species of Wolff ia brasiliensis and columbiana. The first two genera are predominantly floating surface-plants, while the last occurs at all levels, is taken with the plankton, and has been treated in our measurements and enu- merations as a limnetic organism. Wolff ia brasiliensis Weddell. Average number, 2 ; in 1897, 13. It appears irregularly in river planktons from the last of March till January, and is somewhat more abundant in late summer and autumn. The seining operations of fishermen in the river and tributary backwaters have much to do with its appearance in the plankton of the river. Wolff ia columbiana Karsten. Average number, 7; in 1897, 41. With the preceding species. Neither of these species are sufficiently abundant greatly to affect the ecology or quantity of the plankton of the river, though they are of more importance in the backwaters. Owing to their size and duration they compete with the smaller organisms of the phytoplankton, but do not serve as food for any of the zooplankton. PROTOZOA. Average number, 111,731,000. The number of species exceeds 147 ( + 38), distributed as follows: Mastigophora, 60 ( + 10); Rhizop- oda,3l ( + 28); Heliozoa,5; Sporozoa (3) ; Ciliata, 45; and Suctoria, 5, the numbers in parentheses indicating the additional forms whose specific rank was not recognized in the enumerations . The Protozoa occur in great numbers (Table I.) in every collection of the year. Owing to the fact that the totals are a conglomerate of two methods of collecting, of a large number of species of many di- vergent seasonal tendencies, and of both eulimnetic and adventitious forms, their seasonal fluctuations have no particular significance which is not better treated either in connection with the subdivisions of the class or with the individual species. In the totals, traces appear of 63 the vernal pulse, of the midsummer maximum of the chlorophyll- bearing Mastigophora, and of the autumnal- winter wave of Ciliata. The Protozoa, through the Mastigophora, share with the algas the synthetic function in the elaboration of food from inorganic or partially disorganized organic contents of the water. _They utilize decaying organic matter as food, and are thus primary links in the cycle of food relations. Some of them feed upon bacteria, upon alga3, or even upon other animals, and thus become secondary or tertiary links in the chain. MASTIGOPHORA. (Plates I. and II.) Average number, including, without duplication, both silk and filter-paper collections, 95,856,449. In the collections of 1897 they were five times as abundant as a result, in part at least, of the extended low-water period, sewage contamination, and extension of high temperatures during the late autumn of that year (Pt. I., PI. XL). The Mastigophora abound in every collection and occur at all seasons of the year. Four fifths of them occur, however, between the first of April and the last of September. They are predominant- ly chlorophyll-bearing organisms, and have their greatest numbers during the same season in which the land flora attains its growth. They spring into abundance with the opening buds of April, and van- ish from the plankton when frost cuts off the foliage in autumn. There are, it is true, some species, such as Synura, which grow luxuriantly at winter temperatures, but these are generally of the chrysomonad type, with yellowish or brownish chromoplasts. The bright green chlorophyll-bearing flagellates are in the main summer planktonts. Since water temperatures do not fall below 32, the phytoplankton is exempt from this risk of destruction against which the land flora must provide. We find, accordingly, that the most of the Mastigophora are wont to occur in diminished numbers and irregu- larly in the plankton throughout the winter. This appears in the records of the more common species, and fuller examination would doubtless greatly increase the number which thus winter over in reduced numbers. I have already called attention to the fact that there are in 1898-99 recurrent pulses in the Chl0rophyce& and Bacillariacece at 1*5 o t~ O 00 1 t^ ^ I i * T^ -H f! T t| O CN H "ri Q t^ CN ' CN CO > | > 10 -i-j 03 P i CO ON C 00 f H H CN I CN (5 , O r^ 00 _, CN ro >JO T}< t-H Tf to ? i d) ON C i i OJ H 1O NO NO PO CU r j H I/") Q- r O O 4 * 00 O CN O g I 5 I" 2 ^eo" - J hj 1 1 ON NO ON f > NO O fO t- N D t^. '-I O q H (P 00 CN -i '-H a 5- SB t-H ct Q 00 4-> TH 3 - o ct P 3 Pi Oj CIS 3 - > a) oo g> 3 S 3 O j o ^ TJ< NO ro CN ro CN JH ^ 00 ON fO s * P-i-O h CN d**^ H D I 5 <^ l t- O M 00 f" > * -) CN <^ Q H "cS P ON OO 00 - rt P ON 00 JH ^ efl - h . 2 CN f i PO CN CN M i 00 C i u 5 t^ O ^1 ^ < ^> ro *^ > NO U3 0) rt NO M-H "^ On'O 10 T f ro CV^ o Q S 5 C 5 0) z p< Q (U 0) j; H 3 3 ce h el P , i p ON 00 ,-H * 64 ON -H CO O^ ^*~' S CU CN ro a d, Tf ' * -H 11 |I ]?. - CJD ^ 3 3 03 Q 2 15 . I o CO Q s i . S ', ^H in co , o o *^ N M T 1 Q} t-^- ^H NO j t CD O ON * CN O H -j C !3 g 5 -^ ? "^ 2 j i i t^ *. O) ^ ^ ^ H CO OJ IJ p U3 cu f^* ^ O^ S CD NO CO ^ J Q ^^ ^ ^ ^ F^B t -H 0, Q <** 1 , 5 03 Q 00 -H CN -H CN * > ,0 j to oo Ck, f*3 CM a Q O ' P af rn .' 'd Qi TO (U CO 4) Oi (H S .q .1 a, a o J3 ,bp "c3 & 8 J ^ J , 1*11 ~a S 9 ' " "^ (U Q 'cj tc CD 11 "a & ol rt O ffl S +-> c ^^ 65 66 intervals of several weeks, and that such pulses can also be traced back into 1897 as far as the collections were made at weekly inter- vals that is to the early part of July. A similar periodicity on the part of the Mastigophora the greater part of which are also chlorophyll-bearing is even more evident. Not only is this periodicity present in this group, but it coincides approximately in the location of its maxima and in their relative development with that found in the Chlorophycece and BacillariacecB. The following table, which gives the dates of culmination of the pulses of these three groups from July 1, 1897, to April 1, 1899, will serve to demonstrate this point more clearly, and a graphic presentation of the data will be found in Plates I. and II. There are twenty-two of these recurrent pulses in the period from July, 1897, to March, 1899. Of the sixty-six possible maxima only five are missing, or at least not apparent in our data, and but ten culminate on other dates than the one (of collection) most to be expected. These ten in every case culminate either a week prior or subsequent to that in which the other two groups reach their max- ima. These divergences may be due to the error incident to the interval of collection, and their approximation in time is still cor- roborative of the tendency towards recurrent periods of growth. These exceptions are no greater than might be expected to occur in the unstable fluviatile environment and within the large margin of error of the plankton method. There are twenty-one intervals between July 14, 1897, and March 14, 1899, with a range in length of 20 to 42 days and an aver- age of 28.95. The intervals in days with the numbers of instances of each are as follows: 20 (1), 21 (3), 22 (1), 23 (1), 26 (1), 27 (1), 28 (7), 35 (3), and 42 (3), days. The effect of the weekly interval of collection is seen in the preponderances at 21, 28, 35, and perhaps at 42, days. There is evidently a tendency towards the interval of 28 days. Nine of the 21 pulses are grouped about this interval; 6, about that of 21 ; while 3 are at 35 and 3 at 42. If there be such a tendency it is but natural that with a weekly interval of collection there should also appear minor preponderances at 2 1 and 35 days. Traces of a similar rhythm may be found in the period of weekly collections in 1896 (Pt. I., Table III.). In some instances the environmental conditions at these times of departure are such as to suggest that they may have produced the 67 shifting in the position of the maxima. Thus the pulse of January 25, 1898, appears after a 3 5 -day interval, but in the midst of the rising winter flood, to whose effect the delay may be attributed. In both 1896 and 1898 the 28-day rhythm is interrupted at the time of the vernal pulse in April-May. It appears as though these re- current pulses -if such exist were submerged in the greater ver- nal increase. The double summit of the vernal pulse in the curve of the Bacillariacea and Mastigophora (PI. II.) for 1898 suggests the compound character of this pulse in the case of these groups of organisms at least. The time interval in the case of the vernal in- terruption is also significant. In 1898 there are two pulses between March 22 and July 19, at intervals of 42 days a total of 84 days, which is the equivalent in duration of three 28-day intervals. The total number of species of Mastigophora recorded by me from the plankton of the Illinois River is over sixty. This number will be increased to more than seventy if forms not separated in our enumerations be distinguished as separate species. The Protomastigina (including the Bicoscecidce and the Cras- pedomonadida] are well represented in the plankton by passive limnetic species which are principally sessile on other planktonts. These are Bicosoeca lacustris, Salpingceca brunnea, S. minuta, and Diplosiga frequentissima. Asterosiga radiata is a eulimnetic repre- sentative and Anthophysa vegetans an adventitious one. As a group they are more abundant during the warmer part of the year. The Chrysomonadidcz are also well represented, and include the most abundant flagellates of the plankton of the colder months. Synura uvella is quantitatively the largest factor furnished by this group. It is supplemented by Syncrypta volvox, and the various forms of Dinobryon, Uroglena, and Mallomonas. The last two genera have more of a summer range of occurrence, but are not of quantitative importance in the waters of the Illinois. The CryptomonadidcB are represented only by Chilomonas and Cryptomonas, and are of somewhat constant, though of minor, importance quantitatively. The Euglenidce, on the other hand, are, in our waters at least, second to no coordinate group in their quantitative importance. They are individually of relatively large size, and they occur in great numbers throughout the summer months, replacing the Chrysomonadida of the colder seasons of the year. Euglena (6) 68 viridis is the most abundant, and it is associated with other species of the genus, with species of Amblyophis, Phacus, Lepocindis, Chlo- ropeltis, Colacium, and Trachelomonas , especially the latter. The PeridimidcB are quantitatively of considerable importance in the plankton of our Great Lakes (Kofoid, '95), but in the Illinois River they are of little significance, at least the larger forms such as Ceratium. Smaller species such as Peridinium tabulatum and Glenodinium cinctwn are more abundant. As a group they do not show any marked seasonal preferences. The Volvocida, on the other hand, are of more than the usual consequence in the plankton of the Illinois. The group is repre- sented by the curious Chloraster gyrans, by the sporadic and meteor- ic Carteria multifttis, and by the colonial genera Eudorina, Pando- rina, Pleodorina, Platydorina, and Volvox. As a group they are almost exclusively summer planktonts. The Mastigophora as a whole are, next to the BacillariacecB, the most abundant of the synthetic organisms of the plankton. Their quantitative importance has not hitherto been sufficiently demon- strated in the plankton of fresh water, owing it may be to their escape through the silk net in the ordinary methods of collection. It seems quite probable also that they may be present in our warm and fertile waters in much greater abundance than they are in the colder and clearer waters of most lakes. This is especially true of the EuglenidcB and Volvocidcz, perhaps less so of the ChrysomonadidcB and Peridiniidce. DISCUSSION OF SPECIES OF MASTIGOPHORA. Amblyophis viridis Ehrbg.* Average number, 63,014 in 1897. It occurred throughout the summer in 1897, from May to October, with a maximum of 1,440,000 on August 31. Apparently a sum- mer planktont but never very abundant. Anthophysa vegetans (O. F. Mull.) Butschli. This was identi- fied in the plankton of June, 1898. It is very abundant at times on various substrata in stagnating water, and from such places becomes adventitious in detached fragments of colonies in the plankton. Aster osiga radiata Zach. This interesting colonial and limnetic choanoflagellate, described originally from the plankton of German lakes, has been found but a single time in our plankton in the latter 69 part of August, 1896'. It is one of many illustrations of the cosmo- politan distribution of plankton organisms. Bicosceca lacustris J. Clark*. Average number, 112,896. Only one third as abundant in 1896, and four times as many in 1897. This minute flagellate is found in our waters sessile upon the-filaments of Melosira, principally M. granulata var. spinosa. It occurs more frequently upon the dead frustules than upon live ones, and upon those of the shorter form than upon the longer. It has appeared also upon Dinobryon sertularia, Pediastrum pertusum, and Richteri- ella botryoides. It exhibits a considerable range of variation in proportions, in the amount of lateral compression, and in the length of the pedicels. These variable forms are, however, connected with the type as described by Clark, and are not, in my opinion, to be designated as distinct species. Zacharias ('94) has described one of these variants as B. oculata. I regard it as a growth condi- tion of B. lacustris, and not as specifically distinct from it. Its seasonal distribution in 1898 is somewhat peculiar. It appears as two quite symmetrical pulses, the first extending from early in June till the middle of July, and culminating on June 14 at 3,801,600. The approach of this pulse is abrupt and its decline somewhat gradual. The species does not reappear until September 13. The autumnal pulse culminates October 11 at 486,000, then gradually declines, and disappears November 1. There is no record of its occurrence in 1898 outside of these two pulses. In 1897 it is found irregularly from May to August, and in 1896 in February and from May to December, with pulses in May, June, July (2) , August, and October. In 1898 its optimum temperatures appear at 82 and 65, and its pulses in other years do not occur below 57. It thus belongs to the plankton of the warmer months. Its seasonal distribution falls within that of the limits of its host Melosira, and in 1896 and 1898 their vernal pulses coincide, and the same correlation appears in all but one of the pulses of 1896. Not all Melosira pulses, however, are attended by an increase in Bico- sceca. Thus in the late summer and fall of 1897 Melosira fluctuated without any appearance of Bicosceca. In the autumn of 1898 the pulse of Bicosceca on October 1 1 appears on the decline of the Sep- tember pulse of Melosira, in which the host made no corresponding increase. Melosira is thus apparently essential for any marked in- 70 crease of Bicosceca in the plankton, but is not in itself the primary cause for its appearance in the plankton. Carteria multifilis (Fres.) Dill.* Average number, 2,365,384. In 1897 more than one hundred -fold as abundant. This species was recognized only in the autumnal and hiemal planktons, from August till January in 1897-98 and from October to February in 1898-99. It is not easily and with certainty identified by the usual methods of plankton counting, and probably other species of similar habitus may have been included to some extent ; and, on the other hand, many Carteria may have been thrown with the "un- identified" flagellates, especially in earlier years. This species occurs throughout the whole range of temperatures, and its maxi- mum development (6,476,400,000) was attained October 5, 1897, at 70. A pulse prior to this appeared September 7, at 2,846,250,000. From the major pulse in October there is a gradual decline as the minimum temperatures are reached. The remarkable outbreak of Carteria in the autumn of 1897 was associated with unusually low water (Pt. I., PI. XI.) and concen- tration of sewage and decrease in current. The water of the stream was of a livid greenish-yellow tinge, due principally to great numbers of Carteria, which developed to the exclusion or diminution of other chlorophyll-bearing flagellates such as Euglena, and of diatoms such as Melosira. This unusual development seems to have been a dis- turbing factor in the usual seasonal routine of the autumnal plank- ton of that year. The distribution of Carteria in the river was remarkable. It formed great bands or streaks visible near the surface, or masses which in form simulated cloud effects. The distribution was plainly uneven, giving a banded or mottled appearance to the stream. The bands, 10 to 50 meters in width, ran with the channel or current, and their position and form were plainly influenced by these factors. No cause was apparent for the mottled regions. This phenomenon stands in somewhat sharp contrast to the distri- bution of the usual water-bloom upon the river, which is generally composed largely of Euglena. This presents a much more uniform distribution, and unlike the Carteria is plainly visible only when it is accumulated as a superficial scum or film. Carteria was present in such quantity that its distribution was evident at lower levels so far as the turbidity would permit it to be seen. It afforded a 71 striking instance of marked inequalities in distribution within small areas, of at least one plankton organism. Carteria showed great variation in the amount of chlorophyll present. Some individuals were practically colorless. It seems very probable that in the presence of great abundance^ partially decayed organic matter such as occurs in a sewage-laden stream, Carteria may become largely holozoic in its nutrition, as Zumstein ('99) has shown to be the case with Euglena. The literature of fresh-water plankton contains no record of a similar preponderance of Carteria in other localities, though its occurrence has been occa- sionally noted in the plankton. The chemical conditions under which this great pulse of Carteria appeared in the autumn of 1897 can be followed in Part I., Plate XL IV and Table X. The high chlorine and the great increase in free ammonia and nitrites indicate the decay of sewage; the high nitrates and albuminoid ammonia show that there was no lack of some at least of the important sources of food. The two principal pulses appear September 7 (2,846,250,000) and October 5 (6,476,- 400,000), with a minimum of 680,400,000, on September 21, sep- arating them. Both of these pulses are attended by sharp declines in nitrates and nitrites and free ammonia, and very slight decreases in organic nitrogen and albuminoid ammonia. Either the first three substances named or those matters which supply them by their decay, are thus noticeably utilized at the times of these pulses. The relation of the Carteria to the volumetric pulses is (Pt. I., PI. XI.) not a constant one. The Carteria pulse of September 7 lies in a slight depression between two maxima of the volumetric curve, and a week prior to the autumnal culmination on September 14 at 19.8 cm. 3 per m. 3 . It thus appears during the growth period of this volumetric maximum. The second and larger pulse of Carteria, on October 5, coincides with the second volumetric maxi- mum, and in fact fluctuates throughout with it. Though Carteria constitutes but a small part of the actual catch of the silk net, owing to leakage through the silk, it is apparently an important factor in the food cycle which builds up such maxima. Ceratium brevicorne Hempel. This species appeared in small numbers in isolated instances from April through October. It varies towards C. hirundinella, but the small numbers in which it has occurred have not as yet afforded sufficient ground for regard- 72 ing it as a variety of that species. It occurs most frequently in August and September, and is apparently a warm-water planktont. Ceratium cornutwn Ehrbg. was found but once in June, 1896. Ceratium hirundinella O. F. Mull, was not noted in our plank- ton in 1898, but in 1896 was found from June to October, with a pulse of 19,200 on June 6. It was recorded only at temperatures above 57, and is apparently a warm-water planktont. It has but an insignificant part in the potamoplankton of the Illinois River and its backwaters, though quite abundant in the summer plank- ton of Lake Michigan (Kofoid, '95). It seems not to have survived the transit through the sewage-laden waters of Chicago River or to thrive in the conditions prevailing in the Illinois River, though common generally in fresh- water plankton of the temperate zone. Chilomonas paramcecium Ehrbg.* Average number, 555,000. This flagellate, which is frequently abundant in aquaria or stag- nant water, appears also in the plankton of the Illinois River. There is in 1898 a vernal pulse, culminating at 10,800,000 on April 26, and there are scattered records from October to February. Chloraster gyrans Ehrbg. This rare and unique flagellate was found in but two collections in July and August, 1898 and only in small numbers. Chloropeltis monilata Stokes.* Average number, 362,941 in 1897. This is a summer planktont, appearing at irregular inter- vals from the last of May until the middle of September. It was not found in 1898. A maximum of 10,800,000 appears on August 31. Colacium calvum Stein. The attached stage only of this flagel- late was observed , and was recorded only in 1 8 9 6 and 1897. It appears from the middle of April to the first of October, and is usually found upon Polyarthra platyptera. It has occurred occasionally upon several species of Brachionus and upon Chydorus sphczricus. The largest number recorded (162,792) appeared on April 17, 1896, upon Polyarthra, usually upon the body and more rarely upon the oar-like appendages. It is often exceedingly abundant upon the planktonts of backwater ponds. Colacium vesiculosum Ehrbg. This species is much less abun- dant than the preceding species in our waters, and was found only in June and September, upon Cyclops albidus and Polyarthra. 73 Cryptomonas ovata Ehrbg.* Average number, 121,154. This species has been recorded principally in the autumnal or hiemal plankton. It escapes through the silk net readily, and was rarely found in collections of earlier years. In 1895 it occurred from July till the last of October, and in 1898 was common in the December plankton. Dinobryon sertularia Ehrbg. Like most typical planktonts, Dinobryon is an exceedingly variable organism, and the varia- tion finds its expression in the form and proportions of the loricas and in their arrangement and continuity in colonies. Divergences from described and figured species are thus at once apparent, and they have been utilized by systematists, notably by Lemmermann ('00) and by Brunnthaler ('01) as the basis for the establishment of a large number of new species. The validity of these species, in my opinion, must rest ultimately upon careful experimental evidence of their present mutual genetic independence under normal conditions of growth. From my own observations upon large numbers of colonies and individuals distributed through- out the range of their seasonal recurrence in six years in our waters, I am inclined to regard all as belonging to a single species, and the different types as mere growth varieties. The rapidity of growth and the age of the individual or of the colony are, I believe, impor- tant factors in the determination of the form of the lorica, and its various forms are therefore not of specific value, but rather of physiological significance. It is a simple matter to find individuals, or even colonies, conforming to the descriptions of the several species, but it is not so easy to refer all individuals and all colonies to the described types. They intergrade nay, more, two, or even more, "species" are not infrequently combined in the same colony. I have never found all the forms in a single colony, but such com- binations as angulatum-divergens, diver gens-angulatum-stipitatum, sertularia-angulatum, and sertularia-undulatum have been observed by me. These combinations are most frequent in large colonies, and, indeed, the number of "species" in a colony is apparently a function of its size. The slender growing tips are wont to assume the stipitatum type of lorica and colony, and the older loricas at the base to conform to that of sertularia, divergens, or angulatum. Small colonies as a rule belong to a single "species." These com- binations are generally most evident during the maximum period 74 of growth; that is, when Dinobryon is multiplying rapidly, though they may appear at any season of its occurrence. In the enumeration of Dinobryon five types were recognized, and the individuals were assorted to these "species," viz. : D. sertularia, stipitatum, diver gens, angulatum, and undulatum. Some corrobora- tion of the view that we are dealing with a single variable organism and not with five distinct species may be seen in the coincidence of the seasonal distribution, and of the rise, culmination, and decline of the pulses of the five different forms. Since these varieties have such a similar seasonal distribution I shall treat them as a whole, discussing subsequently any individual peculiarities which are noteworthy. The average number of individuals of Dinobryon sertularia, including all its varieties, in 1898 was 1,979,785. In 1897 the average was much smaller (79,352) owing to the few collections in the winter, when it is most abundant, and to its suppression in the prolonged low water of the autumn of that year. The relative frequency of these different varieties for I shall treat them as such is shown by the average per cubic meter for the year in 1898, viz.: D. sertularia, 407,602; D. sertularia var. stipitatum, 603,911; D. sertularia var. divergens, 866,083; D. sertularia var. angulatum, 101,358; D. sertularia var. undulatum, 831. These figures are only approximate, since colonies containing more than one variety have all been included with the predominant variety in the colony, which is usually sertularia or divergens, consequently angulatum and undulatum are more numerous than indicated by these figures. The seasonal distribution of Dinobryon in our waters is well defined, and is sharply limited to the period from November to June. Its earliest recorded appearance was November 8 in 1898, while in 1896 and 1897 it was not found until in December. It lingers well into June in 1896 and 1898 the two years in which the spring collections were of sufficient frequency to trace its decline. In 1898 the latest record was on June 28. Most of the records after May are irregular and sporadic. It is thus absent from the plankton of the Illinois River from the last of June till November or December. In 1895-1896 there was also a winter interval in which no Dino- bryon was recorded during the December- January flood (Pt. I., PI. IX. and X.). In 1897-1898 a similar interval appears, and con- tinues almost to the end of the slow rise of the flood which culmi- 75 nated in March. Rising floods thus do not favor the development of Dinobryon in channel waters of the Illinois. The interval of collection in 1894-95 is too great to trace the seasonal fluctuations of Dinobryon, though there are indications of a maximum pulse on April 29. In 1895-96 there is^ a slight de- velopment in November prior to the rise of December, in which Dinobryon again disappears. A slight pulse of 3,192 appears on the declining flood (Pt. I., PI. X.) on January 25, and declines again with the rise in February to reappear on February 20 at 42,588. Another decline in Dinobryon attends the rise in river levels in February-March, and after a fortnight of falling levels a third pulse of 2,531,280 is seen on March 17. Two other pulses attend the decline of this flood, one upon April 29 (800,064) and the other on May 18 (339,624). On the decline of the June rise of this year a late and unusually large pulse for the season appears (June 11) at 2,438,400. An examination of the hydrograph will indicate that almost without exception these pulses attend the run-off of im- pounded backwaters after recent invasion, or, as on April 29 and May 18, after a temporary check in the run-off. During those times when the channel contributes to the backwaters, that is, dur- ing rising floods, Dinobryon declines in numbers ; and, on the other hand, it reaches its greatest development in channel waters during the run-off of the flood. In 1896-1897 the interval of collection (Pt. I., Table III.) is again too great to trace satisfactorily the fluctuations of Dinobryon. There is a pulse on December 3 of 157,609 and on April 27 of 172,800. In 1897-98 Dinobryon appears first on December 7, with a pulse of 1,807, 200, during a period of low water and ice blockade with no backwater contributions. It declines, and after December 21 does not again return until March 22, when an isolated record appears. The vernal pulse begins April 19 and culminates May 10 at 84,- 841,600 on the declining spring flood (Pt. I., PL XII.). Dinobryon declines at once during a fortnight of rising water, and two minor pulses on the decline of the flood one on June 7 of 70,400 and one on June 28 of 219,840 complete its vernal cycle. The hydrographic conditions in 1898-99 were very different from those of the preceding season, and we find a marked change in the seasonal occurrence of Dinobryon. From November to March 76 there are three rises to overflow stages (Pt. I., PL XII. and XIII.) with intervening declines of a month's duration. There is a pulse of Dinobryon in each of these periods of declining flood. The pulse of 275,200 on December 20 follows the November flood, and it is followed by a minimum of 1,500 on the rising flood of January 10. The numbers slowly increase until a meteoric rise on February 7 to 6,486,700 and on February 14 to 22,621,440 is followed again by another decline, to 25,920 on February 28, with the sudden flood of that week. During the .maximum flood stage in March (Pt. I., PL XIII.) no Dinobryon was recorded, but it reappeared again on March 21. The suspension of our plankton operations interrupted the further tracing of the fluctuations. From the facts above detailed it is very evident that the pulses of Dinobryon occur in channel waters at times when the run-off of impounded backwaters is making its greatest contribution to the river plankton. These are times of greatest stability of the en- vironment in all respects save river level and its sequences. The impounded waters have come from regions of slight current and decaying vegetation, and there has been time in those localities for the decay of sewage and debris, and for the growth of planktonts such as Dinobryon. These conditions of the environment are therefore favorable for the growth pulses of Dinobryon. The phenomenon of pulses of growth is not, however, to be considered as merely the result of declining floods. These afford a favorable environment and doubtless determine within certain limits the time and the extent of the pulse. The phenomenon is one common to most plankton organisms, and occurs in Dinobryon of lakes where floods are of little significance. Any evidence of recurrent minor pulses in Dinobryon at brief intervals is lacking. Dinobryon has been found in our plankton through practically the whole range of temperatures, but it disappears when maximum summer heat is reached and does not return until the water cools to 45 or lower. Large pulses, such as that of February 21, 1899 (22,621,440), have developed at temperatures approximating 32, and largely under the ice. The vernal pulse of April-May has been recorded at temperatures ranging from 60 to 79, but generally nearer the former. No well-defined optimum temperature appears, and the seasonal distribution suggests that the high temperatures 77 of our summer waters are inimical to Dinobryon. That its absence from the plankton at that time is not due merely to low- water con- ditions is shown by the December pulse in 189 7, under the most pro- nounced type of such conditions. Dinobryon is a common planktont in the Great-Xakes (Kofoid, '95) during the summer months, but surface temperatures here rarely exceed 68, and are 10 to 20 below those of the Illinois River. In German lakes Apstein ('96) finds the maximum develop- ment of Dinobryon in June and a continuance through the summer in reduced numbers, but temperatures are also 10 to 20 (F.) lower than in our waters. In the case of D. stipitatum there is a second maximum in August. Lauterborn ('93) finds Dinobryon throughout the winter in the plankton of the Rhine, with a maxi- mum in April-May, with diminished numbers during the summer, and a second maximum in September. The filter-paper collections give very much larger numbers, owing partly to the inclusion of small colonies which escape through the meshes of the silk net in the usual method of collec- tion. The numbers are increased at least thirty-fold if filter col- lections are utilized instead of silk, as above. The size of the colonies in the collections varies greatly, the averages ranging from three to forty-eight cells. The maximum pulse is attended or followed by a considerable decrease in the size of the colony. In the pulse of February 21, 1899, the average num- ber of cells in the colony falls from thirteen to sixteen, during the rise of the pulse, to seven, at its culmination. On the pulse of May 10, 1898, the average is thirteen, and a week later, when the pulse declines from 16,153,600 to 43,200, the average size of the colony drops to three cells. Cysts also are most frequent during and subsequent to maximum development. Dinobryon is some- times covered with large numbers of minute choanoflagellates, probably Salpingosca minuta Kent. Frequently colonies occur in which only the younger cells are alive. Dinobryon is, in the light of its distribution, one of the impor- tant synthetic planktonts of the colder months, and is one of the primal links in the chain of food relations of that season, serving as food for some of the winter Cladocera and Copepoda. The fact that its maxima frequently occur when volumetric minima appear as, for example, on February 21, 1899 indicates that Dinobryon 78 does not directly contribute much, even at its maximum develop- ment, to the volume of the plankton taken in the silk net. On the other hand, its rapid multiplication, as evidenced by its meteoric pulses, may serve to build up a more permanent and bulkier animal plankton, and thus indirectly, in a cumulative way, it may be of considerable quantitative importance. The inclusion of all the variants of Dinobryon as a single species has been favored by Wesenberg-Lund ('00), who regards D. stipi- tatum as the summer form of D. sertularia. In our plankton, D. stipitatum has occurred sporadically in December and March, but it is most abundant during the vernal pulse in April-May. Its distribution thus in the main supports that author's contention in that it is found during the warmer portion of the seasonal cycle of Dinobryon in our waters, though not in our summer plankton. It is not desirable in this connection to enter further into a discussion of problems which have been raised by the splitting up of Dinobryon into so large a number of forms. Lemmermann has found seven- teen species and varieties within the limits of the subgenus Eudino- bryon. A discussion of their validity involves not only some per- plexing problems of synonymy, but also an extensive examination of a large amount of material showing seasonal changes, and, above all, a series of experiments which shall demonstrate the limits of variation within a known line of descent and in the sea- sonal range of environmental conditions. It involves, moreover, the fundamental question of the criterion of species. The papers of Lemmermann ('00) and Brunnthaler ('01) have appeared since my work of enumeration was completed. I recognize among the forms which they have sought to establish the following which occur in our plankton: D. sertularia Ehrbg., D. sertularia var. thyrsoideum (Chodat) Lemm., D. sertularia var. alpinum Imhof, D. protuberans Lemm., D. sociale Ehrbg., D. stipitatum Stein, D. stipitatum var. americanum Brunn., D. stipitatum var. bavaricum (Imhof) Zach., D. elongatum Imhof, D. elongatum var. undulatum Lemm., D. cylindricum Imhof, D. cylindricum var. palustre Lemm., D. cylindricum var. schauinslandii (Lemm.) Lemm., D. cylindri- cum var. ped^forme (Lemm.) Lemm., D. cylindricum var. diver gens (Imhof) Lemm., andD. cylindricum var. angulatum (Seligo) Lemm. As a result of my attempts to refer all of the individuals which I have seen in my work of enumeration to species, I am of the opinion 79 that we are dealing in the case of the species of Dinobryon above cited with a single variable organism, whose extremes of variation only have been regarded as separate species. The connecting links are sufficiently abundant still and the union of several types in a single colony is sufficiently frequent to lend some weight to my con- clusions with regard to those forms which have been under my observation. In the interests of utility as well as in the interests of well-grounded taxonomy, it is extremely desirable that the establishment of new species among variable plankton organ- isms should be attempted with extreme caution and only after the fullest study of the range and conditions of variability. The insta- bility of the taxonomic structures which Brunnthaler and Lemmer- mann have recently raised, is evidenced by the differences in syno- nymic, varietal, and specific rank given to the variants of Dino- bryon by these two systematists, who have but recently mono- graphed the group, largely if not wholly from the systematic point of view. The changing estimate of validity which Lemmermann himself has put upon his own species or varieties for example, schauinslandii, pediforme, and curuatum rgives further evidence that the basis upon which they rest is at the best but slight. It is my firm conviction that the establishment of new species among the organisms of the plankton of fresh water can be satisfactorily accomplished only after careful analysis of the limits of variation within the range of environmental conditions. Stand- ards less comprehensive than this can yield results of but temporary or local value and can lead to but little permanent advance in science, and they bring only perplexity and chaos where order should reign. Diplosiga frequentissima Zach.* Average number, 1,736,538. This minute flagellate is found upon the rays of the colonial diatom Asterionella, often in great numbers and so thickly set as to leave little unoccupied space. It was found in each year at the time of the vernal pulse of Asterionella in April-May, and was as a rule most abundant immediately after the maximum growth of Asterionella had been attained. Beyond an isolated occurrence in January it was not recorded at other times than during the months of April and May. Eudorina elegans Ehrbg. Average number, 14,362. About twice as abundant in 1897. The distribution of this species is 80 somewhat erratic. It has occurred in every month from February through October, but in smaller numbers and sporadically in the colder months. In 1898 its seasonal curve is of characteristic form. It makes its appearance March 1 5 , and is continuously present until the end of September. There is a vernal maximum April 26 of 240,000, but no corresponding autumnal one. In 1898 there are indications of recurrent pulses at brief intervals which coincide in location immediately or approximately with similar ones of Gonium and Pandorina. These pulses occur March IS (3,600), April 5 (2,800), April 26 (240,000), June 14 (60,000), August 2 (8,000), August 23 (3,200), and September 20 (2,000). The minima between these pulses in all cases but one fall below 1,000. In 1897 a vernal pulse was not detected, a maximum of 496,000 occurring August 31, and but three mindr pulses appearing. In 1896 this species appeared in the plankton on February 20, and remained until the end of August with a month's interrup- tion in May- June. There were no marked pulses, exceeding 15,000, in that year. The absence of the spring flood (Pt. I., PI. X.) and the disturbed hydrograph of the summer may account for this suppression of development in Eudorina. The distribu- tion in preceding years is also irregular. Eudorina begins its seasonal development at temperatures but slightly above 32, but any considerable growth is not attained until at least 45 has been reached, and the largest pulses on record have been at the close of the period of maximum summer heat at a temperature of 80, and the vernal pulses have been at 60 or above. The disappearance of Eudorina from the plankton in the early fall, about the tim that foliage is killed by autumnal frosts, has been constant in the different years. Eudorina is not sufficiently abundant to be of any considerable importance in determining directly the volume of the plankton. It serves as food for many of the rotifers, and is itself frequently parasitized by Dangeardia mammillata Schroder, which destroys the cells but leaves the matrix intact. There are times when it is hardly possible to find perfect colonies, and when it is not unusual to see colonies swimming about propelled by one or two surviving cells. Euglena acus Ehrbg.* Average number, 214,807. Found from the middle of March till the first of November, and most abundantly 81 in late summer and early autumn. It escapes through the silk net readily, and no marked pulses in occurrence appear in the erratic data of the filter-paper collections. It is found in the water-bloom, and is predominantly a warm- water planktont. Euglena deses Ehrbg. Occurs occasionally in the plankton and water-bloom during summer months. Euglena elongate Schew.* Average number in 1897, 278,970. It is found irregularly in our plankton and water-bloom from July to October. Originally described from New Zealand. Euglena oxyuris Schmarda.* Average number, 960,769. Next to E. viridis this is the most abundant member of the genus in our plankton. It is abundant during the summer, especially towards its close during low-water conditions, when the water-bloom, in whose formation it shares, is best developed. There is no vernal development, and the fluctuations are but slight in com- parison with those of most organisms of the plankton: There is a slight indication of recurrent pulses at intervals of a few weeks. Its optimum temperature lies near that of maximum summer heat, that is, about 80, though some tendency to run over into autumn months is manifest. Euglena sanguinea Ehrbg. There are only sporadic occurrences of this species in the plankton. It is found along with E. vindis among matted growths of Lemnacea, and on exposed and reeking mud flats, where it forms patches of bright red color often of large extent. It may be only a physiological condition of E. viridis, with which it is always found. It has appeared in the plankton most frequently in September, though found elsewhere throughout the summer. Euglena spirogyra Ehrbg. Found but once in October, in the river plankton. Euglena viridis Ehrbg.* Average number, 1,571,731 ; from silk collections only 8,653. This is the most abundant of the larger green flagellates in our plankton, and constitutes the greater part of the water-bloom of summer months, when it forms towards four p. m. a livid green scum on the immediate surface of the water. Collections of the silk net give no clue to its abundance and shed no light on its seasonal distribution. The filter-paper collections indi- cate its presence from March to December, but in numbers only during the warmer period, from May to October. There is no ver- 82 nal pulse though there are slight traces of minor irregularities, and on September 7, 1897, a single unusual deyelopment of 58,000,000. Its optimum temperatures lie close to the maximum heat of summer months. It is found not only in water-bloom and plankton, but also along shores, on mud banks, and in sequestered pools and bays where temperatures reach 90 and over. Lightly colored and semi- transparent individuals of this and other species of the genus are found frequently in the plankton, suggesting an approach to holo- zoic nutrition in nature, such as Zumstein ('99) has demonstrated experimentally in E. gracilis. Euglena is quantitatively one of the most important links in the chain of food relations of the summer plankton, converting nutrient matters in the water, both organic and inorganic, into food for the Rotifer a and Entomostraca of that season of the year. It in a measure replaces the diatoms, some of which decrease in number or disappear during the warmer months. Glenodinium cinctwn Ehrbg.* Average number, 1,360,192. This species is generally present from the middle of March till the end of September, though sporadic occurrences are found in winter months. There is a pulse on March 29 of 4,260,000 at a temperature of 49, and another August 9 of 25,200,000 at 83. This small planktont usually escapes through the silk net. It may be that several species have been included, as the conditions of plankton enumeration do not permit close scrutiny of such small organisms, lacking prominent structural characteristics. It seems to be a perennial planktont with a wide range of temperature adaptation, and with a growing period approximating that of the land flora of our latitude. Gonium pectorals O. F. Mull. This colonial flagellate has been found in the water-bloom in large numbers, especially in the back- waters. It was taken in the river plankton in 1897 and 1898 in May and again in August and September. These pulses coincide in location with those of Pandorvna and Eudonna. Lepocinclis ovum Ehrbg.* Average number, 401,538; silk 3,719. This species appears in the plankton in April and continues until the end of October, with sporadic appearances in winter months. There is no vernal pulse, and in both 1897 and 1898 maxi- mum numbers, 43,200 and 50,400, occur at the height of midsummer heat in August. In both years there are well-defined recurrent pulses at intervals of three to six weeks to be traced in the silk 83 collections. The optimum temperatures plainly lie near the maxi- mum, that is, about 80, and the season of growth approximates that of the land flora, being limited to the months of April-Septem- ber. This is a variable organism, and a number of species have been described in the genus in recent years. Many of these" occur in our waters, but no attempt has been made to separate them, since they are based on minute characters. Mallomonas plosslii Perty. and M. producta Zach. These two forms will be treated together, as in my opinion they are merely divergent variants perhaps seasonal of a single species. In 1898 M. plosslii was found but three times in June and July and M. producta eight times from May through September. In 1897 the latter only was recorded, and in September and October. In 1896 M. plosslii appeared in July and M. producta in April and August. In 1895 M. producta alone was recorded, and that in November. The data are hardly sufficient for generalization, but so far as they go they indicate that producta is more prevalent in late summer and autumn and plosslii in early summer, the more attenuate form (producta} in the warmer season. Biitschli ('80-' 89) has intimated that there may be some genetic connection between Mallomonas and Synura uvella. Certain features of its occurrence in our plankton lend their support to this view. Synura in our waters is a winter planktont, with December and February or March pulses. Mallomonas is a summer planktont, making its first appearance during the time of the decline of Synura, and when many of the colonies of the latter are breaking up into their individual zooids. Again, the differences in structure and size between the two genera are quite superficial, and might result from the growth attending the free life of a Synura zooid and its prepara- tion for sporulation. It is a noticeable phenomenon that the pro- portion of sporulating individuals of Mallomonas in the plankton is exceptionally large among all plankton organisms. "Free cells" of Synura are plainly referable to that genus by their resemblance, and by the fact that they are often united in clusters of several indi- viduals forming fragments of disintegrating colonies. It may be that some reproductive phase, as conjugation, intervenes between the free-cell condition of Synura and the Mallomonas stage, and that the relatively smaller numbers of the latter are due to the in- frequency of this process. While the features of seasonal distribu- (7) 84 tion, structure, and sporulation thus suggest the possibility that Mallomonas is a free zooid stage leading to sporulation in Synura, they do not demonstrate it, and the genera must stand in statu quo until breeding experiments shall clearly demonstrate the full life- cycle of Synura. Pandorina morum Bory. Average number, 6,957. In 1898 this organism was about half as abundant as Eudorina, but in 1897 it more than equals it. On account of the small size and the motility of the colonies many of them escape through the silk, so that it is not so adequately represented in silk-net collections as Eudorina. It is probably the most important quantitatively of the Volvocidcz in our plankton. It occurs from April to October, with a few spo- radic appearances in March and up to January. Its greatest growth occurs from May to October. There is no predominant vernal pulse in 1898, but a series of smaller ones culminating May 3 (48,400), June 14 (60,000), July 26 (63,200), and August 30 (3,200), all upon declining floods (Pt. I., PL XII.) and coincident with pulses of other VolvocidcB Eudorina and Gonium. In 1897 its seasonal dis- tribution was also similar to that of these genera, exhibiting a max- imum pulse August 31 of 638,000 at 80. In 1896, a year of inter- rupted hydrograph (Pt. I., PL X.), Pandorina attained no marked development. Its optimum temperatures lie at and above 60, and its larger pulses appear during the season of maximum temper- ature, that is, at about 80. Pandorina does not attain any marked autumnal growth, but declines in September, and as a rule dis- appears in October. The period of its growth thus lies within that of the land flora. As in Eudorina, so also here, parasitism by Dangeardia mammil- lata is of frequent occurrence. Pandorina is an important element in the food of summer rotifers such as Brachionus. Peridinium tabulatuni Ehrbg.* Average number, 3,875,769; silk, 3,711. This is a perennial planktont, having been found in every month of the year. Its principal development is, however, reached during warmer months, from May till September. In 1897 the maximum pulse of 172,800 was on August 10, and in 1898 one of 66,800 fell on July 26, the temperatures being 81 and 89 respectively. The only exception to this predominance in warm months is an iso- lated pulse of 2,400 which developed on the declining flood of Febru- ary, 1899 (Pt. I., PL XIII). The absence of any autumnal development 85 of this species is noticeable. Its optimum temperatures lie close to the summer maximum (80), and though perennial, its occurrences at other seasons than late spring and summer are irregular and its numbers few. Its seasonal distribution in German- lakes, as re- ported by Apstein ('96), is similar to that in the Illinois River. The Peridiniidas play but an insignificant part in the plankton of the Illinois River. Phacus longicaudus Ehrbg.* Average number, 61,153; silk, 3 ,03 1 . This species in 1898 made its first appearance in the plankton on March 23 and continued till November 15. The species is small enough to escape through the silk net, and the data from such collections do not fully express its seasonal fluctuations. There is no marked vernal pulse, and there are traces of but a few small ones during the summer, the largest in 1898 being one of 35,200 on September 27. The distribution in previous years is much the same. A well-sus- tained development throughout the warmer months save when rising floods, as that of May, 1898, reduce the numbers indicates that the optimum temperature for the species approaches the summer maximum (80). There are almost no occurrences below 45. This is the most abundant member of the genus in our plank- ton, but it is not quantitatively an important element therein. Phacus pleur one ctes Nitzsch.* Average number, 450,000; silk, 298. It is less abundant (from one fifth to one tenth) than P. longicauda in the catches of the silk net but apparently much more abundant in the filter-paper collections, which may be due in part to its smaller size and greater tendency to escape through the silk in the collections of the net. Its occurrences are even more closely limited to summer months from June till September. There is no vernal development, and the largest numbers occur during the period of maximum heat. Pulses are but feebly defined. It is also a summer planktont. Phacus pyrum Ehrbg. was found but once on August 10, 1897. Phacus triqueter Ehrbg. occurred in small numbers during July and August, 1897. Platydorina caudata Kofoid. Average number, 17. In 1898 this interesting new genus of the Volvo cidoe was found in the plank- ton only in the latter part of July. In 1897 it was much more abun- dant (average number, 21,963) and ranged from July 14 to October 86 12. There was a pulse on July 21 of 18,400 and another on Septem- ber 7 of 600,000. In previous years the occurrences were scattering, but confined to July, August, and early September. It is evidently a summer planktont, whose optimum temperature lies near the max- imum attained by our waters. No record of occurrence below 60 was made. The smaller and younger colonies escape readily through the silk net. Its pulses in 1897 coincide very closely with those of Gonium, Pandorina, Eudorina, and Pleodorina. Pleodorina calif arnica Shaw. Average number, 11. In 1897 this species, in common with other members of the family, was much more abundant than in any other year of our work, stable conditions of low water with the accompanying sewage contamination seeming to favor its development. The earliest record for P. calif ornica in the plankton is May 18, 1896, at 71. This was a year of low^er water and higher temperatures than usual in spring months (Pt. I., PI. X.). In other years P. calif ornica did not appear until June or July. It continues into September, the latest record in 1895 being October 2. In 1897 there were pulses on July 21 (5,600) and September 7 (4,000). The occurrences at other seasons are too scattered to trace the seasonal fluctuations, but there is a well-de- fined predominance during the period of maximum heat. This is evidently a summer planktont, whose optimum temperature lies near 80. Pleodorina illinoisensis Kofoid. Average number, 6,91 7 in 1897. This is somewhat more numerous than the preceding species, and its range of occurrences is quite similar. Its maximum pulse in 1897 (180,000) is on August 31, a week earlier than in other members of the family. These pulses of the VolvocidcB occur (Pt. I., PI. XLIV.) in a depression of nitrates and just prior to the volumetric pulse of September, 1897. This pulse is doubtless built up partly at their expense. Their decline in numbers corresponds with its rise. This is also a summer planktont, and was not recorded below 71. Salping&ca brunnea Stokes.* This species was not recorded in 1898. Average number in 1897, 1,887,356. It occurred on May 25 and July 21, dates of culmination of pulses of Melosira granulata var. spinosa. In August-September a pulse occurs, culminating September 7 at 47,250,000 a week after the culmination of a Mel- osira pulse. In 1896 (silk collections only) it was present through- 87 out most of the summer, attending only approximately the sup- pressed and interrupted pulses of Melosira in that year of disturbed hydrograph. It has been recorded from the latter part of April till the middle of September, and, as a rule, above 60. This beau- tiful little choano flagellate is sessile upon the filaments of Melosira, principally upon the variety spinosa, and but rarely upon M. varians or other planktonts such as Pediastrum. It is often associated with Bicosceca lacustris and is usually found upon the sides of the filaments, the bowl of the transparent brownish lorica being closely sessile upon the diatom. In one instance a lorica was found upon the corner at the end of the filament. The lorica had adapted itself to this novel situation by an angular indentation fitted upon the corner of the diatom. Syncrypta volvox Ehrbg. Average number, 625. This species has a definite and somewhat unusual seasonal distribution. In 1898 it was found from March 1 to April 12, and reappeared Novem- ber 8, attaining a maximum of 13,500 on December 6, and of 43,000 on January 1, declining then to 800 and rising on February 14 to 4,800, and subsequently disappearing in the flood waters of March. It was not recorded in 1897. In 1895 it appeared September 27 and continued for a month, reappearing in February and March, and not occurring after April 10. It has attained its largest develop- 'ment at minimum temperatures under the ice 43,000 January 3, 1899, at 32.7. The greater part of its occurrences in 1898-1899 lie very near this temperature, and but three in all the years lie above 50. It is par excellence a winter planktont, or at least a cold-water one. Its occurrences in 1895-1896 lie near the beginning and the close of the seasonal pulse of Synura. In 1898-1899 the pulses of Syncrypta coincide in location with or immediately follow those of Synura. The resemblance of Syncrypta to small colonies of Synura is striking, and this fact combined with the relation of their seasonal fluctuations raises the query if Syncrypta may not be an encysting stage of the Synura colony. Its life history should be fully worked out. Synura uvella Ehrbg. Average number of colonies, 8,463. The seasonal distribution of this chrysomonad flagellate is some- what similar to that of its near relative Syncrypta. It is a perennial, though predominantly cold-water, planktont. It appears 88 in the December plankton of 1894, but was exterminated from the channel plankton taken in the following February by the stagna- tion attending the long-continued ice blockade. It reappears in April, and again disappears promptly, but does not return until September 12, and not in numbers until October. There are pulses November 20 (506,800) at 42.8, and December 30 (362,520) at 36.5. The December pulse is followed by a decline, with a rise during February to a well-sustained maximum during March, approaching 400,000, and at from 35 to 48. The decline follows in April, and there are only isolated occurrences in small numbers at irregular intervals during the summer. Continuous occurrence begins again in September, and numbers rise rapidly in October. There is a pulse of 542,699 on December 3 at 32.2, and another on March 22, 1897, of 159,500 at 43.8. Synura is very rare indeed in the summer of 1897, and in the prolonged low water, sewage contamination, and higher temperatures of the unusual autumn of that year it does not reappear continuously until October 26, at 59, and does not exceed 1,000 until December 7, at 32. There is a low maximum of 98,700 on December 14 at 36, followed by a decline during the rising flood of January-March, 1898. The slight cessations in the flood invasion (Pt. I., PL XII.) in January and in the second weeks of February and March produce prompt responses in immediate rise in numbers in Synura. Finally, a low maximum of 320,600 is attained upon the crest of the March flood, on the 29th, at 49. This is followed by a decline during April and a few scattered appearances during the summer. Synura returns at the end of October and rapidly mounts to a pulse of 1,999,500 on November 29 at 35 with the first decline of the November overflow (Pt. I., PI. XII.). A second pulse of 2,764,800 on December 20 at 33, under the ice, gives way to a decline to 51,600 towards the end of January, 1899, during rising water. On February 14 another pulse (348,800) appears at 32.5, under heavy ice, and declines again in the sudden flood of the last days of February, but recovers quickly with a maximum pulse of 898,800 on March 7 at 32.8. Within a fortnight this falls to the low level of 9,800, but its further history was not followed. From these data it is evident that in our waters at least Synura is limited to the months from October to April, except isolated and irregular occurrences of small numbers during the summer. Its 89 optimum temperatures lie below 50, and its greatest development has taken place in minimum temperatures under the ice. Rising floods and disturbed hydrographies conditions tend to reduce its mimbers or to suppress its development, while declining floods initiate increase in numbers and favor the appearance of pulses. A "late" autumn delays the appearance of Synura. Not only are colonies of Synura found in the collections, but at times large numbers of free cells make their appearance. These are released by the breaking up of colonies, and occur in all degrees of isolation. It seems to be a natural phenomenon, and occurs most abundantly with or immediately after the crest of the pulse. Thus the pulse of December 29 (1,999,500 colonies) was attended by 21,600,000* free cells on that date. A week later there were 1,693,500 colonies and 57,600,000 free cells. There are in the rec- ords several instances of meteoric increases of free cells at other times than at those of apparent pulses. It does not seem possible from the data at hand to determine whether this is due to environ- mental influences or to the accidents of collection and subsequent handling. In the discussions of Mallomonas and Syncrypta, sugges- tions have been made that these organisms may be stages in the life cycle of Synura. Synura is the largest and by far the most important synthetic organism of the winter plankton. It shares appreciably in the winter volumetric pulses as, for example, those of December, 1898 (Pt. I., PI. XII.). Its fluctuations do not seem to produce any marked effect upon the nitrates, possibly because the latter are present in excess of the needs of Synura. In the winter of 1898 nitrates are high, 1.25 parts per million with the pulse of 1,999,500 colonies on November 29, but decline rapidly to .1 on December 13 with a fall of Synura to 78,000. On December 20, Synura rises to 2,764,800, but the ni- trates rise only to .35. It is evident that the nitrates are not the only factor regulating the fluctuations of Synura. Marsson ('00) reports Synura as abundant in the winter plankton of lakes about Berlin, and Brunnthaler ('00) finds it in the winter plankton of the Danube. There is, however, no recorded instance in which Synura forms so prominent a part of the plankton of a body of water as it does of that of the Illinois River. It may be that a closer analysis than has yet been given the potamoplankton of other streams will reveal its prominence there also. It is present (Kofoid 90 '95) in the summer plankton of the Great Lakes at temperatures 15 to 20 below the summer maximum of the Illinois River. Trachelomonas acuminata Schmarda.* Average number, 1,094,- 615 ; silk, 873. This species appears in the plankton in April or May and continues into October or November. There is no vernal pulse, and the data are too irregular to trace the seasonal fluctuations. The greater numbers occur during the period of maximum heat. Excepting a single occurrence in February, this species has been found only above 40, and its period of continuous appearance from May to October lies above 60. It is evidently a summer plank- tont. Trachelomonas hispida Stein.* Average number, 1,002,115 ; silk, 1,251. This is a perennial organism, found in every month of the year but in larger numbers during the warmer months. It was more abundant than usual in the winter of 1897-98 following the low water and unusual development of the previous fall. There are no large pulses in 1898, but in 1897 there is indication of a vernal max- imum on April 27 and an autumnal one of 85,500,000 on September 7. The data are too irregular to trace the seasonal fluctuations in detail. There is no doubt, however, from the evidence at hand that this is a predominantly warm-water planktont similar to the other members of the genus. Trachelomonas volvocina Ehrbg.* Average number, 17,672,692; silk, 7,162. This is the most abundant species of the genus and is found throughout the year in almost every collection. It is most abundant from May to October, during the period of maximum heat. There are no well-defined vernal or autumnal pulses, but recurrent maxima during the summer are to be found in both 1897 and 1898. There are four such pulses in the former year, and in the latter five, as follows: May 17 at 64 (14,400,000), 'june 21 at 77 (147,600,000), July 19 at 84 (86,400,000), August 9 at 83 (252,000,000), and October 4 at 71.5 (11,700,000). The periods of greatest growth thus lie above 60 and the optimum is near 80. None of these pulses coincides with a volumetric maximum of the silk-net catches (Pt. I., PL XII.). They usually follow these maxima at intervals of one or two weeks a phenomenon often observed in other synthetic species. It may be explained by the decrease in animals which feed upon the organ- isms in question. These volumetric pulses are predominantly 91 animal in their composition, and when they decline the organisms upon which the disappearing animals were feeding have an oppor- tunity to multiply with less decimation in their ranks. This species is one of the most abundant of the synthetic organ- isms in the summer plankton, and next to Euglena is the foremost among the synthetic elements of the food cycle of the plankton. The presence of many light-colored or even colorless forms (forma hyalina Kl.) justifies the suspicion that members of this genus, like those of its near relative Euglena, adopt holozoic nutrition in the presence of abundant organic matter suitable for food. This species, as well as the others above listed, is exceedingly variable in the proportions of the lorica, in its color, and in the development of the neck. It is very desirable that its life history and the full limits of its variation be determined before many more new species are proposed in the genus. In addition to the forms above listed, the following have been noted as present in small numbers in the summer plankton, viz. : T. armata Ehrbg., T. caudata Ehrbg., T. torta Stokes, T. urceolata Stokes, and T. volvocina var. rugulosa Kl. Uroglena americana Calkins. This species was found in small numbers in July and September, 1897, and in January, 1899. Uroglena radiata Calkins. This species was found in January, 1896 ; in April and May, 1897 ; and in March and April, 1898. There was a vernal pulse of 15,279 on April 29, 1896. Uroglena volvox Ehrbg. This species was found sparingly in the spring plankton in 1896. Uroglena is one of the few organisms which the usual method of plankton collection and preservation fails to keep in fair condition for subsequent indentification. The gelatinous matrix is easily crushed, and debris adheres to it so as to obscure it beyond recognition. Judging from the frequency of Uroglena in the living plankton it is very probable that the genus is much more abundantly represented in the Illinois River than the data at hand indicate. The genus seems to prefer the cooler waters of autumn and spring to those of midsummer. Volvox aureus Ehrbg. This species was found from March to August, but in small numbers and irregularly. Volvox globator L. This was somew T hat more abundant than the previous species, and was found more frequently, especially during 92 1895 and 1896. It occurred from the first of May till the end of August, but always in small numbers. It is occasionally abundant in backwaters where there is much vegetation. In addition to the Mastigophora above listed there were many individuals belonging to unidentified species . They were as a rule the smaller forms, which are not readily identified in preserved material and under the conditions of plankton enumeration. They consti- tute about twenty-six per cent, of the total Mastigophora enu- merated. In silty planktons their number is relatively somewhat larger on account of the difficulties attending the determination of species in such material. These unidentified flagellates occur in every collection, and are somewhat more abundant in the summer months. RHIZOPODA. Average number, 55,364, including filter-paper collections; 23,826 without them. This group of Protozoa is numerically of less importance than the ciliates or flagellates, but its quantitative significance is greater than the numbers of individuals indicate. This is due to the relatively large size of the Rhizopoda, and also to the fact that plankton collections afford only an irregular and in- complete record of the rhizopodan fauna of any body of water, and give but an imperfect idea of the part which these organisms play in the total economy of the lake or stream. This results from the fact that they are as a rule largely bottom or shore-loving species, and are generally either adventitious or temporary constituents of the plankton. The seasonal distribution of the total Rhizopoda in the Illinois River gives evidence of the adventitious or temporary nature of the contributions of the group to the plankton. There are pulses in 1898 on January 25 (66,388), February 22 (141,524), August 23 (36,800), September 27 (59,200), and November 15 (42,000), all of which appear on rising water and are largely adventitious, their presence in the plankton being due to the disturbances of currents, waves, and the like. There are pulses on May 10 (49,800), June 28 (37,000), and July 19 (28,800) which cannot be traced to any general hydrographic condition. These, as will be suggested in the discussion of the seasonal fluctuations of individual species, are probably due to the temporary adoption of a limnetic habit on the 93 part of some of the rhizopods,orto the appearance of limnetic forms, varieties, or species according to the systematic value placed upon these eulimnetic individuals. I am inclined myself to regard them as seasonal forms of species which are predominantly of the bottom or littoral fauna, which have multiplied rapidly under the stimulus of abundant food. Owing to this fact, to the storage in their tissues of the products of metabolism, such as gas and oil vacuoles which tend to lighten their specific gravity, and to the frailer structure of their shells under conditions of rapid multiplication, they abandon their customary benthal or littoral habitat and assume temporarily a limnetic distribution in the plankton where they con- tinue to find abundant food. Their appearance here under these circumstances is a result of their physiological condition, and with its cessation they decline, as shown by their pulse-like occurrences. Whatever the systematic valuation placed upon these limnetic forms may be, there is no doubt of their occurrence. They have appeared in every year of our operations, but were most prevalent in 1897, a year of most stable conditions, and also in the quieter backwaters, and on the declining spring flood or June rise when hydro- graphic conditions are less catastrophic than those of early flood stages. In 1897 there was a pulse of 68,400 (silk-net only) on August 8 and another of 1,268,400 on September 7, both in stable conditions and almost exclusively of limnetic types, differing in this respect from the pulse of 141,524 on February 22, 1898, which was pre- dominantly of an adventitious character, resulting from the flood of that period (Pt. I., PI. XII.). The contrast in the numbers of Rhizopoda in the plankton during warm and cold seasons of the year is very striking in 1897. The average per m 3 , per collection from May 1 to October 1, that is, above 60, is 161,045, omitting all filter-paper collections, while in the seven months of lower tempera- tures this average is only 4,771. During the warmer period the June rise was the only hydrographic disturbance (Pt. I., PI. XL) to which any adventitious increase might be attributed. This con- trast is less evident in 1898, when the summer hydrograph was more disturbed. These larger numbers during warmer months may be attributed in part to the greater numbers of the Rhizopoda in their littoral habitat, and in part, doubtless, to the fact that at low water the shore and bottom fauna are brought into more intimate relation with the plankton, and in the river the disturbance of these regions 94 by current, waves, seines, boats, and fish make relatively larger contributions at low-water stages to the diversification of the plankton. In addition to these factors, however, there is abun- dant indication that many individuals assume during the warmer months a eulimnetic habit, and that some of the Rhizopoda become, for the time being at least, typical, though temporary, planktonts. It naturally follows that in so far as the plankton is concerned, the Rhizopoda exhibit a seasonal preference for the warmer months above 60! Maximum numbers were attained only at the higher temperatures save in those instances where they attend winter floods. In a measure the seasonal distribution of the Rhizopoda in the plankton reflects that of the group in its normal habitat ; but at the best the picture is incomplete. The Rhizopoda have important relations in the economy of the plankton. They feed upon diatoms, desmids, the smaller algae, and even the chlorophyll-bearing Mastigophora such as Trachelomonas and Carteria. Their occurrences in the plankton do not exhibit any striking correlation with those of the groups named. The great pulse of September 7, 1897, for example (PI. II.), lies in a depression of the diatoms and coincides with pulses of Chlorophycece and Mastigophora, and that of August 10 (68,400) exhibits a similar relation, the diatoms rising the following week as the Rhizopoda fall. In 1898 the pulse of Rhizopoda on June 28 of 37,000 (Table I.) culminates a fortnight after that of the diatoms and Chlorophycece and a week after that of the Mastigophora. It thus is intercalated between the June and July pulses of these chlorophyll-bearing organisms (PI. II.). The Rhizopoda pulse of July 19 (28,800), on the other hand, occurs with the coincident pulses of the three groups named (PL II.). The immediate diluent effect of flood waters upon the plankton combined with their tendency to increase the number of adventitious Rhizopoda results at times in the intercalation of their pulses with those of the chlorophyll-bearing organisms whose relative numbers are reduced by the dilution. The data evidently do not afford any adequate solution of the inter- calations of the Rhizopoda with other organisms. The Rhizopoda are very frequently found in the digestive tract of limnetic rotifers, but I have never noted the Entomostraca feed- ing upon them. They are important elements in the food of young 95 fish (Forbes, '80) such as the Catostomidce and some of the Silu- rida and minnows. I have found them in great abundance in the intestine of the adult gizzard-shad (Dorosoma), and in the contents of the digestive tract of the German carp (Cyprinus carpio). In the pages which follow, the seasonal distribution, or occur- rence in the plankton, of thirty-one Rhizopoda is discussed, and the presence in the plankton of the Illinois of twenty-eight other rhizopodan forms which have been recognized by other writers as of specific rank is noted. This by no means exhausts the rhizopo- dan fauna of the environment which was the field of this investiga- tion. A continued study of the plankton itself would doubtless greatly extend the list of adventitious forms from the shore and bottom, and a more careful analysis of the variants, especially in the Difflugia globulosa-lobostoma group, would still further increase the richness of the fauna from the systematic point of view. Hem- pel ('99) lists sixteen species from this locality, and Penard ('02), in discussion, remarks: "Une pareille pauvrete dans une region riche en organismes de toute nature, est une impossibilitie mat 6- rielle." However, neither Hempel's paper nor the present one pretends to give a full account of all the Rhizopoda of the region. He dealt largely with plankton collections, and the present paper deals with them exclusively. There is but little in plankton literature which gives with any fulness the seasonal distribution of the Rhizopoda, or indicates that they are of any considerable importance in the economy of the plankton. The importance which they acquired in the plankton of the Illinois is no doubt in part due to the nature of the environ- ment with which we are dealing. The somewhat sporadic and meteoric character of their appearances in our waters leads to the inference that full seasonal analyses of the plankton of other bodies of water at brief intervals may reveal a greater prevalence of the Rhizopoda in the plankton than has hitherto been detected. DISCUSSION OF SPECIES OF RHIZOPODA. Amoeba Umax Duj. This was frequently abundant in the water- bloom of midsummer, but was not identified in the plankton collections. Amoeba proteus Rosel. Average number, 342. The individuals here assigned to A. proteus include those taken in our plank- 96 ton which belong to the type of A. radiosa Ehrbg., a type which presents no distinctions sufficiently well-defined to separate it spe- cifically from the first-named form. It seems probable that A. radiosa includes small individuals of A. proteus which are not, at the time of observation, creeping upon a substratum ; that is, they are limnetic, floating free with filamentous pseudopodia character- istic of that condition. Verworn ('97) has shown that A. proteus takes the radiosa form in weakly alkaline solutions. Pond water rich in algae may have an alkaline reaction (Knauthe, '98) in bright sunlight. Larger individuals, distinctly referable to the A. proteus type when taken in the plankton, possess at times the slender pseu- dopodia of the A. radiosa type as well as the blunter ones charac- teristic of the A. proteus form. I see no valid reason for separating the two as distinct species. Most of the Amoeba recorded from the plankton collections belong to the A. proteus type, the smaller ones belonging to the radiosa type probably escaping through the meshes of the silk net. This species w T as found in 30 of the 180 collections examined, being observed in all months of the year except May, November, and December. The conditions attending its occurrence suggest that it is not, habitually at least, an active planktont at all seasons of its occurrence, but rather a tycholimnetic member, an invader from the littoral or bottom fauna, or a temporary accession during the warmer months. In the first place, both the number of occur- rences and the numbers of individuals found are small, and the seasonal distribution, plotted from the data of the collections of the five years, is exceedingly irregular. Furthermore, 17 of the 30 occurrences happened on rising floods, when the fauna of the bot- tom and shore of both the river and its tributaries is most mingled with the plankton. Further evidence of the agency of floods in introducing Amceba into the plankton is brought to light by a com- parison of its occurrences in 1897 and 1898. As shown by Plates XL and XII. , Part I., the hydrograph of 1897 is much less irregular than that of 1898, the latter year exhibiting repeated fluctuations in level due to floods. As a result we find Amoeba occurring rela- tively (to the number of collections) almost twice as often in 1898 as it did in 1897. It may also be significant that Amceba was not found in November and December, months of unusual stability in river levels. There is, however, a suggestion in the data of distri- 97 bution (see Table I.) that Amoeba may become an active member of the plankton during the warmer seasons, like other Rhizopoda, as a result, perhaps, of the formation of gas or oil vacuoles in its proto-' plasm. Of the 30 occurrences, 21 fall between ApriLlS and Octo- ber 17, with water temperatures of 58 and 56, respectively. Of these 21 occurrences in warm waters but 8 accompany flood inva- sions, while all of the 9 occurrences during the colder months are in connection with such disturbances. Finally, the maximum num- ber per cubic meter (6,400) was found July 21 in clear waters, free from the debris of flood invasion. In conclusion, it seems probable that Amoeba in warmer seasons of the year (above 56) may adopt a limnetic habit. There is, however, the possibility that local and minor disturbances of the water due to current, waves, etc., are the occasion of its presence in the plankton in the absence of flood conditions. Jennings ('OOa) reports both A. proteus and A. radiosa in the open water of Lake Erie. The range of temperature of river water in which Amoeba was found was from 32 to 89 the full extremes observed by us in the river at Havana. The temperature at the maximum occur- rence, July 21, 1897, was 82. It is perhaps significant that 14 of the 30 occurrences of Amoeba were between June 21 and Sep- tember 6, the period of maximum heat, the river averaging almost 80 apparently the optimum temperature for the occur- rence of Amoeba in the plankton in this locality. The relative numbers of individuals found in the various collections of the five years are too irregular to suggest any conclusions as to a seasonal cycle. Amoeba verrucosa Ehrbg. Average number, 19. This species was found but three times in the plankton, once each in May, August, and September, occurring but singly, and in each case in flood waters. It is apparently a tycholimnetic member of the plank- ton. The temperature limits of its recorded occurrence in the plankton were 5.8 and 82 respectively. Arcella. This genus is represented in the plankton by four species and two varieties which, like most of the Rhizopoda, are exceedingly variable, grading in some instances into each other by occasional 98 individuals which present intermediate characters. The majority of the individuals were taken in a living condition, though many empty shells were found. The conditions of the examination of the plankton and the opacity of many of the shells made it impossible to distinguish the dead shells in all cases. The records include many dead shells. Arcella costata Ehrbg. Average number, 48. For the purposes of this paper I have included here all those individuals which possess an angular or ribbed shell. Leidy ('79) refers such forms to A. vulgaris. Individuals of this type are rare, occurring infrequently and in small numbers. It was recorded but 18 times in the 180 collections, and the largest number per cubic meter was only 1,187. As in the other species of the genus, the warmer months are favored, fourteen occurrences falling in June-September in water at 70 or above. The other four records are one each in April, October, November, and December. The seasonal range of this form in the plankton thus falls in the main within the period of the maximum abundance of A. vulgaris, of which species it may be but a variant. Arcella discoides Ehrbg. Average number, 972. This prevalent species is not in all instances easily separated from A. vulgaris. Indeed, even Leidy ('79) states that it graduates into A. vulgaris, and that he views it as the variety of this species in "which the shell presents a greater proportionate reduction in height compared with the breadth." In the enumeration of our plankton catches, the larger, flatter, and unornamented individuals have been referred to this species. Both the brownish and the hyaline forms should probably, for reasons hereafter given, be included here, and they are so grouped in the present discussion. Thus considered, A. discoides is the most abundant member of the river plankton be- longing to this genus, including two thirds of all the individuals observed. This species occurred in almost two thirds of the collections, hav- ing been recorded in 115 of the 180, and more frequently and in larger numbers in the latter half of the five years than it was in the earlier period. This is in part explained by the unusual fluctua- tions of the river levels in 1898, during the maximum summer occurrence of the species. Like the other species of the genus, A. discoides has a period of maximum occurrence in the latter part of summer, as is shown in Table I. Of the 115 occurrences, 55 were in 99 June-September, in water at or above 70, while in the remaining eight months there were but 60 occurrences. This contrast is heightened by the ratio of occurrences to the total number of collec- tions, which in the period from June to September inclusive is 55 to 68 and in the remainder of the year only 60 to It2. - The num- ber per cubic meter is also higher during this warm period, averag- ing for a single occurrence 1,376 to 1,028 for one in the remainder of the year. The average for the colder months falls to 850 if the large accessions attending the floods of February and November are omitted in the totals. The same causes efficient in determining the summer maximum in other Rhizopoda of the plankton are doubtless operative here, and as in A. vulgaris the impetus of the summer in- crease is carried over into the autumn, causing a slight increase in numbers as compared with the numbers at corresponding temper- atures in the spring months. It seems probable that high temper- atures favor its occurrence in the plankton, not, however, directly, but because of greater abundance of food under those conditions, greater metabolism, and the storage of the products as oil or gas vacuoles which tend to lower the specific gravity and thus to bring the animal into the plankton. The adventitious occurrence of A. discoides in the plankton is shown by the fact that 45 of the 115 occurrences are with rising flood waters. The greater part of them lie in the colder months; in fact, nine tenths of the occurrences between October and May are correlated with flood movements. For reasons above given, how- ever, A. discoides may be regarded as temporarily adopting a lim- netic habit during warm months as a result of its physiological condition ; at least many individuals of the species exhibit this habit during the warmer months. The data do not indicate that the open water is at any time the center of distribution of the species. There are no indications of recurrent pulses in the species and, as might be expected in case of adventitious planktonts, but little evidence of a characteristic seasonal distribution. There is some evidence that the summer is the period of most active multiplica- tion, and that an exceedingly transparent and hyaline form other- wise resembling A. discoides is the young of this species. In 1898 . separate records were kept of the two types with the result that they were about equally abundant 24,159 and 26,387 for the brown and hyaline types respectively. (8) 100 With but few exceptions the seasonal distribution exhibited by the hyaline form was very similar in time and numbers to that of the brown form. Both occurred more frequently and in larger numbers in the warmer months, and irregularly and in small numbers in the colder waters. Both entered in larger numbers with flood waters. The differences though slight are suggestive. The hyaline form was less frequent than the brown both in occurrences and numbers dur- ing cold weather, and summer floods sometimes brought a rela- tively larger number of the hyaline type. These are conditions that might be expected if the latter is only the young (that is, the daughter organism occupying the new shell after fission of the oc- cupant of the old) of Arcella discoides. In warmer months food is more abundant and, presumably, fission more frequent. For this reason the young individuals abound at that time. Owing to the difference in the specific gravity of the two, the hyaline type is more readily transported by flood waters. Though not con- clusive, the data here presented seem to favor the view that the hyaline form is only a stage in the life history of the individual Arcella discoides. The species A. artocrea Leidy and A. polypora Penard occur also in our waters, but were included with A. discoides in the enumera- tion. Typical representatives of these species are not, however, present in any numbers Arcella mitrata Leidy was found but once on Aug. 1, 1895, in small numbers, at 78.5. Arcella stellata Perty. Under this designation are included only those individuals which have well-defined prolongations on the margin of the shell. Only a single occurrence in small numbers (48 per cubic meter) was recorded for the typical A . stellata July 29, 1895, at a temperature of 75.5. Arcella vulgaris Ehrbg. Average number, 1,098. This species is somewhat more abundant than A . discoides, but occurred in fewer collections. It is a somewhat common planktont, whose seasonal distribution exhibits some irregularities attributable in part, as in the case of other members of the genus, to flood conditions. It was found in 61 of the 180 collections examined, and in approximately one third of those made in each year, excepting in 1894, when it was not recorded, and in 1898, in which year it was found in about 101 half the collections, the river levels for this latter year being subject to more than the usual disturbance. Arcella vulgaris is found throughout the whole year, with a marked predominance of occurrences during the warmer months, June to September inclusive, for during this period, irr which a total of 68 collections were made, this species was found in the plankton 34 times. If the month of October be included, the ratio is 44 oc- currences in 83 collections, while in the remaining 97 collections, from November to June, only 17 occurrences were recorded. Of the 10 occurrences in October, 7 were in water at or above 55. The season of frequency in the plankton thus ranges from June through October. In both frequency of occurrence and in numbers of individuals (see Table I.) there is an apparent maximum in August, preceded by an increase in June and July and followed by a decline in September and October. Arcella vulgaris thus seems to be a late summer planktont. The continuance into October may in part be due to the temperature conditions above cited, and perhaps also to constant seining of the river by fishermen in the low- water stages at that time, causing repeated disturbances of the bottom and shores, where Arcella habitually lives. This maximum frequency of Arcella during the warmer months in the plankton is, however, probably due to the formation of gas or oil vacuoles in the plasma under the conditions of higher temperatures. Their flotation is thus facilitated, and they become, in a way, semi-active but temporary planktonts. That floods are also in part responsible for the presence of Arcella in the plankton is evident from the fact that 32 of the 61 occurrences come with rapidly rising waters, or shortly after rapid rises, during the interval of rapid decline. The larger numbers of individuals also appear in flood- waters, occurrences of more than 1000 per cubic meter happening 10 times with floods to only 4 in more stable conditions. The maximum occurrence, 25,272 per cubic meter, came with the flood of February, 1898, indicating the presence of this species in large numbers, even under winter conditions, in some local environment tributary to the flood plankton. The average number per cubic meter in the 61 collections con- taining Arcella was 1,260; and the maximum, 25, 272, as above noted. This species occurred in only 10 collections in stable conditions of the river, when the temperature of the water was below 55. The 102 average number of individuals in these cases was, however, only 230 per cubic meter as against 1,443 when the temperature was above 55, or, if below, when floods prevailed. The seasonal and numerical distribution of occurrences and individuals alike point to the agency of floods and higher temperatures in the introduction of Arcella into the plankton from its usual habitat, the bottom and the shore. This species occurred in water ranging in temperature from 32 to 89. Being a bottom form, the plankton data do not afford a satisfactory basis for determining its true seasonal distribution and optimum temperature. The maximum number found, 25,272, was in water at 32; but this was an isolated occurrence in a flood, and serves only to illustrate the irregularity of distribution in the plankton of tycholimnetic organisms. Centropyxis aculeaca Stein. Average number, 570. This species has appeared in collections in every month of the year, but its sequence is frequently interrupted and its numbers are quite irregu- lar. Practically without exception all the larger occurrences attend rising flood waters. It is evidently adventitious at all seasons of the year. Centropyxis aculeata var. ecornis (Ehrbg.) Leidy. Average number, 604. In former years this species was less frequent than the preceding species. Its appearances in the plankton tend to coincide with those of C. aculeata (Table I.), and are doubtless due to the same causes. Thus in the February flood of 1898 there is a pulse of 12,636 of C. aculeata and one of 9,477 of var. ecornis. C. l&vigata Penard seems to be identical with this variety. The data concerning both C. aculeata and its variety ecornis are too irregular to throw any light on the seasonal cycle of these adventi- tious planktonts. Cochliopodium bi limbo sum (Auerbach) Leidy. Average number, 1,384. This species was found in the plankton during 1898 in irregular numbers in 27 of the 52 collections. The distribution of the occurrences affoids indubitable proof of their close dependence upon flood waters. In 15 of the 27 cases Cochliopodium appeared with a rising river, and in all but 6 cases, in periods of considerable movement in river levels (cf. Table I. with PL XII., Pt. I.), such as the rising flood of January and February and the repeated minor 103 fluctuations of August and the following months. The year 1898 was one of unusual irregularity in the hydrograph (Pt. I., PL XII.), especially at the lower stages of the river, at which times this rhizopod appeared most frequently. Its maximum occurrence, 20,898 per cubic meter on Jan. 25, accompanied a rise" of 0.6 of a foot in 24 hours. At other times the numbers range from 100 to 8,000 per cubic meter, their irregularity affording additional ground for regarding this species as an adventitious planktont. Cochlio podium was present in water ranging from 32.1 to 89, the maximum number observed being found in water almost at the freezing point, when the river was full of running ice. That this is the optimum temperature for this organism is not, however, to be inferred, since, as has been shown above, this species is adventi- tious in the plankton. Plankton collections do not afford adequate data for determining the seasonal cycle of the organisms habitually living upon the bottom. This species was not found, though careful search was made for it, in the winter collections of 1899. Its absence from the records of years previous to 1898 may in part be due to a failure to observe it in the silt-polluted collections in which it is most apt to occur. Cyphoderia margaritacea Ehrbg. Average number, 198. This species has occurred in every month but February. In 1898, the majority of the occurrences and three fourths of the numbers ap- peared between May 1 and October 1 at temperatures above 60. It was never abundant at any time, though there is this indication of its increased numbers during the warmer season. It is not an im- portant element in our plankton. Apstein ('96) found it somewhat irregularly in the plankton of German lakes. In our waters it exhibits no marked dependency upon floods for its presence in the plankton, though it is probably capable of assuming the limnetic habit in the warmer season. Cyphoderia trochus Penard appeared occasionally with the pre- ceding form, from which it is distinguished by its conical horn on the fundus and by its larger scales. Difflugia. This genus is the most abundant one of the Rhizopoda in the plankton of the Illinois River, and is a factor of quantitative 104 importance in its economy. It includes a number of forms notorious for their variability and for the difficulty with which specific dis- tinctions can be applied. I shall discuss the species as they were enumerated, and shall correlate my work with Penard's ('02) recent elaborate analysis of the species so far as I can with the aid of my notes in the absence of the collections. Opinion as to the validity of the species is expressly withheld excepting in those instances in which it is formally stated. Difflugia acuminata Ehrbg. Average number, 315. This spe- cies has occurred in every month of the year and in 83 out of 180 collections. In 1898, two thirds of the occurrences and three fourths of the individuals were taken between May 1 and October 30, at temperatures above 70. In this year there are six recurrent pulses from June to November, but all but one of these are found on rapidly rising flood waters, and they bear no constant relation to the pulses of diatoms previously noted, with which in some instances they are intercalated, though this is not regular or constant. Similar tendencies to appear with floods and in greater numbers and more frequently in summer can be detected in records of other years. It was more than twice as abundant in 1896 a year of interrupted hydrograph (Pt. I., PI. X.) as in 1898. This is one of the larger and heavier rhizopods, and its occurrence in the plankton is doubt- less adventitious, due to floods and currents, and its greater numbers and frequency in the summer may result from its greater abundance at that season in its natural habitat, the shore and bottom, and perhaps, also, from its lighter specific gravity during the warmer season. An illustration of this appears on the rising flood of June, 1897, when the maximum number recorded (10,000 per m. 3 ) oc- curred. The shell of this species is exceedingly variable in size, constitu- ent particles, and proportions. A number of forms separated by Penard ('02) and others as distinct species were grouped under D. acuminata in the enumeration. The greater number of these belong to the type designated by this name by Penard ('02). D. acuminata var. inflata Penard and the somewhat similar D. elegans Penard are not uncommon. D. acuminata var. umbilicata Penard, D. elegans var. teres Penard, D. curmcaulis Penard, D. lance olata Penard, and D. scalpellum Penard occur also, but are rare. 105 Difflugia bicuspidata Rhumbler. Average number, 76. A sep- arate record was kept of this bicuspid type in the later years of our collections. Penard ('02) regards it as a synonym of his D. elegans, though it would seem to be as worthy of specific distinction as many other variants to which he accords this raak_ It varies greatly in the relative development of the accessory "horn," which is sometimes but a mere elevation near the base of the main horn. Individuals with equal and symmetrical horns represent the other extreme. In a few cases tricuspid individuals have been seen, evidencing a tendency to vary towards the type found in D. varians Penard and D. fragosa Hempel. This form was about one fourth as abundant as D. acuminata, and eight of the ten occurrences fall between May and October, usu- ally with D. acuminata and presumably for the same reasons. Difflugiq constricta Ehrbg. Average number, 46. This species occurs irregularly at all seasons of the year without marked prefer- ence for the warmer months, and often, but not always, with flood waters. It occurs throughout the whole range of temperatures, and the largest number (2,778 per m. 3 ) appeared during the decline of the spring flood. Data are too infrequent to establish any seasonal routine. This species varies greatly, and is connected by an unbroken series of variants with the genus Centropyxis. Penard ('02) also notes the existence of this connection, and states that after careful search he was unable to find any constant distinction w r hich would suffice for its separation. In my enumeration only the elongated and smooth individuals were referred to this species. The spinose forms were referred to Centropyxis aculeata, and tho.se similar in form to the spinose type ; but those free from spines, to C. aculeata var. ecornis. Difflugia corona Wallich. Average number, 36. In 1896, when the hydrograph was much disturbed, the average number was more than twice as great. This superb species was found in every month of the year except December, but never in large numbers. Its large size (200-300 fi), and its heavy shell militate against its pres- ence in the plankton, and its occurrences are irregular and its num- bers few. There is no marked preference for warmer months, and four fifths of its occurrences are in rising flood waters. It is plainly 106 an adventitious planktont. The data are too irregular to trace its seasonal distribution. As a species it is as well defined as any in the genus. It is not in our waters connected by intermediate forms with other species. Its assignment to D. lobostoma by Schewiakoff ('93) is not in my opinion justifiable unless we regard all forms of Difflugia as belonging to one species. Difflugia fragosa Hempel. Average number, 25; in 1896 over 100. This species occurred in every month of the year but Febru- ary, though three fifths of the records and the majority of the in- dividuals were found between May and October at temperatures above 60. The data are too irregular to trace the seasonal history of the organism, but they suffice to suggest the agency of floods at all times and of high temperatures during the summer, as factors in the occurrence of the species in the plankton. The shell of this form is relatively to that of other species rather heavy, and this fact combined with the irregularity of its occurrence seems to justify the conclusion that it is largely adventitious at all seasons of the year. The species exhibits a great deal of variation in the development of the central spine Hempel ('99, Fig. 1) and in the number and arrangement of spines in the accessory circlet. The mammillate form of the central spine figured by Hempel is not usually present. Individuals in which the central spine is but feebly developed seem to connect this species with D. varians, recently described by Penard ('02). Otherwise, and in our waters, the species is well delimited. Difflugia globulosa Duj. Average number. 7,194; in 1897, 47,329, the larger number in this year being in part due to a remark- able pulse of 1,240,000 early in September. This is the most abundant of all the rhizopods in our plankton, occurring most frequently and in largest numbers. It is found in every month of the year, and in 1898 appeared in every collection except four in De- cember. With a few exceptions in the autumn of 1898 (Table I.), no large development (exceeding 10,000 per m. 3 ) has taken place earlier than May or later than September that is, at temperatures below 60. The occurrences are most continuous and the numbers of individuals are largest during the warmer period between the months named. The largest pulse, that of 1,240,000 on September 107 7, 1897, was at 80. A pulse of 48,000 on November 22 at 40 gives evidence of considerable range in adaptation to temperatures. In Table I. the seasonal distribution of D. globulosa is given in full. It differs from that of previous years mainly in the fact that the summer pulses do not here have the amplitude reached in other years; for example, in 1896 (252,000) and 1897 (1,240,000). It is characterized by considerable irregularity caused by somewhat abrupt pulses at irregular intervals. A comparison of these occur- rences with the hydrographic conditions (Pt. I., PL XII.) indicates that in the colder months increase in numbers in the plankton at- tends flood waters only, as, for example, in January, February, late October, and November. In the summer, pulses may also come with floods. For example, that of 252,000 on May 25, 1896, ap- peared on the upward slope of the June rise of the year, and that of 80,000 on June 28, 1897, came with the belated June rise of that year. On the other hand, some of the minor fluctuations appear on declining floods, and the maximum one of our records, that of Sept. 7, 1897, came in the midst of the most prolonged period of stable low water (Pt. I., PL XI.) found in the six years of our operations. From these facts it is evident that floods are efficient in increasing the number of D. globulosa in the plankton, and that the amplitude of the pulses to which they contribute is much greater in the warmer months (above 60) than in the colder ones as a result, perhaps, of the greater numbers present in their normal habitat, the shores and bottom, and also as a result of their readier flotation at this season. In so far as their presence is due to floods they are adventitious. On the other hand, it is very probable that they become temporarily eulimnetic in habit during the summer months. The evidence for this lies in their greater numbers in a period which is predominantly one of greater stability. Thus in 1898, in the 22 collections between May 1 and October 1, the average number present is 9,731, while in the remaining seven months of colder weather the number is only 5 ,200. Additional evidence arises from the fact that pulses of unusual magnitude have occurred quite independently of any factor such as flood or other disturbance which might cause their adventitious introduction into the plankton. Thus on Sept. 7, 1897, there is a symmetrical pulse whose rise and decline occupy four weeks, as shown in the following table. The total change in river levels in this period of four weeks (Pt. I., PL 108 Date Number per m. 3 Turbidity (in meters) Silt (in cm. 3 ) Stage of river above low water August 24 4,800 .37 .15 1.8 August 31 112,000 .33 .19 1.8 September 7 1 240,000 .15 .45 1.8 September 14 106,000 .33 1.04 2.0 September 21 800 .35 trace 2.0 XL) was only a fall of .1 and a rise of .2 of a foot changes due to wind and the operation of the locks in the dams at either end of the pool. The estimated percentage of silt is near the minimum from a trace to 5 per cent. and the turbidity was no greater than is customary (Pt. I., Table III.) in our waters during periods of abun- dant plankton such as this (Pt. I., PI. XI.). Beyond the presence of these rhizopods there was nothing in the plankton to suggest that the bottom had been stirred up any more than usual. No environmental factor is apparent to which we can attribute this wave of Difflugia in the plankton. It is due, I believe, to their own physiological condition. This was a time of prolonged low water and great sewage contamination, and of remarkable development of water-bloom, chlorophyll-bearing flagellates, unicellular algae, and some diatoms, all elements in the food of Difflugia. In the open water Difflugia could find abundant sustenance and thus maintain itself there. It is not strange, then, that we find it in these warm waters, richly charged with its food, assuming for the time a eulim- netic habit, perhaps as a result of rapid growth and lighter shells, and of increased metabolism with reserve products which lighten the specific gravity and so facilitate flotation. This species is found throughout the whole range of temperatures,. There are indications that its optimum lies above 60, and perhaps near the maximum, 80. This may, however, be the result of the effect of temperature upon the food supply of the organism. In any case the plankton data can not suffice to follow the complete seasonal cycle of an organism which is either an adventitious or but a tem- porary constituent. 109 The question of specific limits and variation in this organism is one of exceeding difficulty, and I see no satisfactory solution for it until some one attacks the problem by a study of the variation by modern quantitative methods, and endeavors by breeding under control to establish the limits of variation within the^iormal range of seasonal changes of the environment. When this is done, some more satisfactory criterion for species in this group of planktonts will be feasible than the present condition affords, in which slight differences from previous descriptions are held to be valid for specific distinctions. Thus, in recent years, species of plankton Difflugia have been described by Heuscher ('85) (D. urceolata var. helvetica) from Swiss lakes ; by Zacharias ('97) (D. hydrostatica) from Lake Plon ; by Garbini ('98) (D. cydotellind) from Italian lakes; by Levander ('00) (D. lobostoma var. limneticd) from Finnish waters; and by Min- kiewitsch ('98) (D. planktonicd) from Russian waters. All of these forms occur in the Illinois River, and there are others equally worthy of specific designation in our plankton as yet undescribed. They occur most abundantly at the times of the pulses, especially of those in stable conditions. In my opinion they are all mere limnetic varieties of D. globulosa or D. lobostoma, the form of the shell and its constituent particles being modified by the habit of life in which these individuals of the seasonal cycle are found. They occur at times of abundant food, rapid multiplication, and limnetic environ- ment. Their shells are accordingly lighter, more chitinous and transparent, and the foreign particles adherent to them partake of the nature of those of the silt in suspension. This, however, is merely an opinion based upon an examination of the statistics of occurrences, and upon the work of plankton enumeration in which all individuals must be assigned to some species. This is at least a different point of view from that of the systematist, who may, per- haps, lay more stress upon divergences from described types and less upon links connecting such variants. For the sake of genuine progress in the science it would seem to the writer extremely desir- able that more attention be given to the question of variation and less to the description of new species under criteria now in vogue. It may be desirable, indeed necessary, to distinguish such forms in the plankton. It would be both safe and conservative to designate them as forms, or, at the most, as varieties. 110 The location of the pulses of D. globulosa bears no constant rela- tion to those of other organisms, owing, in part, at least, to the irregularities of the floods upon which some of them seem to depend. The great pulse of Sept. 7, 1897, is intercalated between two pulses of diatoms and other chlorophyll-bearing organisms, and some others bear a similar relation to their food supply, while some co- incide with an increase in these synthetic organisms (cf. Table I. and PL II.). Difflugia globulosa and the following species were reported by Smith ('94) in the plankton of Lake St. Clair; by Jennings ('OOa) in that of Lake Erie; and were common in the plankton of Lake Michigan (Kofoid '95). Difflugia of the forms included here under D. globulosa and D. lobostoma have been reported by many authors from various European lakes and rivers, but in no reported instance do they reach the numbers or importance in the plankton that they do in the Illinois. Full records of their seasonal distribution may, however, bring such importance to light. Difflugia lobostoma Leidy. Average number, 1,158. In the total of all collections it is about one fifth as abundant as D. globu- losa. Like that species it occurs throughout the whole year in almost every collection (Table I.), and the fluctuations in its occur- rence follow very closely those just described for D. globulosa in the direction of their movement. The amplitude of the pulses is less, as a rule, and their culminations and limits are coincident, or at least approximate. Thus, on Sept. 7, 1897, D. lobostoma attains only 24,000, and the pulse of D. globulosa on June 28 (80,000) is attended by one of 96,000 in D. lobostoma in the next collection, on July 14. There are in this species also the same influx into the plankton with floods, and increase in numbers at temperatures above 60. There are 954 per collection per cubic meter below this temperature to 1,436 during the warmer months in 1898. There are also pulses during the warmer months, in stable conditions, coincident with those of D. globulosa. Similar causes presumably contribute to these results in both species. Difflugia lobostoma is also exceedingly variable in proportions, in the texture of the shell and the degree of incision, and in the num- ber of lobes about the mouth. Two, three, and even four have been noted, and they vary greatly in depth, in regularity, in perfection of their development, and in the structural border which sometimes Ill forms their margin. Chitinous, brownish, or more or less trans- parent shells are abundant when pulses occur. Forms which connect this species with D. globulosa have been observed. In- cluded with D. lobostoma are forms which have since been described by Penard ('02) as D. gramen, D. gramen var. acETom, and D. lithopUtes, though I have not found in the Illinois plankton any of the last-named with the peculiar tipped horns found by Penard upon many individuals of his species. Difflugia pristis Penard (?). A small Difflugia was found occa- sionally in the filter-paper collections in the colder months, but only from November to March. It was often dark, or even blackish, resembling in this respect Penard's D. pristis. Individuals not thus darkened approach more nearly D. fallax Penard and D. puleoc Penard. Difflugia pyriformis Perty. Average number, 368. This species occurred in every month except January, but generally in small numbers and irregularly. The largest number taken 12,000, on May 25, 1896 came with the flood at that time (Pt. I., PI. X.), and all the large occurrences of 1898 came with rapidly rising water (cf. Table I: and Pt. I., PI. XII.). There are no indications of pulses during stable conditions, and we must conclude that the species is purely adventitious in our plankton. It is one of the largest species with a heavy shell, and its flotation is impeded thereby. This species is exceedingly variable. The following varieties or variants, given specific rank by some writers, have been noted, and are included with D. pyriformis in the enumeration: D. pyriformis var. nodosa Leidy, D. pyriformis var. daviformis Penard, D. pyriformis var. venusta Penard, and D. pyriformis var. lacustris Penard. A more slender and smoothly contoured form than the last is not uncommon. D. capreolata Penard and D. bacillifera Penard were also found , but are rare. Difflugia rubescens Penard was taken but once on May 25, 1896. Difflugia tuberculosa Hempel was also found but once in the planktons enumerated, though Hempel ('99) reports it as appearing occasionally from August to November in 1895. Difflugia urceolata Carter was taken only in April and May, 1896, in small numbers at temperatures of 66- 80. 112 Dinamcsba mirabilis Leidy was found in the plankton but once Apr. 12, 1898, in small numbers, at 52. Euglypha alveolata Duj. was found in small numbers in the plankton, but only on Nov. 1, 1898, and March 14, 1899, at tempera- tures of 45 and 36. Euglypha ciliata Ehrbg. appeared in the filter-paper collections in 1897, in July, August, and November, in small numbers at tem- peratures ranging from 80 to 48. This is said by Penard ('02) to be predominantly a sphagnum species, but widely distributed elsewhere in small numbers. Euglypha Icevis Perty. This minute rhizopod was found in the filter-paper collection of Oct. 4, 1898, at 72. Nebela collaris Leidy was found only once on June 25, 1898, at 32. Pontigulasia incisa Rhumbler. This curious rhizopod occurred in the plankton in July and August, 1895, and again in August and September, 1897, at temperatures of 75- 85. Both occurrences were in stable conditions, and the temporary adoption of the lim- netic habit is suggested by their appearance at these times. Two other records in 1897 on March 22 and November 9, at 44 and 50 extend the seasonal range of the species. These occurrences attended rising water and were apparently adventitious. Trinema enchelys (Ehrbg.) Leidy. Average number, 158. This little cosmopolite rhizopod of the sphagnum fauna was found but eight times in the plankton. The individuals observed were all dark- ened by the granular food vacuoles to such a degree that structural details were obscured. It was noted only in the somewhat turbu- lent years of 1898 and 1899, though on account of its small size and the obscurity of its structure it may have been overlooked in previ- ous collections. The few occurrences are insufficient to establish any seasonal routine. They were at both extremes of the tempera- ture range and in all seasons but spring, with a predominance i 'n late summer and fall. The species is evidently adventitious in the plankton, as shown by irregular distribution and small numbers, and by the fact that its occurrences coincide in all instances but one with rising water. 113 HELIOZOA. The Heliozoa of the plankton of the Illinois are few both in number of species and of indiyiduals. They apparently play but a small part in the economy of the plankton. The average number for 1898 was but 4,883. Their occurrences are confined -in ^the main to midsummer and early autumn. But four species were identified, though several others remain undetermined for lack of sufficient material, especially of the living forms. Apstein ('96) reports Heliozoa in considerable numbers in German lakes, with maxima in July- August. It is probable that these delicate forms are fre- quently crushed in manipulation or hidden in silt in our collections. DISCUSSION OF SPECIES OF HELIOZOA. -Actinophrys sol Ehrbg. Average number, 62. This species occurred irregularly from April to the early part of November at temperatures above 46. It was recorded most frequently in the latter part of the summer, the largest number (28,000) appearing Sept. 7, 1897, at 80. Actinosph&rium eichhornii (Ehrbg.) Stein. Recorded a few times, from July to October, at maximum temperatures (75- 80), but always in small numbers. Endophrys rotatoriorum Przesm. This heliozoan (?) has been recently described by Przesmycki ( '01) as parasitic, during a part of its existence, in Philodina and Hydatina. A parasite resembling this parasitic stage of Endophrys was observed by me in a bdelloid rotifer (Rotifer tardus) on several occasions, but it was never abun- dant, nor was its connection with any free-swimming condition noted. The heliozoan affinities of this organism seem very ques- tionable. Nuclearia delicatula Cienk. Average number, 4,760. This species in 1898 appeared first on June 21, attained a pulse of 78,400 on August 9 at 82 and another abrupt one of 65,600 on September 27 at 73, and made its last appearance October 25 at 48. Occur- rences in previous years are confined to midsummer. Its optimum conditions of temperature obviously lie near the summer maximum, and its lower limits near 50. Its appearance in the plankton is not traceable to flood conditions, and it is apparently eulimnetic in our waters. 114 Hempel ( '99) reports Raphidiophrys pallida Ehrbg. and R. elegans Hertwig and Less, in the plankton of Quiver Lake adjoining the river, and I have found an undetermined species of Acanthocystis and a small heliozoan resembling Nuclearia in the river plankton. SPOROZOA. Triactinomyxon sp. In the plankton collections of each year there have been found free limnetic spores which unquestionably belong to that highly aberrant and peculiar group of organisms described by Stole ('99) as Actinomyxidia and regarded by him as Mesozoa, but later referred by Mrazek ('00) Caullery and Mesnil ('04), and Leger ('04) to the Myxosporidia. The organisms de- scribed by Stole were parasitic in fresh-w r ater oligochaetes, and it is not improbable that the limnetic spores taken in our plankton collections are derived from parasites in some of the numerous aquatic oligochaetes, or other invertebrates, found along the bottom and shores of the stream. The species here referred to Triactinomyxon differs in some details from T. ignotum Stole. It was found in the course of the six years at least once in every month of the year, but most regularly in May-September, and rarely and in small numbers in the colder months. Its transparency and long, slender, radiating, tripod-like arms give it a typically limnetic habit. Actinomyxidia, gen. et sp. indet. Clusters of eight, or less, cylindrical spores radiating from a common center and bearing a marked resemblance in structural features to those of Triactinomyx- on, but lacking any anchor-like projections, were found sparingly in the plankton in June-September. The distinctively limnetic habit of these spore stages in the life- history of these parasites is unique among the Sporozoa, and has not, to my knowledge, been before noted. Many of the rotifers of the summer plankton, especially Brachi- onus and an occasional Asplanchna, have been heavily parasitized internally by small sac-like bodies, often pear-shaped, with the smaller end attached to the lorica, or of spherical or flattened form. They occur in such numbers at times as to be a menace to the rotifer population. They are usually most abundant in any given species at the time of, or subsequent to, its maximum occurrence. It 115 was not unusual to find as high as ten or fifteen per cent, of the individuals parasitized, and a number of empty loricas bearing addi- tional testimony to their destructive agency. Bertram ('92) describes these structures as " parasitische Schlauche" in the body cavity of rotifers, and Przesmycki ('01) works out their life history, and describes the organisms as Dimoe- rium hyalinum, but does not designate their systematic position or affinities. There are, however, marked suggestions of sporozoan affinities in the organism found in the rotifers of the Illinois plankton, which seems to be identical with that described by Przesmycki ('01). Obviously it is difficult to take a census of such internal para- sites. A record was kept, however, of the number of parasitized individuals in each species of rotifer, and references will be made to these results in the discussion of the hosts. Dimcerium appeared in both summer and winter rotifers, and its seasonal distribution naturally depends upon the number of available hosts. It was in consequence most abundant during the midsummer and autumn months. CILIATA. Average number, 15,812,346, including filter-paper collections. If these be excluded and the silk catches only averaged, the number will fall to less than a tenth of this sum. The ciliates are found in the plankton of the Illinois throughout the whole year, and as' a whole they do not exhibit any common seasonal predominance. The analysis of the distribution of the individual species which follows, exhibits two diverse tendencies which affect the distribution of the totals. These are the vernal and autumnal pulses of the Tintinnidce, represented by Codonella cratera and Tintinnidium fluviatile, and the autumnal-winter occurrence of a large number of species during the height of the sewage contamination and bacterial development. The dominant species in this ciliate wave are Carchesium lachmanni, Epistylis, Amphileptus, Lionotus, Plagiopyla nasuta, Glaucoma scintillans, Stentor niger, and 5. c&ruleus. Some species, as Halteria grandinetta, have a wider seasonal distribution, and others, as Vorticella, Trichodina, Zoothamnium, Pyxicola affinis, and many others, are adventitious in the plankton. Still others, as Rhabdo- styla, Cothurniopsis vaga, Operculana, and similar peritrichan parasites, are passive members of the plankton. The actively (9) 116 limnetic ciliates are very few. As such we may include Codonella cratera, Tintinnidium fluviatile, and possibly Stentor niger. Car- chesium lachmanni and Epistylis enter the plankton only in the form of detached and often moribund zooids, and thus are not typical planktonts, though of quantitative importance in our plank- ton in the colder months. A large number of species not here reported occur in our collections made elsewhere than in the river channel, especially in places where the decay of large quan- tities of organic matter is in progress. This is not a condition normally found in the open water of lakes, though it may occur along their shores, where vegetation is found, or in regions of sewage contamination. In the waters of the Illinois, on the other hand, the current, combined with sewage and industrial wastes and the organic detritus from the richest of fertile prairies, provides a suitable environment, even in the open water, for the support of a ciliate fauna of a magnitude somewhat unusual in fresh- water plankton. This fauna is present also in the back- waters, but is less abundant there than in the river itself. These species occur in greatest numbers of individuals in our plankton dur- ing the winter months at minimum temperatures, rising in November as the temperature falls below 50, and declining again as it rises to this point in April. As shown by the bacteriological investigations of Jordan ('00) and Burrill ('02 and '04), the bacterial pulse attend- ing the decay of the sewage and wastes at Peoria does not reach Havana during the warmer months (see table on p. 231, Pt. I.), but when temperatures pass below 50 in November the increase in bacteria is marked. The decay is less rapid at low temperatures, and the process is still going on when the water in the channel passes Havana during the prevalence of low temperatures, and the ciliates that thrive in such an environment abound in the plankton at that time. The temperature limits of these ciliates of the period of bacterial development thus seem to lie between 50 and 32. An examination of the plankton in the river at several points between Peoria and Havana at intervals throughout a year, will reveal how far the component species of this ciliate fauna are governed in their seasonal distribution in the plankton at Havana, respectively, by conditions of temperature and by the state of sewage contamination. The work of Roux ('01) upon the Ciliata about Geneva would seem to 117 indicate that many species of the fauna of stagnant water are more abundant in that region during the winter months. Owing to the difference in food conditions attendant upon the increase of sewage and bacteria during the colder months in the Illinois River, it is impossible to determine from the data at hand the relative efficiency of the two elements of temperature and food in regulating the seasonal occurrences of our ciliates. Here, as elsewhere, the disastrous effect of sudden floods can be traced. The number of ciliates (Table I.) drops as floods rise, and recovers as the waters fall again. For this reason the winter occur- rences of the total ciliates are subject to considerable disturbances in the winter floods of the several years. The combination of the two methods of collection and of the two groups of ciliates, typical and adventitious, causes further irregularities (Table I.) in the sea- sonal distribution of totals. In the Illinois River, for reasons given above, the Ciliata occupy a place in the economy of the plankton of more than the usual im- portance. They feed principally upon bacteria, decaying organic matter, and the smaller algae, and are themselves eaten by the rotifers. I have found no evidence that they are utilized by the Entomostraca. They thus become active agents in the reduction of sewage and in the destruction of the bacteria of decay, in the purification of sewage-laden waters, and in the transfer of the matter in sewage to higher forms of animal life. The ciliates found in the Illinois include all the important species reported in the plankton of fresh water, and the list is somewhat larger than hitherto recorded in quantitative plankton collections in river or lake waters. These organisms escape readily through the silk net by reason of their small size, and in some instances the larger species, by reason of their mobility and flexibility, escape through the silk where less motile organisms of equal size are re- tained. By experiment I have found that well -shrunken silk bolting - cloth whose meshes average about 30-45/1 will not retain Paramecium whose diameter is 40-70/*. It may be that supple- mentary methods of collection which will correct the error of leakage will show that the Ciliata are of wider occurrence in the plankton than has hitherto been found to be the case. 118 DISCUSSION OF SPECIES OF CILIATA. Amphileptus spp. Average number, 630. Amphileptus is a well- defined winter planktont in the river at Havana, and it affords a striking instance of the interdependency of organisms in the plank- ton. It feeds upon the heads of Carchesium lachmanni, engulfing the head in situ and encysting during digestion. Such heads, joined to the colony or free in the plankton, have been found in our waters. Its seasonal distribution at Havana is almost identical (Table I.) with that of Carchesium, upon which it feeds. Thus in 1897-98 Car- chesium was continuously present in the plankton from October 26 to May 10, with a pulse on December 7 of 283,800, and one on February 8 of 197,600. Amphileptus appears October 26 ; continues, with interruptions, to May 17; and has pulses December 7 and January 25, the latter reaching 13,545. In 1898-99 both appear early in October and have coincident pulses on November 22 and January 24. In 1895-96 the interdependence is even more striking, Carchesium reaching a greater development in this winter, with a pulse of 964,600 on November 27, and Amphileptus reaching 14,469 . on this date and 14,835 a week later. Both species decline during the flood which follows, and rise during March to culminations, on the 24th, of 104,535 and 3,636, respectively. In 1898, Amphileptus disappears on April 12 at 52, save for an isolated occurrence May 17 at 64. It does not reappear until October 18 at 52. In 1897, it reappeared October 26 at 59, and in 1895-96 its limits were 45 and 48, with the exception of one occurrence, April 17, at 66. Carchesium occurs irregularly and sparingly during summer months, and Amphileptus was not taken in the plankton during that period. Its occurrence in the plank- ton is limited in the main to temperatures below 50, but this limitation may be due primarily to the reduced numbers, at higher temperatures, of the organism upon which it feeds. It appears during the period of greatest sewage-contamination and bacterial development in the river at Havana. Roux ('01) finds Amphilep- tus most abundant in stagnant waters about Geneva in the winter months. Aspidisca costata (Duj.) Stein. Found in the plankton but once Jan. 11, 1898, at 32. Bursaria truncatella O. F. Mull. Average number, 23. This large ciliate was found in the plankton at irregular intervals and in 119 small numbers. It was found six times in March; twice in January and April; and once in February, July, and November. Its ap- pearance in the plankton is thus predominantly in winter months and at temperatures below 45, though it occurs in the extremes of temperature conditions. Carchesium lachmanni S. Kent. Average number, 26,546. This is normally an attached species, and its appearance in the plankton is due to the detachment of the heads. Small fragments of colonies are also found, but the greater number are isolated heads. The detachment seems to be a physiological process of the organism and not merely the result of accidents. It is thus a detached and an adventitious planktont. Many of the heads taken in the plankton are in a moribund condition. For example, in a pulse of March, 1896, the following proportions were recorded. Date Total Carchesium per m.s Per cent, normal Per cent, moribund 1896 March 17 60,420 55 45 " 24 104,535 48 52 " 30 47 571 53 47 April 10 16 688 39 61 Enumerations were based on the total number of heads, both normal and moribund. The colonies are sessile, and adhere in vast numbers to any substratum furnishing a suitable place for attach- ment submerged vegetation, brush, sticks, and fishermen's nets. The latter sometimes become so clogged with Carchesium and. floating mats of Crenothrix and Beggiatoa as to break down in the current of the river. How far the number of free heads in the plankton is an index of the development of the species in the stream can not be determined from the data at hand. This species has been taken in the plankton in every month of the year, but its occurrences between the early part of May and 120 October 1 that is, above 60 are irregular and the numbers few (Table I.). It is thus predominantly a cold-water planktont. Winter collections in 1894-95 and 1896-97 were too few to trace its seasonal movements. In 1896-97 it appeared November 5, rose to a maximum of 964,600 on November 27, and declined in the December- January flood (Pt. I., PI. IX.) almost to extinction, but recovered during its decline to a minor pulse of 16,160 on January 30. It again fell off in numbers during the floods of February (Pt. I., PI. X.), but rose during the decline of March to a maximum of 104,535 on March 17. Numbers become smaller and occurrences irregular after May 1. In 1897, Carchesium increased rapidly in late October to a small pulse of 13,200 on November 2, with a decline in the following fort- night, and a pulse culminating December 7 at 283,800, with subse- quent decline. The fluctuations during 1898 may be followed in Table I. The numbers increase during the slowly rising flood of January to a maximum of 197,600 on February 8 at 32, and decline again during the more rapid rise (Pt. I., PI. XII.) of the next three weeks. Stable conditions in early March bring about a pulse of 89,600 on March 15, and numbers decline again to 2,400 as the flood passes its maximum in the early part of April. As the levels fall another pulse of 99,200 appears April 26, from which a descent to minimum numbers which prevail during the summer takes place within a fortnight. The floods, especially sudden ones, seem thus to interfere with the appearance of Carchesium in the plankton, while gradual rises, as that of November, 1898, are not so detri- mental. The table of bacterial occurrences (Jordan, '00) in the Illinois at Havana and Pekin given on p. 231, Part I., indicates that the bac- terial development consequent upon the sewage and industrial wastes of Peoria extends down the river to Havana during the colder months of the year. The occurrence of Carchesium in the plankton is thus coincident with that of greatest sewage pollution and bacterial development at Havana. Carchesium is much more abundant in the channel of the river, where sewage pollution is greatest, than it is in the adjacent backwaters. It seems probable that the bacteria either directly or indirectly contribute towards its development, constituting, it may be, an important element in its food. Flood waters, which dilute the sewage (cf . hydrograph and chlorine 121 in PL XLV. of Part I.) might for this reason tend to interfere with the development of Carchesium, and thus cut off the source from which the plankton individuals arise. I am not able, however, to trace any close correlation between the fluctuations of the chem- ical matters indicative of sewage and sewage decay and those of Carchesium. In the stable hydrographic conditions of 1897 we find a symmetrical pulse of considerable dimensions rising from 2,200 on November 9 to 283,800 on December 7, and declining to 26,500 on January 11, 1898. Stable low water with an ice blockade (Pt. I., PI. XI. and XII.) characterize this season. ~Nr> explanation for the fluctuation is suggested in the physical environment. The chemical condition of the water, was, however, greatly disturbed (Pt. I., PL XLIV.). The fivefold increase in free ammonia is indic- ative of approaching stagnation under the ice, and the threefold increase in chlorine marks the sewage concentration. Approaching stagnation might have caused the decline of Carchesium, or it may be a specific reproductive cycle of the organism which combines with the external factors of the environment to produce such a wave of occurrence. Chilodon cucullulus Ehrbg. Average number, 102. This species was found in the plankton in January and February during the bac- terial increase. It was also found in July. It escapes through the silk net, and does not ordinarily appear in plankton collections, though abundant wherever decay is active. Codonella cratera (Leidy). Average number, 101,024 or 452,500*. This is the most abundant of the ciliates in our plankton, consti- tuting about one third of their total number. It appears in every month of the year, and in 1898 it was recorded in every collection but one, that of December 13 (Table I.). It is sub- ject to great fluctuations in numbers, its maximum occurrences tend- ing to appear in April, May, or June, and again in September or October. Minimum numbers prevail during the winter, when many of the shells are empty, and the midsummer interval is subject to pulses of varying amplitude. Spring pulses were detected as follows : in 1895, on April 29 (16,324) at 64; in 1896, on April 24 (562,152) at 72;inl897,onApril27(470,000)at60;andinl898,onMay3(736,000) at 60. These vernal pulses coincide with or approximate closely to the dates of the spring volumetric pulses. This somewhat remark- able approximation of dates near the end of April may be the result, 122 in part at least, of the dates of collection ; but after allowance is made for this, the species still exhibits a seasonal cycle of remarkable regu- larity. The autumnal pulse is of less amplitude, and of less regu- larity in location as to time and temperature. In 1894 it appears September 4 (14,000) at 78; in 1895, on September 12 (5,840) at 81; in 1896, on August 29 (58,800) at 74 or October 14 (63,200) at 57; in 1897, on October 5 (204,400) at 71; and in 1898, on September 27 (92,800) at 73. The midsummer pulses are, as a rule (Table I.), of less amplitude than the vernal or autumnal ones. In 1896 and 1898 exceptions to this statement appear in two large developments which follow in each case upon the decline of the June rise. In 1896 (Pt. I., PI. X.) this pulse (152,400) came June 11, and in 1898 (Pt. I., PI. XII.) it came (1,499,200) June 7 at 78 and exceeded in amplitude the re- corded vernal pulse. In both cases the pulse was recorded as occur- ring at an interval of a week after the crest of the June rise had passed. The character and sequence of these pulses is well shown in Table I. The occurrence of Codonella in abundance in the purer backwaters and in the plankton of our Great Lakes (Kofoid, '95) indicates that it is not dependent upon the sewage bacteria directly for food for its development in our waters. The appearance of the greatest pulses during a period of considerable sewage dilution still further indicates its independence of sewage bacteria. A comparison of the fluctua- tions of the totals of the chlorophyll-bearing organisms with those of Codonella affords some evidence of a correlation between the two. Of 39 pulses which can be traced, in our records in the chlorophyll- bearing organisms, 21 precede and 13 coincide with those of Codo- nella, while in the remaining 5 instances the multiplication of Codo- nella precedes that of the phytoplankton as a whole. Thus in the main the pulses of Codonella follow, or coincide with, those of the phytoplankton. The evidence of this sequence may be followed in Table I. by a comparison of the records of Codonella with those of the total phytoplankton. The sequence indicates that the food of Codo- nella may be found in the phytoplankton, and that these recurrent periods of growth have some connection with the conditions of nu- trition. The seasonal cycle of Codonella is closely followed by the other member of the family found in our plankton Tintinnidium fluviatile. 123 Codonella occurs throughout the whole range of temperatures. The winter minimum and the decline during the maximum temperatures of summer, combined with the presence of vernal and autumnal, or late summer, pulses, indicate that the optimum conditions for this organism lie neither in winter nor in summer. The spring pulse was at temperatures of 60-72, and the autumnal one at a wider range of 57-78. Permanent increase in numbers does not begin (Table I.) until March 15 at 46, and the permanent falling off is found on November 15 at 41. The optimum temperatures in our waters thus lie near 60-70, and conditions favoring growth are limited to a range of 10-15 upon either side of the optimum. This species readily escapes through the silk net on account of its small size and its motility, and such collections give at the best in- complete evidence of its seasonal distribution. The amplitude of its fluctuations is thus reduced, and owing to the irregularity of the error arising from leakage, the reduction is not proportionally distrib- uted throughout the year. Tests made of the loss of Codonella by leakage through the silk indicated that but one was retained to twenty-four found in the filtrate . Codonella was counted in both the silk and filter-paper collections, with the result that in 1897 the totals for the year (omitting one date on which the filter collection contained an unusually large number of Codonella) showed one Codonella in the silk to twenty-five in the filter collection. In 1898, however, the ratio was one to four and a half. The error in the filter collection is large, but data seem to justify the conclusion that only a small proportion of the Codonella is retained within the silk net. The proportion for the whole period of collection by the two methods (August 3, '97, to March 28, '99) is one to seven, if one date on which aberrantly large numbers appear in the filter collections be omitted. This species is a typical planktont, and is apparently the same as C. lacustris Entz, by which name it is designated by European writers. Leidy's name, however, has priority according to the accepted rules of nomenclature. It is an exceedingly variable organism, at least in the form, proportions, and size of the shell, in the degree of its con- striction, and in the foreign particles which fill its matrix. The rings or bands which ornament the orifice vary in their number, width, and relative proportions, and in the perfection of their development. The intergradation which these variants exhibit is sufficient to my mind to make their elevation to specific rank unjustifiable. 124 Codonella is an important element in the food of many of the lim- netic rotifers, especially Asplanchna. Codonella is a common constituent in the plankton of our own Great Lakes (Smith, '94; Kofoid, '95; Jennings, 'OOa), and has been reported from most European waters. Apstein ('96) finds in German lakes major pulses in spring and autumn and minor ones in midsummer. Lauterborn ('94) reports Codonella in the plankton of the Rhine, and Schorler ('00) in that of the Elbe, but neither follows its seasonal history. Coleps hirtus Ehrbg. Average number, 1 3 . This species occurred in the plankton collections irregularly and in small numbers, princi- pally in autumn months during the height of the bacterial develop- ment. It escapes through the silk readily. Colpoda cucullus Ehrbg*. Average number, 9,615. This species appears in the plankton principally during the colder months of bacterial predominance, from November to April, and occasionally during the summer. Cotkurniopsis vaga (Schrk.) Blochmann was found in both 1898 and 1899 on Canthocamptus. Didinium nasutum (O. F. Miill.) Stein*. Average number, 12,692. This species also is found in the plankton during winter months, especially in November and December during the bacterial increase. It was also found in midsummer. Epistylis spp. Average number, 2,020. The free heads or frag- ments of colonies of one, or possibly of several, unidentified species of Epistylis, or it may be of Opercularia also, were associated with Car- chesium lachmanni in the plankton during the colder months, but in much smaller numbers (1 to 13 in 1898). Identification in most cases was impracticable, though in some instances E. flavicans Ehrbg. was determined, and it seems probable that most of the winter forms at least belong to this species. Hempel ('99) reports E. plicatilis on snails, and various other aquatic animals have been found infested with colonies of undetermined species of Epistylis. The distribution of Epistylis in the plankton (Table I.) is in its limits somewhat like that of Carchesium. It is more abundant and more continuously present during the period from November to June (at temperatures below 60) than in the intervening warmer months. It is found throughout the whole range of temperatures. Its pulses coincide with those of Carchesium when they occur, but they are not 125 always found in Epistylis when they appear in Carchesium. This degree of similarity in the seasonal cycle of the two genera is indica- tive of their correlation with the same environmental factors, the principal one of which is the increase in bacteria attending the colder months. Euplotes char on (O. F. Mull.) Ehrbg. was taken but once in the plankton August 23, 1898. Euplotes patella Ehrbg*. Average number, 2,888. It was found in small numbers and at irregular intervals from April to December throughout the full range of temperatures. It was most frequently taken in the summer. Glaucoma scintillans Ehrbg.* Average number, 39,615. This species was taken in the plankton from the middle of October till the middle of April. It was present in larger numbers and more contin- uously in December and February. It is thus a member of the plankton during the time of bacterial increase. Halteria grandinella O. F. Mull.* Average number, 255,769. The seasonal distribution of this species in the plankton does not show the limitation to the winter months noted so frequently in other ciliates. It was found in every month of the year but May, in largest numbers in July and August, and most continuously in December and January. The data are too few and irregular to determine any pre- dominance as to season or temperature. Holophrya simplex Schew. was found in small numbers in the filter collections of December, February, and March in the winter of 1896-97 at temperatures from 32 to 44. Leucophrydium putrinum Roux. Average number, 525. This species was recorded July -September, 1898, during the low-water period, at temperatures from 89 to 63. It was described by Roux ('99) from stagnant water, but in our plankton no conditions of stag- nation attend its presence, though sewage contamination is great and decaying organic matter abundant. Lionotus spp. Average number, 94. With Amphileptus in the winter plankton there occur a number of other, smaller, gymnostome ciliates which in best-preserved specimens resemble Lionotus. A few occurring in March and April, 1898, were found to be L. fasciola Ehrbg. , and it is probable that most of the individuals belong to this species, though exact identification is difficult with plankton mate- rial. The seasonal distribution of Lionotus coincides very closely 126 with that of Amphileptus. The species appear in November or De- cember and continue through March in temperatures below. 50, but the numbers retained by the silk net are too small to trace their sea- sonal routine. Their seasonal distribution in the plankton "coincides with the period of greatest access of sewage and bacterial increase in the river at Havana. Roux ('01) finds this genus well represented in the fauna of swamps, and most abundant in October and March. Loxodes rostrum Ehrbg. was identified but once March 22, 1897, at 44. Nassula ntbens Perty occurred July 30, 1897, at 84. Opercularia articulata Goldf. This species is parasitic upon aquatic Coleoptera. In the plankton of June 28, 1897, eleven colonies or fragments of a colony were found, the largest with 115 zooids. Opercularia nutans (Ehrbg.). Average number of zooids, 60. In the plankton this species was found attached to Alona afjinis in January, 1898, and to Cyclops in April and August. Opercularia not specifically determined were found free in the plankton in June and July ; in November, attached to Canihocamptus ; in January, attached to Brachionus and even to the eggs of this species. An unidentified form was also found upon Cyclops. Ophryoglena atra Lieberk. Five irregular occurrences of this species in small numbers were recorded in 1899 from January to the middle of March. Paramecium spp. Average number, 41. Paramecium was found 18 times in the plankton. Two of these instances were in May and August at temperatures of 64 and 79, and the remainder were between November 20 and March 30 at temperatures below 48. Most of the occurrences are in midwinter at minimum temperatures under the ice. P. aurelia (O. F. Mull.) has been found in the river waters (Hempel, '99), but not all taken in the plankton belong to this species. Specific determinations are not easily made with accuracy in preserved plankton material. In our plankton, Paramecium is present principally during the period of greatest contamination by sewage. Plagiopyla nasuta Stein*. Average number, 1,181,000 during the winter of 1898-99 from November 29 to March 28. This species was not recognized in the plankton of previous winters. It reaches a pulse of 11,520,000 on January 3, 1899, at 32.2 under the ice. 127 Levander ('94) finds it in numbers under the ice in Finnish waters. On account of its motility and small size it readily escapes through the silk net. Pleuronema chrysalis (Ehrbg.) Stein. Average number, 9. Re- corded only in January, 1898, at minimum temperatures. Prorodon farctus Clap, and Lach. Only a few scattered occur- rences from the last of September to the first of March at tempera- tures from 73 to minimum. An unidentified species of Prorodon was also found irregularly from November to April. Pyxicola affinis S. Kent. Average number, 58. This species is usually attached to aquatic plants, especially to Lemna. It has been found in the summer plankton from June to August during maximum temperatures, especially in 1896, when recurrent floods brought much Lemna from the backwaters into the river. It was found October 18 at 52, attached to Melosira varians. Rhabdostyla spp. Average number, 1 10. Peritrichan ciliates re- ferred to this genus have been noted on Cyclops, Canthocamptus, Oligoch&ta, and even in considerable numbers upon the body, append- ages, and eggs of Polyarthra platyptera. They have appeared thus passively in the plankton during winter months from December to March, especially in 1899. Stentor casruleus Ehrbg. Average number, 882. This species presents a characteristic seasonal distribution in our plankton. Its numbers are never very large, and its full cycle can not always be traced in the records. It is a planktont of the colder season in our waters. But three records one July 28, 1896, at 82, one August 3 of the same year at 80, and a third, August 15, 1894, at 84 lie outside of the period between September 1 and May 1. In 1898 (Table I.) the autumn cycle begins September 6 at 79, but in both 1895 and 1897 the species does not appear until late in November or in December at 34 or below. In years prior to 1898 the numbers were small and irregular, but on January 21, 1898, the maximum number of 28,800 was reached at 34, under the ice, during the slowly rising flood of that month (Pt. I., PI. XII.). It accompanied an increase in Stentor niger, and there are indications elsewhere that the two species may fluctuate together. The high (Pt. I., PI. XLV.) chlorine (38.), nitrites (.175), and free ammonia (4.6) at the season of greatest development in the plankton are in- dicative of conditions approaching stagnation. The appearance of 128 this species in stagnant water has often been observed. Roux ('01) finds it especially abundant in September, October, and February in stagnant waters about Geneva. Stentor niger Ehrbg. Average number, 3,124. In our waters this species also is a winter planktont (Table I.). There have been but four records of occurrence between May 1 and September 1 . In 1895-96 the species appeared November 14 at 44 and reached a maximum of 68,635 December 18, after three weeks of minimum temperatures and approaching stagnation under the ice. Numbers declined in the December- January flood (Pt. I., PI. X.), but rose again in March, as the flood declined, to 39,087 on the 24th at 40. It disappeared from the plankton April 30 at 70 and did not re- appear until November 1 7 , from which time it continued until March 22. In 1897-98 it returned September 21 at 71, attained a maxi- mum of 42,000 November 23 at 43, declined during December, and rose to 47,000 on January 21 at 34 under the ice, and in the con- ditions approaching stagnation described in connection with the dis- cussion of S. c&ruleus. A decline in numbers continued until April 12 at 52. Favorable conditions for growth are thus found in our waters between 32 and 50, and the optimum seems to lie near 40 or below. This species reaches its greatest development in our waters during the time of greatest sewage pollution and bacterial development. It is known as a bog-water species, and was found by Roux ('01) in stagnant waters about Geneva during the colder months. Hempel ('99) reports this species as 5. igneus (?), but from the descriptions of Roux ('01) I am inclined to consider it as 5. niger Ehrbg. It may be that both species are included in our data, but they are predomi- nantly of the niger type. They include also individuals of the black- ish variety 5. igneus var. juliginosus Forbes, which, it would seem from Roux's description of these species, should be transferred to 5. niger. The fuliginosus form was very abundant in the margins of Pine and Round lakes, Michigan (Kofoid, '95), during the summer in surface temperatures of 61-70, where sewage contamination was but slight. Stentor polymorphus (O. F. Mull.) Ehrbg. was found sparingly in July and August during maximum temperatures. Hempel ('99) reports S. barretti Barrett and 5. roeselii Ehrbg. from the river, but I have not identified them in the plankton collections. 129 Strombidium viride Stein was found in small numbers in January- March, 1899, at minimum temperatures. Stylonychia mytilus (O. F. Mull.) Ehrbg. was found in the plankton sparingly from September to February, and once in June. Tintinnidium fluviatile Stein. Average number, 22,590 or 1,640,- 192*. This species is somewhat sharply limited to the warmer months in its seasonal distribution. In 1898 (Table I.) it makes its appearance April 4 at 49, reaches a maximum of 720,000 May 3 at 60, and has three decreasing pulses ; one of 1 04, 000^ oa June 14 at 80, one of 95,200 on August 2 at 79, and one of 22,400 on September 27 at 73, and disappears from the plankton October 18 at 52. The records in previous years are more irregular, though traces of vernal and midsummer pulses can be found in the records. Filter-paper catches indicate that only one in eighty of this species is retained by the silk. They also locate the pulses as approximately coincident with those of the silk collections. Apstein ('96) finds Tintinnidium to be a spring planktont with its maximum in April in Lake Plon, while Seligo ('00) finds it in lakes near Danzig in the autumn, with a maximum in September. In our own waters in 1896 the autumnal pulse in August-September exceeds the vernal one. The gelatinous lorica of this species is subject to great variation in its size and proportions, and especially in the region about the aper- ture. A somewhat thimble-shaped form was described by Hempel ('96) as T.illinoisensis, the specific distinctions being based wholly on the lorica. This form intergrades with the typical lorica of T. fluviatile Stein, and should not in my opinion be given specific rank. Trachelius ovum Ehrbg. Average number in 1895, 847. This species did not occur in 1898 but was rather common in November- December, 1895, reaching a maximum of 10,695 on December 4 at 32.5. Isolated appearances in small numbers in December and January of other years have been recorded. In our waters it is thus a winter planktont. Stagnation conditions under the ice were approaching (Pt. I., PI. XLIII.) when the pulse of 1895 occurred in the Illinois River. Apstein ('96) found it, however, in Lake Plon with a maximum in May- June, disappearing in the summer and returning again in November. Trichodina pediculus Ehrbg. Average number, 1; in 1897, 874. This species is normally found upon Hydra, on the gills and skin of 130 amphibians, and on young fish. It appears in the plankton during the summer months in every year except 1898, a single record only being made in that year. The earliest record was on June 1 1 , and the latest on November 3 1 . The whole temperature range is practically included in these occurrences, though the species disappears within a few weeks after the temperature falls below 50. It usually appears in small numbers and irregularly, and no pulses like those of typical plank- tonts can be traced. A free life in the plankton is apparently not its usual habit. Zacharias ('00) has recently called attention to its appearance in the plankton in German waters. Vorticella rhabdostyloides Kell. Average number, 61. This little Vorticella is found attached in small clusters to Anabozna spir aides and occasionally to other members of the phytoplankton. It is some- what common in the waters of Lake Michigan, but is rare in spring months in the Illinois River. Vorticella spp. Average number, 7,843. At irregular intervals from April to November isolated individuals and small clusters at- tached to bits of debris in the silt were taken in the plankton. They were most abundant at temperatures above 50. The irregularity in their occurrences indicates that they are adventitious in the plank- ton. Identifications of plankton material are impracticable except in strongly marked species. Hempel ('99) has found V. campanula Ehrbg., V. microstoma Ehrbg., and V. similis Stokes in the river and its adjacent waters. Zoothamnium arbuscula Ehrbg. A few colonies were taken in August and September in 1896 in the plankton, probably adventitious during the disturbed hydrograph of that year (Pt. I., PL X.). The preceding list of 45 species does not complete the catalog of the ciliate constituents of the plankton, though it includes all of the species of quantitative importance during the years of our operations. The residium of unidentified ciliates, which, excluding the partial identifications in the above list, does not often exceed two per cent, of the total individual ciliates, includes principally isolated individ- uals of species difficult of identification or others whose preservation did not permit it, and a considerable number of small ciliates and of forms ectoparasitic upon Entomostraca and other planktonts. Most of these organisms are either adventitious or passive members of the plankton, and further study of the littoral region, of stagnating 131 waters, and of these parasitic forms will reveal the great richness of the ciliate fauna in this aquatic environment. SUCTORIA. Average number, 332. This class is not quantitatively im- portant in the plankton, being represented, in so far as our records go, only by adventitious or passive planktonts. No limnetic species has as yet been found in the Illinois. An examination of the littoral region during the prevalence of ciliates wilt probably yield a rich suctorian fauna. DISCUSSION OF SPECIES OF SUCTORIA. Acineta linguifera Clap, and Lach. This species is usually found on aquatic Coleoptera. A single occurrence of an unattached indi- vidual was recorded June 21, 1898. Metacineta mystacina Ehrbg. Average number, 301. This species occurred in the plankton from March till October in 1898 and in the winter months of 1899, at irregular intervals and in small numbers (Table I.). Most of its occurrences attend flood invasions, and it is evidently adventitious. It is frequently attached in the plankton to minute particles of debris. This species varies greatly in the size of the lorica. Sand ('01) gives the range in height as from 3 3-7 00 -/i. The variation in proportions has given rise to a number of descriptions of new species by Stokes ('88 and '94) and Maskell ('87), but an examination of a series of individuals such as appear in the plankton shows that they intergrade so closely that specific distinctions can not be maintained for the variants. Meta- cineta appears throughout the whole range of temperatures, no seasonal predominance appearing in the records. Podophrya fixa O. F. Mull. Average number, 12. This species is also adventitious in the plankton. It was recorded in March and September at 37 and 73. Cysts were noted January 21. Tokophrya quadripartite Clap, and Lach. Average number, 4. Adventitious in the plankton in March and November. Hempel ('99) finds it most abundant in May and June, associated with Epistylis plicatilis and Opercularia irritabilis on crayfish, insect larvae, and turtles. Tokophrya cydopum Clap, and Lach. Found occasionally upon Cyclops during spring and summer. (10) 132 PORIFERA. Spongilla spp. Average number of spicules, 772. The identifi- cation of fresh-water sponges by isolated spicules is practically impossible, and, moreover, the sponge fauna of the Illinois River is as yet practically unknown. No attempt, therefore, was made to identify the species to which the spicules which occur in our plank- ton collections belong. They belong to the genus Spongilla in part, and were usually the simple sarcode forms, the gemmules or their spicules not appearing in the plankton. They occurred in all months of the year, and were found in 46 per cent, of the collections. They are adventitious, and their occurrence in the plankton is there- fore dependent in part upon hydrographic conditions. Records in December and January are few (3) and always occur on rising floods. In February and March, months of rising floods, they are increased (8 and 7), but decline again in April- June (3,5, and 5), months of predominantly declining water and more stable conditions. In midsummer and autumn months (July to November) they again occur more frequently (8 to 12), probably as a result of proximity to the season of greatest growth and frequency of sponges in the river and its backwaters. Here also they occur most frequently in years of greatest hydrographic disturbance, as, for example, in 1898. The adventitious relation which they bear to the plankton is also seen in their erratic and irregular numbers. The maximum record (16,000 per m. 3 ) was made June 28, 1897, on the rising flood; the next in size, on August 10 in stable low water. In both instances the plankton was probably taken from water in which as a result of some local disturbance the remains of some disintegrating sponge had been distributed. Living sponges are found in considerable abundance on submerged brush and timbers in the channel and backwaters during the summer months, and feed on the smaller organisms of the plankton, being one of its depleting agencies. CCELENTERATA. Hydra fusca L. Average number, 39. Hydra occurred in about 16 per cent, of our channel collections a percentage which would be considerably increased if the whole of each collection had been examined for it, or if backwater collections should be included. With one exception the 28 occurrences recorded, all fall in May-September 133 at temperatures rarely below 70. The earliest record in channel waters was on May 1, 1896, at 68.75, and the latest on November 15, 1897, at 47. Of the 28 records in channel waters the months from May to September have, respectively, 6, 3, 10, 7, and 1 record, and there is 1 in November. Hydra is thus a late vernal and a summer planktont in our waters. Observations in the field and a cursory examination of the col- lections made in the backwaters have indicated that Hydra is often very abundant on the vegetation. It is also limnejtic in habit, floating with the foot attached to the surface film and tentacles widely extended; or, without attachment, in the deeper strata of water. A similar limnetic habit was often observed in the case of Hydra in channel waters, especially on still warm days when the surface was unruffled. Hydra \vas generally more abundant in the plankton in May or in early summer. The maximum record in channel waters was 3,200 per m. 3 on July 21, 1897, the error of dilution being, however, large in this record. In Quiver Lake on May 8, 1896, a maximum record of 5,335 per m. 3 was made, the error of dilution being very small. This was during a vernal plankton pulse (8.14 cm. 3 per m. 3 ) in these waters, when the food of Hydra was present in considerable abundance. Hydra viridis L. was seen 'frequently in spring-fed backwaters and in laboratory aquaria, but was never recognized in plankton collections made in channel or backwaters. The limnetic habit noted in H. fusca was not observed in the case of this species. . PLATYHELMINTHES. TURBELLARIA. Numerically and from the volumetric standpoint the Turbellana are not of great significance in the plankton of fresh waters as a rule. However, in some seasons and under certain conditions Stenostoma becomes very abundant, as, for example, in autumn months in back- waters, and generally where decaying vegetation abounds. In the autumn of 1895 the plankton in the relict pools of Flag Lake consisted almost entirely of Synura uvella, Stenostoma leucops, and Entomos- traca. 134 The average number in channel waters is 103 per m. 3 , and, as might be expected, their occurrences are erratic in seasonal distri- bution and their numbers are irregular. They occurred in channel waters in eveiy month of the year and throughout the whole seasonal range in temperatures. The numbers in 1898 were larger and occur- rences more frequent in May, during the run-off of the spring flood, and smaller and more erratic during the rest of the year. In the total of all collections enumerated the percentage of occurrences was highest in June (60 per cent.), July (83 per cent.), August (48 per cent.), and October (47 per cent.), and lowest in colder months, when it rarely rises above 30 per cent. The numbers are also larger in the warmer months, a maximum record of 19,250 per m. 3 on September 4, 1894, following a slight rise in river levels at low stages. The adventitious character of the Turbellaria in channel plankton is sug- gested by the erratic data, but the adaptability, at least of certain species, to the limnetic habit under certain conditions is also indi- cated by the large numbers. The identification of the Turbellaria in plankton collections is not feasible in the course of the usual methods of examination of pre- served plankton. Accordingly no effort was made to identify the individuals occurring in our catches. Many of them were evidently rhabdoccele turbellarians, and of these probably many were Stenos- toma leucops. The genus Vortex was also represented. Mesostomum ehrenbergii O. Schmidt was taken in small numbers on August 26, 1895, along the shores of the river in vegetation. This identification is that of Dr. W. McM. Woodworth ('97). Stenostoma leucops O. Schmidt. Average number, 21. By far the greater proportion of the turbellarians in our collections probably belong to this species. The statements made regarding the group as a whole therefore probably apply to this species. TREMATODA. Many of our predaceous fishes and other aquatic vertebrates are infested to an extraordinary degree by flukes parasitic in the intestine or other viscera. This, in conjunction with the fact that the fish markets are located in house-boats along the stream and their refuse generally cast directly into the channel, is sufficient to account for the few adventitious adult distomes which have been noted in our plank- 135 ton collections. They have occurred singly in February and July, but were not identified. The free-swimming larval stages or cercaria of unidentified trem- atodes were also found singly in August, September, and October. Aspidogaster conchicola v. Baer, which occurs abundantly in the mantle cavity and pericardium of many of the Unionidcz (see Kelly, '99), which form great beds on the river bottom, was taken in an immature condition in the plankton on June 27. Cotylaspis insignis Leidy, likewise a parasite of the Unionidce, associated with Aspidogaster but confined principally to the mantle chamber, was taken in the plankton on February 4. CESTODA. Tetrarhynchus sp. was -adventitious in the plankton on June 27, and doubtless of similar origin to the adult trematodes above noted. NEMERTINI. Fresh-water nemerteans were definitely identified as such in the plankton on only two occasions, July 23, 1894, and March 22, 1897. They were doubtless adventitious from the shore or bottom, where they are most abundant. NEMATELMINTHES. NEMATODA. The free-living nematode worms are predominantly shore and bottom forms, living in the midst of the decaying organic matter of the bottom ooze. In a habitat such as ours, where the quantity of this decaying matter is very great, the nematodes are correspondingly abundant, and, owing to the unstable hydrographic conditions, they find many opportunities of joining the plankton temporarily. Ac- cordingly we find that nematodes are met most frequently and in largest numbers in rising flood waters, when the bottom deposits of tributaries and the main stream are carried in channel waters as silt. Thus, in the month of March nematodes occurred in 13 of the 15 collections examined, with an average number per m. 3 of 465, while in August they were found in but 8 of 21 collections, and averaged only 186 per m. 3 . So, also, in the winter flood of 1895-96 nematodes were found in the plankton almost continuously till the middle of 136 April, while in the more stable conditions of the preceding year they were found in only one third of the collections. In 1897 most of the 31 collections examined were made in stable conditions, and nema- todes were found in but 5 of these, and 4 of these 5 were made in rising flood waters. In 1898, a- year of greater hydrographic dis- turbance, nematodes occurred in 31 of the 52 collections, averaging 318 per m. 3 to 82 in 1897. Of the 31 occurrences in 1898 all but 6 were in recent flood waters. The hydrographic conditions attending the presence of nematodes in the plankton thus indicate that they are adventitious in the plankton. Further evidence of this is to be found in their erratic numbers. Thus, on February 20, 1896, none was re- corded, and on the 25th their numbers rose in flood waters to the maximum record for all of our collections 18,422 per m. 3 No effort was made to determine the species of these nematodes. A considerable variety of forms awaits the labors of some courageous systematist. ACANTHOCEPHALA. These worms are found abundantly in the Catostomida and other limophagous fishes of the Illinois River, and in many of the water- fowl which feed in its waters. A chance occurrence of a single specimen in the plankton on August 3, 1896, is probably to be ac- counted for as in the case of other intestinal parasites. ANNULATA. OLIGOCH. d P & & o CO AH to CD CN l^ CN (A w CO 4J o3 P. VH t-i Bj OS I-) S S P PH Q O d CO d P ja PH CN CN r^ O . C 3 ^ ^H CM d r* 1 ro O *5' ^ H D CN ON CO CN *-l ^H CN b . NC CN CN *-H o3 P a .03: c d i i > ' C3 0) tH *. rf to NO t^ OO ON ON ON ON ON ON ON OO OO OO OO OO CO O 00 o CN ^ o pH o " - - TjH ^ t O ^_ CO ro P d 0) d OJ O CN d (D P P p ^ o CO o * o NO to O CN d V - - ~ ^^ ON CO to IO * ON Tj< ON ^ 1 CN 0) CN ON O tn 43 n3 ^ K) ^ P O ; o to O O CN to 00 * i d ~ . fc NO O 10 to CN * ON ^, ON m ^ CN CN '-' "03 ' >N ^ >~. >, P "3 ^ "3 3 IH 03 0) ON ON 00 00 NO ON 00 ON 00 00 ON 00 146 147 the seasonal distribution of individual species and these recurrent plant pulses will be discussed in connection with the various species wherever the data are available. For the present it will suffice to call attention to such correlation as exists between fluctuations of the phytoplankton and the total Ploima. The table on the preceding page gives the location and amplitude of the maxima of the ploiman pulses, and a graphic presentation of the seasonal curve of distribution of the total Rotifera will be found in Plates III. and IV. On comparison of the ploiman pulses with those of the chlorophyll-bearing organisms, graphically presented in Plates I. and II., it will be found that 15 of the 33 pulses of Ploima contained within the period covered by the plates coincide in location with the plant pulses ; that 12 follow at the next collection, usually a week later, and 3 within a fortnight ; while only 3 of the 33 exhibit no such correlation. The data suggest strongly the agency of the plant pulses in building up the Ploima, and that the food relations are fundamental in the fluctuations of these planktonts. DISCUSSION OF SPECIES OF PLOIMA. Anuraza aculeata Ehrbg. Average number, 1,839. In 1898 this species has a very well-defined and characteristic seasonal distribution (Table I.). It first appears March 8 at 37, increases to a maximum of 45,200 on May 10 at 61. then declines, and disappears June 14 at 83. The curve of its occurrence in this year is a very symmetrical one. It reappears on December 27 at 32, and there are scattered occurrences through the winter months of 1899. Records in other years suggest in the main a similar distribution. In 1896 it first appeared January 6, rose to a pulse of 6,550 on May 8 at 76, and, on the decline of the June rise, there was a second and larger pulse of 29,600 on June 17 at 76. It reappeared on Decem- ber 29, and in 1897 reached a vernal maximum of 22,400 on May 25 at 66, then disappeared, and was not again noted in the following winter nor until March 8. In 1894 the last vernal record was made June 12, and on September 4, at 78, there was an autumnal pulse of 13,825 a phenomenon not repeated in subsequent years. The normal course of its seasonal distribution in the river plankton seems to be as follows: reappearance in December when minimum temperatures have been reached; slow multiplication during the winter, and a well-defined pulse on the decline of the spring flood in (in April-May with the possibility of a second on the June rise; and prompt and complete disappearance when maximum summer tem- peratures are established. Low water in the autumn seems to inter- fere with an autumnal pulse. In 1894 there was a well-sustained rise in September (Pt. I., PL VIII.) and a pulse of A. aculeata. In 1896, however, no pulse occurred in the high water of the autumn. No midwinter occurrences followed the very low water of 1897. It is thus in channel waters a vernal planktont, with its temperature optimum near 70 but below the summer maximum. Hempel's statement ('99) that it is a "winter species" is borne out by its presence from December through the winte'r, but its numerical distribution ranks it at once with the vernal organisms. Lauterborn ('94) finds it abundantly in winter months in the Rhine, and Ap- stein ('96) speaks of it as a "Sommerform," absent from Lake Plon from November till March, and with maxima from April to July in different bodies of water where it continues through the summer and till October, and then disappears. Summer temperatures in these waters, however, are not recorded by him above 21 C. (69.8 F.), which is about the temperature at the time of the vernal maxi- mum in the. Illinois, and at least 10 F. below that of the summer maximum in our waters. Jennings ('94, '96, and '00) records it as abundant in the summer plankton of Lake Erie, Lake Michigan, and some inland lakes of Michigan. These waters also are somewhat cooler (5-10 F.) than those of the Illinois River in midsummer. Temperature, it seems, must have a decided effect upon the seasonal distribution of this organism in our waters, though the chemical conditions and food supply may also enter as factors in the summer suppression of the species. Females carrying usually a single egg appeared in 1898 early in April, and were most abundant during the maximum of the pulse. On an average, less than a fourth of the females were ovigerous. Empty loricae appeared May 10 (4,800) and 17 (3,200) at the crest and decline of the spring pulse, and the same phenomenon of deca- dence was noted in previous years during this period. Outbreaks of parasites were not recorded for the species, and the decline is to be attributed to cessation of reproduction and to the death and destruction of the individuals by the more usual causes. This species is quite 'variable, but no effort was made to follow its seasonal history. The type form is by far the most abundant. 149 A. aculeata var. valga Ehrbg. was seen frequently. A. serrulata Ehrbg., regarded by Weber ('98) as a variety of A. aculeata, was recorded Jan. 24, 1899, and found by Hempel ('99) in December. It seems to be rare in our plankton. Forms approaching A. aculeata var. brevispina Gosse were also noted, but they, too, are rare, being recorded only in February and March, 1899. A. aculeata var. curvicornis Ehrbg. was noted April 29, 1896, at 70. Anurcea cochlearis Gosse. Average number, 69,393, distributed as follows: A. cochlearis (sensu strictu) together with A. cochlearis var. macracantha Lauterborn, 9,421; A. cochlearis var. tecta Gosse, 15,432; and forms with posterior spine of intermediate length between cochlearis and tecta which include A. cochlearis var. stipitata Ehrbg., 44,540. Numerically this is one of our important species, containing over one ninth of all the rotifers in 1898. It is surpassed only by Brachionus bakeri (with varieties included) , Polyarthra, and SynchcBta. Average number of eggs, 32,358. This is a perennial planktont, appearing in every month of the year throughout the whole range of temperature. Its entire absence in August, 1898 (Table I.), is not paralleled in any other year. In 1897, for example, there is a well-developed pulse of 45,600 on August 24. In 1894, 1895 , and 1896 there is a midsummer minimum of a few weeks' duration in July, August, or September, but it is irregular in its location. While the appearance of sexual cycles was not traced by the records of males and winter eggs, a matter of some difficulty and uncertainty in preserved plankton material, the existence of such cycles is suggested by the recurrent pulses of occurrence in this species (Table I.). It is possible that the species is poly cyclic in our waters. The pulses in 1898 are well defined, in fact, somewhat better than in previous years. The following table gives the num- bers in the pulses in the several years and the dates and tempera- tures at which the maxima occurred. All of the large pulses save those of November and December and one at the close of October (Oct. 25, 1898, 28,500) lie at tempera- tures above 60. The vernal pulse of April-May is the largest and appears between 60 and 70, and the amplitude diminishes as the period of maximum heat progresses, though in 1898 there was a recurrence of larger numbers as temperatures fell. The optimum 150 PULSES OF ANUR^EA COCHLEARIS. Year Date Temp. No. Date Temp. No. Date Temp. No. 1896 May 8 76 100,870 June 11 73 95,200 July 2 28 81 81 12,800 17,600 1898 May 10 62 1,145,600 June 21 77 372,800 July 19 84 17,200 Year Date Temp. No. Date Temp. No. Date Temp. No. 1894 1895 Sept. 4 Sept. 23 78 76 7,350 1,521 Aug. 21 83 17,805 Nov. 20 44 1,120 1896 1897 1898 Aug. 21 Aug. 24 79 78 5,600 45,600 Sept. 16 Oct. 5 Sept. 27 Oct. 25 71 70 73 48 6,224 4,800 54,400 28,500 Dec. 29 35 3,840 Nov. 21 40 10,000 conditions seem thus to be found in the river at temperatures some- what below the maximum, between 60 and 70. The phenomena of recurrent pulses are distinctly traceable in the seasonal distribution of this species, not only in 1898 (Table I) but also in preceding years. The large May and June pulses of 1898 appear on the declines of the spring and the June rise, respectively; the pulse of September 27 is in a falling river; and that of October 25, on a slowly rising flood (Pt. I., PI. XII.). In 1897 (Pt. L, PL XI.) the first two pulses attend the spring flood and June rise in like manner, but the two subsequent pulses are in stable low water. In 1896 five of the seven pulses lie on the declines of the recurrent floods of that year and two in rising waters (cf. PI. X. of Pt. I. and the table just given). In 1894 and 1895 the pulses appear either in falling water or in the earliest stages of the rise. The number of pulses on declining waters is somewhat greater than the relative number of days of this condition w 7 ould lead us to expect, and it seems probable that optimum conditions for the appearance of larger numbers of Anuraza cochlearis are to be found in such hydro- graphic conditions. The run-off of impounded backwaters is one of the favorable phases during flood decline. On the other hand. 151 the distribution of the pulses with reference to the floods and the appearance of pulses during rising water suggest the operation of other factors than the one arising from contribution from back- waters. The pulse must be dependent to a large extent upon food supply of the organism, and a correlation between its periods of multiplica- tion and the pulses of its food, the chlorophyll-bearing organisms, is to be expected. A comparison of the seasonal distribution in 1898 (Table I.) and the pulses of chlorophyll-bearing organisms (PL II.) reveals the fact that three of the A. cockle aris pulses coincide with those of the plants constituting their food, and the other three coincide in part only, the remainder of the chlorophyll-bearing groups reaching their culmination a week prior to that of the rotifer. In 1897 the three pulses of A. cochlearis which lie in the common period (PI. II.) all culminate a week (in one case in part in fourteen days) after the maximum of the plants in question. In 1896, three pulses coincide and three follow in the subsequent collection; and in 1895, two coincide and two follow. Collections at daily intervals would be necessary to follow the correlation more accurately. It is probable from these juxtapositions and sequences in the A. cochlearis-algse pulses that we are dealing with a food relation. Multiplication of algae leads to increase of Anur&a, which, in turn, reduces the algae, and then itself declines until the food planktonts again increase. Anurcsa cochlearis is exceedingly variable in the length of the posterior spine, in the development and degree of curvature of the anterior spines, in the arrangement of the areas of the lorica, and in the degree of its ornamentation by small spinules. The separation of these varieties where every individual must be assigned to some one of them, is a matter of some difficulty owing to the presence of intergrading individuals. The characters which signalize var. hispida Lauterborn and var. irregularis Lauterborn are not quickly recognized under the conditions of rapid plankton enumeration, and no effort was made to trace their seasonal distribution in our plank- ton. Lauterborn's var. macracantha was included with the type form his var. typica in our records. These two include those individuals with medium-sized and longer posterior spines. In our waters the variety macracantha is relatively rare, at least as figured by Lauterborn ('98). Indeed, both the type and this variety consti- 152 tute less than a seventh of the total representatives of the species. Their distribution throughout the year (Table I.) accords with the results obtained by Lauterborn ('98), who found that the average length of the posterior spine from January to May and from October to December was from 78 to 48/i, while from June to September it was from 28.5 to 21 //. In Table I. it will be seen that the longer- spined forms which I have referred to A. cochlearis var. macracantha and var. typica occur in the plankton from January to May 3 1 , and then disappear, returning again, in small numbers, October. 25. The short-spined variety referred by me to A. cochlearis var. stipitata and the spineless var. tecta are, on the other hand, continued during the summer. The natural result would be that the average length of the spines in the species as a whole would fall during the summer months. It is apparent that this tendency on the part of A. cochlearis to become shorter and smaller during the summer months does not bear out the contention of Wesenberg-Lund ('98) that winter individuals are smaller and summer ones larger among perennial rotifers. He reports var. tecta as "die Hauptform des Winters " in several Danish lakes, and the variety with a long horn as a summer form, found in July-August. Of these varieties, macracantha, typica, and stipitata intergrade in our waters with numerous connecting links, while var. tecta is not connected with the other forms by many individuals with inter- mediate characters. Lauterborn ('98) also notes the greater inde- pendence of this variety in the waters of the Rhine. In Table I. the seasonal distribution of these three varieties, the .long-spined (typica and macracantha}, the short-spined (stipitata), and the spineless (tecta} are given separately. It will be noted that the long-spined form has the distribution above mentioned, that var. tecta runs throughout the whole year, and that var. stipitata is absent in midwinter and is a common summer form. The relative numbers of the varieties fluctuate in different years. For example, var. tecta was relatively but one fourth as abundant in 1897 as in 1898. As shown in Table I., whenever coincidently present in the plankton all the varieties respond to the causes which produce the rhythm of occurrence, the rise, culmination, and decline of the pulses being much alike in all of the varieties. About three eighths of the females noted in 1898 were ovigerous, carrying as a rule but a single egg. Instances of two eggs were 153 noted, but they are rare. The greatest proportion of egg-bearing females appears during the rise of the pulse, as is seen in the follow- ing table, which gives the data of the vernal pulse in 1898. From ANUR^EA COCHLEARIS. Date No. of ovigerous females Total females Total eggs Ratio of eggs to individuals No. of dead April 12 800 2,200 800 1 2 75 April 19 6,400 15,200 8,800 1 1.73 400 April 26 45 000 137 800 65 000 1 2 12 3 200 May 3 . 536,000 1,022,400 552,200 1 1.85 9 600 May 10 489 600' 1 145 600 643 200 1 1 78 99 200 May 17.... 110,400 434,800 160 000 1 2 71 100 000 May 24 6,000 21,200 7,200 1 2.94 1,800 May 31. 3 000 11 200 * 3,400 1 3 29 1 800 April 12 to the crest of the pulse on May 10 (not inclusive) the aver- age ratio of eggs to individuals was 1 to 1.87. From the crest to thp foot of the decline inclusive the ratio is 1 to 2.98. The number of empty loricus is given below, and it will be noted that on the week prior to the crest of the pulse there w r ere 107 living to one dead ; on the crest itself, one to twelve ; while the week following the crest of the pulse there was an empty lorica for every 4.3 living females. Rapid multiplication thus attends the rise of the pulse and rapid destruction its decline. Parasites were very rarely observed in this species. The decline of a pulse is thus due to the cessation of reproduction and a relatively heavy death rate. Apstein ('96) finds that in Lake Plon Anuraa reaches its maxi- mum in July and is at its minimum in April. It is everywhere common in the German w r aters. A. tecta, on the other hand, was found only in the smaller lakes and in great numbers, replacing cochlearis in warmer months to some extent. Lauterborn ('98) regards it as the most abundant rotifer in the Rhine. Our statistical records do not show that this is the case in the Illinois, for it is here 154 surpassed by several other species. Zimmer ('99) finds that this species is the most common winter rotifer in the plankton of the Oder, with a maximum in the spring and a predominance of var. tecta from July to September. Schorler ('00) finds it to be the most common rotifer in the Elbe from April to November ; and Skor- ikow ('97) finds it in the Udy, in Russia, throughout the summer in great numbers, but surpassed by Synchczta, Polyarthra, and Brachionus angularis. The variety tecta greatly exceeds var. stipitata in these waters. Seligo ('00) finds it throughout the year in Prussian lakes near Danzig, with a maximum in May. There are indications, in his data, of recurrent pulses during the summer, but his interval of collection is too great to follow their history. Burck- hardt ('OOa) finds it throughout the year in Swiss waters, with its single maximum in August. Jennings ('94, '96, and '00) reports it in the summer plankton of Lake Michigan and Lake Erie and of inland waters of Michigan. Anuraza hypelasma Gosse. Average number of females, 2,390; of eggs, 1,917. This species has a very definite limitation to a period extending from early in June to the first days of November. There are but two records outside of these limits a single female and egg on Jan. 11, 1898, and another upon April 19 of the same year. The probabilities of occurrence in very small numbers at all tempera- tures is thus indicated. The following table gives the data of pulses and temperatures. All of the pulses save one occur at temperatures above 70, and with this exception the species declines rapidly and disappears shortly after temperatures pass below 60. It is plainly, in our waters, a summer planktont, with its optimum temperature close to the summer maximum. This species takes no share in the vernal pulse, and there is no satisfactory evidence of any fluctuation corresponding to it at any other season. There are three or four pulses in each summer, and the species is apparently poly cyclic, for winter eggs were found in 1898 either at the maximum of the pulse or the week or fortnight following. Thus 24,000 winter eggs were recorded on Sept. 27, 1898, the date of the maximum of the Septem- ber pulse. The parthenogenetic eggs preponderate during the rise of the pulses in a very marked manner in this species. For example, in this September pulse 55,400 eggs were recorded during its rise to 500 during its decline. . In like manner, in the case of the 155 PULSES OF ANUR/EA HYPELASMA. VpvQf First record I Dulses Date Temp. Date Temp. No. 1896 June 2 7 80 June 2 7 80 1 ,200 1897 June 28 75 July 14 79 10,400 1898 June 14 83 June 2~1 77 9,600 Year Pulses Last record Date Temp. No. Date Temp. No. Date Temp. 1896 Aug. 15 " 29 81 74 2,000 3,600 Sept. 30 58 1897 Aug. 31 80 20,000 Oct. 5 71 23,200 Nov. 2 55 1898 Aug. 16 77 16,000 Sept. 27 Oct. 18 73 52 43 , 200. 13,500 Nov. 1 45 August pulse 15,200 eggs were found on the rise to 4,000 on the decline. The location of the pulses of A. hypelasma is of special interest. It will be seen in Table I. that they occur in 1898 in the same col- lections in which the pulses of the other species of Anuraa and many other rotifers occur, or in collections but a week removed. They coincide in general with dates of the ploiman maxima noted in the opening discussion, and exhibit the same correlation with hydro- graphic conditions and intercalation with the pulses of chlorophyll- bearing organisms which were noted in the general discussion and have been found in preceding species. The comparison with Anur&a of the cochlearis group affords a curious instance of an entire sup- pression (Table I.) of one species of a genus (cochlearis} in the month of August and the occurrence of a normal pulse in another (hypelas- ma}. Comparison of the distribution of cochlearis in previous summers would lead us to expect a cochlearis pulse in August, 1898, 156 but none appears in this interval, while hypelasma runs a normal course of recurrent pulses throughout the summer. This August pulse of hypelasma (Table I.) culminates August 16, just a week after the symmetrical and well-defined pulse of chlorophyll-bearing organisms (PI. II.) of August 9. With a single exception, all of the pulses of 1896 and 1897, indi- cated in the table, fall a week later than, or coincide with, the pulses of chlorophyll-bearing organisms, as in 1898 t This species has not occupied a prominent place in the literature of fresh-water plankton. Weber ('98) finds it rare in Swiss waters in the summer. Lauterborn ('93) classes it with the monocyclic summer forms in the plankton of the Rhine, though he states in a footnote that he had" found winter eggs once in June. It is probably poly cyclic in our waters. Skorikow ('96) finds it in the summer plankton of the river Udy, in Russia, but it is not mentioned by other investigators of the potamoplankton of Europe. Apstein ('96) does not report it from Lake Plon. Asplanchna brightwellii Gosse. Average number, of adults 2,079, of eggs, 396; averages in 1897, 16,161 and 2,156. This is a poly- cyclic perennial planktont in our waters. It has been found in every month of the year, but the greater numbers and more con- tinuous occurrences lie between May 1 and October 30. In 1898 (Table I.) all but 200 of the 108,120 recorded, lie within these limits, and all but 260 above 60. In previous years approximately the same limits are found. The following table gives the data of pulses and temperatures. PULSES OF ASPLANCHNA BRIGHTWELLII. Year Date Temp. No. Date Temp. No. 1894 1895 June 19 June 27 80 80 6,678 1,600 1896 May 1 70 1,788 1897 1898 May 5 60 20,800 June 21 77 1,100 157 PULSES OF ASPLANCHNA. BRIGHTWELLII Continued. Year Date Temp. No. Date Temp. No. Date Temp. No. 1894 July 30 82 19,398 1895 July 29 75 1,344 Aug. 12 79 118,206 Nov. 14 45 1,725 1896 Aug. 2 1 79 1,200 1897 July 21 83 3,200 Aug. 10 81 5,200 Sept.- 9 . 80 284,000 1898 Aug. 2 79 23,200 Aug. 23 81 4,000 Sept. 27 73 6,400 It will be seen from this table that all the pulses save one, and that one (Nov. 14, 1895) poorly denned, lie between 60 and the maximum temperatures, indicating an optimum near the summer maximum. There is in this species no prominent vernal pulse such as that found in Anurcea, and the highest numbers were reached during the height of the warm season. The evidence of the polycyclic character of the seasonal distribu- tion of this species is shown in the following table, which gives the occurrences of ovigerous females, males, and winter eggs in 1898. It will be noted that ovigerous females are more numerous during the rise of the pulse; that the males appear just before, during, and after the culmination of the pulse ; and that winter eggs are absent only during the rise of the pulse, and appear at or after its culmina- tion and during the decline. The data given afford a fine illustration of the seasonal distribution of polycyclic rotifers, and of the relation of the sexual cycle to the number and character of the representa- tives of the species in the plankton. The growth of the pulse .results from a rapid succession of parthenogenetic generations in the course of about two weeks, and it culminates with or shortly after a pulse in the food supply. The decrease in food supply is attended by the appearance of males and winter eggs, a decrease in ovigerous females, and a decline of the species. With the recurrence of the food supply the parthenogenetic cycle again begins. The same course of events is run in each recurrent pulse. Food supply rather than temperature seems to be the determining factor in this rhythm. 158 ASPLANCHNA BRIGHTWELLII. Date Males Females without eggs Ovigerous females Winter eggs May 3 3,200 12,800 " 10 8,000 4,800 8,000 1 600 " 17 1 600 5 600 4 000 100 "24 400 200 " 31 200 400 June 7 ... 200 14 21 800 300 28 100 Tulv 5 120 40 120 12 o o 19 40 240 " 26 240 12 400 5 260 60 August 2 4 000 7 200 12 000 5 600 9 80 800 16 800 800 23 3 200 800 60 30. . . ... 1 600 800 1 600 September 6 o 13 o 20 540 600 27 3 200 3 200 October 4 500 1 000 11 1 000 500 159 An examination of the location of the pulses of Asplanchna brightwellii shows (Table I.) that in 1898 one coincided with the pulse of chlorophyll-bearing organisms (PI. II.) and the remaining four followed it either in a week or fortnight. In previous years two pulses coincide with and five follow those of chlorophyll-bearing organisms, and a single ill-defined one (Nov. 14, 1895) precedes. This species is not wholly herbivorous in its feeding habits. Codonella, Difflugia, and even other rotifers such as Brachionus and Anur i -i M ^ "* *nJ i: 5 | ON CN OO PHC 'd I^ CN ~H CN ^ ""' CN <^ -i t/3 bo bo rt CN CN CN "s B ^ ^x, (U CN CO (U ON NO IO 00 ON ^ N_^T ^-H CN NO CN CO f- * CO CN *-l ** ' l^ NO T}< NO CO ^ ^O bo r-~ ^f NO in CO 00 V> bo O\ TH f-< *-< ^f* ^ 0) 2 Tf to Ci> NO ^H O CN 00 (^ 1 o xy-> NO ^t* O -^^ NO CN *-n '-l *-* CN ^ to . co NO t-- O to ^ s bo 1^ CN CN "* ON NO '!' bo OO '-I CO IO ON M 8"^ O CO O t~ NO O CN CO ^ O NO 00 NO "C "to ON ^ ^n 03 cn bo bo O O O -r* CN CN CO o g aj to O O O O 00 -< **t ** ** I* ** H 169 170 The reducing effect of the recurrent floods of 1896 may be traced in the smaller numbers recorded in this year; and the larger num- bers of 1897 may be referred to the more stable conditions then prevailing. The very small numbers of 1898 may also be due to disturbed hydrographic conditions of that year. The number is much smaller than in 1896, when the hydrograph was even more disturbed, but in this latter year there was more run-off of impounded backwaters during the occurrence of B. bakeri,axuA this would tend to favor their appearance in channel waters. The occurrences and numbers of this species (as a whole) are everywhere somewhat irregular, so that pulses of occurrence are somewhat ill defined. Several such pulses are indicated in 1898, and others recur in the records of previous years. As suggested by the data of 1898 (Table I.), the several varieties share in these pulses. The evidence upon this point is much more striking in other years, when numbers are larger. For example, in the following table note the pulse of 26,800 on August 23, 1897. o ^j to Date I to .S ^ to Total to o s to to ^> S 8 p S fc jt E to . -o O to ** -o * Aug. 10 o o 200 200 " 18 1,200 200 600 2 000 5 200 7 400 " 23 o 7 800 1 800 3 400 2 200 11 600 26 800 " 28 200 400 200 1 000 1 800 In their location these pulses exhibit as a rule the same relation of coincidence or sequence to the pulses of chlorophyll-bearing organ- isms noted in some other species, and they frequently coincide with those of other Ploima, but not always. This is perhaps the most variable of the rotifers of the plankton. At least its variations affect the fixed processes of the lorica and are thus quickly and easily appreciated. The species, in common with 171 B. pala, B. angularis, and probably B. urceolaris, has a variety in fact, several varieties with two posterior spines which are usually symmetrically placed but not always symmetrically developed. The form without posterior spines (var. dumorbicularis Skorikow) inter- grades with these, and a series might be formed with complete intergradations linking this in turn with var. rhenanus Lauterborn, in which the spines are but slightly and often unequally developed. From this we pass, by a slight elongation of the posterior spines, to var. brevispinus Ehrbg., thence to the type in which- the spines as figured by Rousselet ('97) are directed .posteriorly with but slight curvature. From this we may pass toward variants in which the symmetry is preserved, but the spines are much elongated and curved outwardly. The anterior spines in such individuals are also more elongated and exhibit a similar outward curvature (var. melhemi Barrois and v. Daday). Extreme types of this curvature sometimes occur (J5. falcatus Zach.). In another direction we find the bilateral symmetry of the processes, both anterior and posterior, to some extent lost as a result of differences in the curvature of the spines (var. tuberculus Turner). There are also differences in the surface markings of the lorica which have been utilized as specific distinctions. Kertesz ('94) describes as B. granulatus a species with a minutely pustulate surface, and Turner's B. tuberculus takes its name from this same feature. It seems questionable, however, if these surface markings are even of varietal value. Individuals without spines, in which the transverse diameter is relatively large (var. obesus Barrois and v. Daday) , are also found. In assorting the individuals belonging to this variable group I have arranged them under the following heads: bakeri O. F. Mull., bidentata Anderson (non bidentatus Kertesz), brevispinus Ehrbg., cluniorbicularis Skor., melhemi Barrois and v. Daday, obesus Barrois and v. Daday, rhenanus Lauterborn, and tuberculus Turner. The number might have been increased. The individuals referred to var. melhemi include many if not all of the long-spined specimens such as Rousselet ('97) has referred to the type, the latter designa- tion having been given to individuals intermediate between this and brevispinus. The variety tuberculus includes the asymmetrical individuals, regardless of the surface markings. I will now briefly compare the seasonal distribution of these varieties and note any peculiarities which mark them individually : 172 Brachionus bakeri O. F. Mull., type form. Average number, 2. As shown in table on p. 1 93 (MS.), this form is much more abundant in previous years though it is relatively rare, ranking sixth in the list of seven forms recognized. The most of the records fall prior to the middle of August, and it seems to be an early rather than a late summer form. Brachionus bakeri var. obesus Barrois and v. Daday. Average number of females, 41 ; of eggs, 62. The proportion of egg-bearing to non-egg-bearing females 2 to 3 in all records is larger than in any other variety. It seems probable that the lateral expansion which marks this variety may be only the result of rapid reproduc- tion. In common with most of the other varieties this one occurs at the time of the pulses, but it is last in the list of seven, and the numbers are too small to trace its seasonal preferences with cer- tainty. Brachionus bakeri var. bidentatus Anderson (non Kertesz). Found once August 5, 1895, at 78. Brachionus bakeri var. cluniorbicularis Skor. Average number of females, 90; of eggs, 95. This also was more abundant in all previous years. This variety is, next to tuber culus, the most abundant of the varieties in our plankton. The two stand at opposite extremes of the series of varieties, the former being least modified, and the latter most, especially in the direction of asym- metry. It includes about one third of all the individuals of the species. The ratio in the grand total of females to eggs carried 11,708 to 5,976 is somewhat less than the average in the entire species. This variety is distributed throughout the whole seasonal range of the species with no marked predominance in any particular part of it. It is wholly absent in the early summer of 1 897 , but very abundant in late summer of that year, though not in other years. The autumn of 1897 was one of long-continued high temperatures (Pt. I., PL XL), and under those conditions this variety constituted two thirds of the individuals belonging to the species. If we add to it the representatives of rhenanus, obesus, and brevispinus we have a total of 15,400 individuals with no posterior spines, or with spines but slightly developed, in contrast with only 2,200 with such well- developed spines referred to varieties melhemi and tuberculus. The conditions of temperature were those in which according to the 173 hypothesis of Wesenberg-Lund ('00) we should expect a predomi- nance of the long-spined forms. Brachionus bakeri var. rhenanus Laut. Average number of females, 118; of eggs, 138; but more abundant in previous years. This is the third in numbers on the list of seven varieties, being surpassed only by duniorbicularis and tuberculus. It includes about one sixth of the individuals referred to this species. ' It is found throughout the whole range of the seasonal distribution of the species and exhibits the same peculiarities noted in-cluniorbicularis, to which it is very closely related. The proportion of females to eggs noted in this variety is very large; 5,284 to 5,485 in the grand total. Brachionus bakeri var. brevispinus Ehrbg. Average number of females, 795 ; of eggs, 390; but somewhat more abundant in previ- ous years. It was found throughout the whole seasonal range of the species, but not quite so abundantly in the latter as in the earlier half of the summer, resembling in this particular the type. The number of eggs carried in this species is in relation to the number of females less than usual 3,906 to 795. Brachionus bakeri var. melhemi Barrois and v. Daday. Average number of females, 49; of eggs, 49. More abundant in previous years, especially in 1894, when it constituted over a fifth of the individuals (25,764) in the largest pulse recorded for the species as a whole 122,958 on July 30. In the aggregate in all years it includes only about a ninth of the individuals referred to the species. This form was originally described from Syria, but it is found in great perfection in our plankton, even in the extreme type described by Zacharias ('98b) as B. falcatus. It occurs throughout the whole seasonal range of the species, its distribution being somewhat similar to that of tuberculus. I do not find any constant tendency limiting its occurrence to any part of the seasonal range. Brachionus bakeri var. tuberculus Turner. Average number of females, 155 ; of eggs, 42 ; but very much more abundant in previous years, especially in 1894, when it constituted almost half (55,332) of the largest pulse of the species (122,958). This, the most divergent of all the varieties, constitutes over a third of all the individuals referred to the species. It occurs throughout the whole seasonal range of the species, though the larger numbers were found in 1894-97 in the earlier part or middle of the summer. I find nothing 174 in a comparison of the seasonal distribution of these more decidedly spinous varieties of B. bakeri with that of the smoother forms, such as cluniorbicularis , which indicates any correlation with temperature conditions of a nature to support Wesenberg-Lund's suggestion that the elongation of the processes of plankton organisms arises in response to the lessened buoyancy of the water during higher tem- peratures. Forms with and without such processes are found among the varieties of this species, and both occur indiscriminately through- out the whole range of seasonal occurrence, and, so far as I can see, the statistical data of their distribution with respect to temperature afford no evidence of a correlation of spinosity and high tempera- tures in this species. Other factors doubtless enter into this problem and obscure this response if it exists. B. bakeri is everywhere widely distributed in fresh water. Its occurrence in the plankton of open waters has not, however, been a matter of frequent note. In fact there is some reason to think that it is largely confined to shallow warm waters where vegetation is close at hand, or where at least the flagellates and smaller algae abound, as they do in water fertilized by decaying vegetation or other organic matter. There is, it seems, no reason for regarding this species as merely adventitious in our plankton. It bears all the characteristics of a true limnetic organism in our environment. Its presence in the plankton is not due to floods or other disturbances which might carry it from a littoral region into the open water. It exhibits characteristic pulses, and is found everywhere in summer in company with typical planktonts in open water. Zacharias ('98) records it in some German ponds and streams, and Weber ('98) in Swiss marshes in the warmer months. Stenroos ('98) also finds it in the summer plankton of littoral and open waters in the shallow Nurmijarvi Lake in Finland. Jennings ('00) reports it as one of the commonest rotifers in East Harbor, Lake Erie, and in the swamps on the islands. In land-locked pools short-spined varieties were found, and in swamps the long-spined. Speaking of this difference, Jennings says " Possibly the different form found in these pools is due to the greater concentration of various salts in this water or to some kindred factor." In our own region both varieties occur at the same time in the same environments, channel and backwaters alike, and such factors as Jennings suggests to explain the appearance of the varieties cannot well be operative here in 175 channel waters. Schorler ('00) reports the species as sporadic in the Elbe, and Skorikow ('97) finds both B. bakeri and its variety brevispinus sparingly in the Udy in summer months. This species in common with other Brachionida: was infested by Bimcerium hyalinum Przesm., and occasionally by a filamentous fungus-like growth. Empty loricae were wont to appear with the culmination of a pulse and subsequently. No males were identified as belonging to this species, and attached male eggs were recorded only late in September, 1897, at the close of an unusual pulse. They were found on var. cluniorbicularis and rhenanus. Females with winter eggs were not at any time recorded for this species. It may be that some of the free winter eggs referred to the genus Brachionus (Table I.) belong to this species. The recurrent pulses are similar to those of known poly cyclic species, and we may infer the probability of such a phenomenon in B. bakeri, though conclusive proof of its occurrence is not found in the statistical records. Brachionus budapestinensis v. Daday. Average number of females, 4,211; of eggs (carried), 740. This is one of the most sharply defined species of Brachionus and a typical planktont of open waters. It has, moreover, a sharply limited seasonal distribu- tion in which it is apparently poly cyclic. The appended table gives the dates and temperatures of appearance and disappearance and the pulses in the several years. In the main, the period of occurrence is practically from the end of June till the early part of October and above 60. A record in May, 1896, and an isolated one in December of the same year, indicate an extension of this period, but such occurrences are rare and irregular and the numbers small. This abrupt decline in 1898 as temperatures pass 60 (PI. XII., Pt. I., and Table I.) is paralleled in previous years. The normal seasonal routine seems to be as follows: The species reappears in the plankton in May- June at 70, rising slowly to its first pulse (average, 26,104) in July, with a larger pulse (average, 184,453) in the following month during the maximum heat, and a much smaller one (average, 10,044) in Sep- tember, followed immediately by an abrupt decline. The average temperature of the larger pulses lies close to the season's maximum, while the latest pulse, at the lower temperature (72.2) averages but 10,044. These data all indicate that this is a midsummer planktont, with its optimum temperature near the summer's maximum. The 1 i_, 1 00000 CN ro *-i OC CN ^ . t^* ro O ^O CN ^j ' -(-> h ' -(->' . -I-J P CU CJ ^ CU ^ CD co c/2 o W 6 *o o ^o o o * g CO CN O CN O ^r H rj* lo Tt* ^O CN 00 t/3 CN - r^ CO o- OO H *"- w ~H ro rj< r^ <5 P CD CN CN CN ^-H O aS bo r^> ^"ex P O > . O CN \O &H O 2! to CO ^O X~** IO NO CO w *-H -H ro 00 CN C/3 _) & ex i O O O O O o co O OO ^ < "^t 1 -^ O O. CD CO CO t^- 00 00 CO * I f" 1 CO ON CO T ~ H CN O ON $ CN rH <~O '-H P ) 1 i^ CD >^ CD C C fT" C ^T" C 33 yS 3 3 ro Cj i > i i Si i i > i i CD P be _C ii 4 'M 4-> a a ) I ( CD' to O * ^ IO vO t CO G "i ON ON ON ON ON P 00 00 OO 00 00 <; 176 177 relation of hydrographic conditions to the relative development of pulses in different years is seen on a comparison of the record for 1896 and 1897, the former (Pt. I., PI. X.) being a year of recurrent floods and the latter (Pt. I., PI. XI.) one of stable conditions through the greater part of the seasonal distribution of the species in question. The average numbers in these two years were 3,105 and 31,306, respectively, and the average amplitude of the pulses 18,250 and 97,200, showing, respectively, a ten- or five-fold increase in the latter year. The extension of the heated termjnto September in 1897, is reflected in the large September pulse (552,000) and in the extension of the period of occurrence into October. The locations of the pulses of Brachionus budapestinensis in 1898 correspond with those of the Ploima in general. They likewise coincide with or follow those of the chlorophyll-bearing organisms (cf. PI. I. and II. with III. and IV. and Table I.). Similar relations are apparent in 1896 and 1897 but are less evident in prior years. They suggest an interrelationship of the pulses in this species with the fluctuations in the food supply. Males, male eggs, and winter eggs were not recorded, but the recurrent pulses in this species are so similar to those in other rotifers in which the evidence of the occurrence of sexual reproduction at the culmination of each pulse has been found, that the inference may be made that this species likewise is poly cyclic in our waters. Females carrying one or two summer eggs have been found in greatest abundance during the rise of the pulse, and only in small numbers, if at all, during its decline. This species is subject to some variation in the development of surface ornamentation, in the ratio of width and length, and in the curvature of the median spines. It is usually somewhat more slender than figured originally by v. Daday ('85) or even by Hempel ('96) , who described a form somewhat more slender than that figured by v. Daday, as B. punctatus. Shortly afterwards Skorikow ('96) described the same species as B. lineatus from Russian waters. The name given by v. Daday has, priority, and as neither the Russian nor the American forms are to my mind well enough set off to merit even varietal distinction, I have used the name given by v. Daday, and have included under it both wide and narrow forms and those with incurved or outcurved median spines. The fact that their common record of seasonal distribution forms a seasonal curve of 178 typical character is corroborative of the view, though not conclusive, that we are dealing with a single species and not with several. This species has not been widely reported in the fresh-water plankton. It is evidently a planktont of warmer waters, and for that reason may have escaped notice, since the cooler waters have been the more thoroughly explored. Thus it was not found by Weber ('98) in Swiss waters in his thorough explorations about Geneva, nor by Jennings ('94, '96, '00) in the Great Lakes or inland waters of Michigan. It has, however, been recorded by Skorikow ('97) in the plankton of the Udy River, in Russia, where it was exceeded in number by only two species of its genus, B. pala and B. angularis, ranking tenth in numbers among all the rotifers. His data of frequency from July to October suggest several recurrent pulses. It has likewise been found by Lauterborn ('98) in the plankton of the Rhine, where he classes it with the stenothermal planktonts. Zacharias ('98) finds it in ponds near Leipzig, and it was originally described by v. Daday ('85) from Hungarian waters, and again noted there by Kertesz ('94). Fuller exploration of the summer plankton in warmer regions will doubtless extend the record of its range. Brachionus militaris Ehrbg. Average number of females, 147 ; of eggs (carried), 98. In previous years the species was much more abundant, the averages in 1897 being 1,412 females and 523 eggs, and in 1896, 1,288 females and 576 eggs. This greater development in years prior to 1898 is evident in many of the Brachionidcz. The following table gives the dates of first and last records in each season, and the location, temperature, and amplitude of the pulses in the several years. This is evidently a summer planktont with well-defined limits. These limits appear much less evident in 1898 (Table I.) than in prior years. In 1896 and 1897, for example, the species is almost continuously present in the plankton from the time of its first appearance until the last record for the season. All of the records save two lie above 70, and the average temperatures at which the pulses occur are all at or above 80. Its optimum thus lies near the summer maximum. The lower limits are not definitely established owing to insufficient collections in periods of rise and decline, but they seem to lie near 70, with small numbers lingering to 60. a 1 O O O O O O 00 ON 00 *-i O co 0) f--* r> 10 t~* \O t^* o o H u ^p O O ^-H ^O t^ CU ^ O C/2 Cfi C/3 C/i O C/3 6 10 O O O CN \C O O r~- Tp O fN 1 H o o o o 00 *-i O CN t~- 00 00 c*0 Tf CN *-l C HJ a3 Q OH OH M g) C/2 W ^ << ^f O O O CN Tp O O \O ON CN OO ^H LO t^ Tp C/3 OH a O O O O 00 ON CN -H Jf - 00 00 PH H ^-i CN 10 O js a Q bb_ bi bi O O o O o ^H O O O CN 6 *H 1 >!>-.>-. Q 3 3 : "3 "3 2 e o o o o o OO O CO OO Tp o ^ >.. c rJ "3 ^ 1 1 1 > 1 > 1 1 s 3 r 3 H ** -LD MO t^ 00 O> Q\ Q\ QN QN OO 00 CO 00 CO (13) 179 180 This species has never developed large pulses in the channel waters of the Illinois. Hempel's statement ('99) that it is "the most abundant species of the genus" can apply only to certain collections in vegetation-rich backwaters, for in the river it is sur- passed in the totals of occurrences in the statistical records by eight other forms of firachionus, namely, variabilis, pala, amphiceros, dorcas, rubens, budapestinensis, duniorbicularis, and tuberculus. I found it in very great abundance in the July-August plankton of Crystal Lake, a shallow warm pond rich in vegetation, formed by damming a small creek tributary ta the Wabash system, near Urbana, 111. From the relatively small numbers, the slight ampli- tude of the pulses, and their somewhat irregular development I am inclined to think that the centers of distribution of this species are not in the open water of the river and its backwaters, but more in the vegetation of warm, shallow regions such as the margins of our bottom-land lakes. It is thus to some extent adventitious in our plankton. The pulses of this species are relatively so small that they do not contribute an appreciable amount to the total ploiman pulses, nor do more than 50 per cent, of their number coincide with such general pulses, though they are sometimes found during their rise. The greater part of them coincide with the pulses of chlorophyll-bearing organisms (PL I. and II.), suggesting a food relationship. This species is one of the best-defined in the genus, though in the character of its asymmetry it varies toward B. bakeri var. tuberculus Turner. It exhibits some variation in the degree of asymmetry, in the curvature of the spines, and in the surface markings. The indi- cations of pulses suggest a poly cyclic habit, but no evidence in the way of males, male eggs, or winter eggs was recorded which will substantiate the inference. A female carrying a winter egg w r as found Sept. 21, 1-897, at the close of the period of occurrence. Fe- males with one, two, or three summer eggs were found throughout the summer and in somewhat larger numbers during the rise of the pulses. Brachionus mollis Hempel. Average number of females, 137; of eggs, 10. More abundant in previous years, the average in 1897 being 1,092 and 277, and in 1896, 428 and 56. This likewise is a summer planktont. The earliest record of its appearance in the plankton is June 17, 1896, at 76; and the latest, 181 October 17, 1894, at 58. With but two exceptions the species was taken only above 70, and the period of most continuous occurrence and largest numbers is near the summer maximum of 80. The optimum is thus near the summer maximum. This species was never taken in the plankton in large numbers, the greatest being on Sept. 14, 1897 (20,000), at 84. On account of the small numbers .and somewhat irregular occurrences the phenomenon of recurrent pulses is here less apparent than it is in more abundant species. The appended table records the best-defined ones. Thesejpulses share in the general ploiman pulses in only about 50 per cent, of the cases, and the most of them coincide with or follow shortly after the pulses of chlorophyll-bearing organisms. PULSES OF BRACHIONUS MOLLIS Year Date Temp. No. Date Temp. No. 1895 July 6 81 742 Sept. 5 75 954 1896 July 18 79 1,200 Aug. 21 \ 79 8 , 400 1897 July 30 85 1 1 , 600 Sept. 7 80 20,000 1898 Aug. 23 81 800 Sept. 27 73 4,800 So far as I am aware this species has not been found in other waters than the Illinois River and its adjacent backwaters. Hempel ('99) reports it as most abundant in the marshy environment of Flag Lake. Brachionus pala Ehrbg. Average number, including all varie- ties: females, 19,969; eggs, 25,974. The following table gives the average number, in the several years, of the varieties here included, and it will serve to show their relative frequency. This is the most abundant species of the genus in our waters, the grand total of all occurrences exceeding 9,000,000. As a whole the species was much more abundant in the stable year 1897 (180,998), and less abundant, all things considered, in the disturbed conditions of 1896 (36,665). As a whole the type form pala is less abundant than amphiceros. It forms but 28 per cent, of the total, as compared with 68 per cent, included in the latter variety. Dorcas forms less O 00 CO ON 00 ro 1 H *-i CN ON O OO CO (- c/3 VO ON ^ CM ON ,_! +3 bjQ , O kJO ^f t OO VO ( r . (^) CO vO CM fij rt 3? rO OO vo OO J^ 'i CM vo ON VO CN CO vO ON ON VO 00 "o '^ ^H VO VO O ON ^f ^H ^^ ^ CO CO '~~ l 00 9 CM V! bo bo O CM I O CM '-H CM -f 10 | w o s ' o O 00 O O 10 ON O Tf CM o r^ 00 O bo p 9 ^ * Includes free eggs of other varieties also. culmination of a pulse is very large. For example, the sudden ver- nal pulse of 716,982 on April 24 is accompanied by 28,584 free winter eggs. The pulse declines to 22,224 on April 29, and the free winter eggs rise to 95,841, and the empty loricas to 26,114. Females carry 1-5 summer eggs, and 1-8, or even more, male eggs. There is great variation in the size of the summer eggs, these and -the male eggs appearing almost to intergrade. Brachionus pala, including B. amphiceros, is a common constitu- ent of the plankton of shallow warm waters. It has not been reported from the larger and cooler lake waters by Apstein ('96), Burckhardt ('00 and 'OOa), or Jennings ('94, '96, and '00). Zacha- rias ('98) and Marsson ('00) find it in the summer plankton of smaller lakes and ponds in Germany. Seligo ( '00) records it from April to October, with a maximum in August, in Prussian lakes ; and Lauterborn ( '98a) finds it to be perennial and polycyclic in -the Rhine. Schorler ('00) reports both pala and amphiceros from the Elbe, the former being abundant in May and sporadic during the summer, while the latter was abundant in April, June, and Septem- ber, and rare at other times during the warmer months. Zimmer 189 ('99) finds amphiceros in the Oder, where it appears in April and. increases until the end of August or the first of September, when it is the most abundant animal in the plankton. In no one of these instances was the examination so long continued or made at such short intervals as in the case of the exploration of the Illinois. The diversity exhibited in these different waters may be paralleled by the fluctuations from year to year in the Illinois, and from all the data it may be inferred that the organism is probably perennial and polycyclic, the number of pulses depending upon local conditions, primarily of the food supply. Brachionus pala var. amphiceros Ehrbg. Average number of females, 17,071; of eggs, 5,103. The numbers were much larger (158,299 and 35,392) in the stable conditions of 1897, and still smaller (5,430 and 715) in the disturbed conditions of 1896. The seasonal distribution of this variety with respect to that of the type constitutes the chief point of interest in the records. It is present throughout the whole range of temperatures, shares in the vernal and autumnal pulses noted for the species as a whole, but constitutes a much greater proportion of the amphiceros-pala group during the warmer months than it does in the colder ones. Thus, as shown in the accompanying table, the proportion which amphi- SEASONAL DISTRIBUTION OF BRACHIONUS PALA AND B. PALA VAR. AMPHICEROS. Year June 1 to Oct. 1 Oct. 1 to June 1 pala amphiceros pala amphiceros No. \-> j-> G PH g No. t-i -4J & C PH g No. IH w cu c PH g No. t-c --> bo W O O t^ O O CN O CN 'e 1 o to T^I t- o oo ro *-H VO u ( oi I '< > O I-H ro O CN c, O c ) i co ^ ro to C ) cd OJ 4-> O EH ^ to vo t^ OO ON ON ON ON ON 00 00 00 00 OO 196 197 ber 1, but is continuously present in the winter of 1898-99 from December 6 till March 28, when collections ceased. Male eggs were recorded but once April, 29, 1895 and there is no other evidence of the cycles of reproduction beyond the pulses in numbers. They suggest a polycyclic habit with major pulses in spring and fall. It is apparent that conditions affect these cycles greatly, as is seen, for example, in the contrast between the earlier years, with low water in the spring, and the later ones, when high water was longer continued. This variety, rubens, has not been widely reported in the plank- ton. Skorikow ( '96) finds it in June in the River Udy, and Kertesz ( '94) reports it from Budapest, while Stenroos ( '98) finds it in the littoral fauna of Lake Nurmijarvi in Finland, and also in the plank- ton in July and August. Brackionus urceolaris var. bursarius Barrois and v. Daday. Average number of individuals, 206; of eggs, 33. This is a sum- mer variety, and forms but a small part less than one per cent. of the total number of individuals referred to the species. Brachionus variabilis Hempel. This species was found but once in 1898, but was more abundant in former years (see table on oppo- site page). The largest development which it attained in the Illinois was a pulse of 168,222 on August 15, 1894, at 84. The largest number in subsequent years was 5,200 per m. 3 on August 8, 1896. It may be significant of the connection of this form with the urceo- laris-rubens group that the great pulse of 1894 was coincident with an unusual development of rubens on that date. This species is a summer form, the earliest record being May 24, 1898, at 74, and the latest September 25, 1895, at 73. Its opti- mum temperatures lie near the summer maximum. If this form should prove to be merely a spinous variety of B. urceolaris it will afford another illustration of spinous varieties of Brachionus appear- ing at high temperatures, in accordance with the hypothesis of Wesenberg-Lund ('00). In Table I. there is given for 1898 the seasonal distribution of the free winter eggs of Brachionus. It will be seen that they occur throughout practically the whole year, with some increase after the times of the April-May and September pulses. Cathy pna leontina Turner. Average number, 47, in 1896, a year of disturbed hydrograph ; less abundant in previous years, and not 198 recorded in subsequent ones. Earliest record, June 17, at 76; and latest, October 2, at 63. Always present in small numbers and evidently adventitious. Cathy pna luna (Ehrbg.) Gosse. Average number, 47. Found in every month but November, though always in small numbers and irregularly. All but six of the thirty-three records fall between April 1 and October 3 and above 50. Over half of all the individu- als were found in 1896. This fact, together with the nature of the seasonal distribution, indicates plainly its adventitious character. Cathypna rusticula Gosse. Found once, March 22, 1897, at 44. Not previously reported from American waters. Ccelopus porcellus Gosse. Average number, 106. From March to September, at 37 to 80, and apparently adventitious. Colurus bicuspidatus Ehrbg. Average number, 274. This species is apparently a winter planktont. In 1897 it appeared first November 9, at 50, and was found somewhat irregularly through the winter until May 17, at 64. There is a pulse March 15, at 46, of 6,400. Ovigerous females were found during the rise of the pulse, and males on April 12, on its decline. A few scattered records were made in the following winter, beginning November 8, at 46. It occurs in the plankton during flood season and may be adventitious. Colurus obtusus Gosse. Average number, 38. In small numbers and irregularly in March and April at temperatures below 50, and in September at 73. Hempel ('99) lists also C. deflexus Ehrbg. Diglena circinator Gosse. Average number, 121, in 1896, a year when many adventitious rotifers were brought into the plankton by disturbed hydrographic conditions. All the records lie between April 29, at 70, and July 28, at 81. An ovigerous female was found in July. The species is adventitious in the plankton. Diglena forcipata Ehrbg. was recorded once October 12, 1897, at 65. Diglena giraffa Gosse was observed but once in the river plank- ton. ^Not before recorded from American waters. Diglena grandis Ehrbg. was recorded in July and September at 76 and 79. Diglena uncinata Milne was found August 12, 1898, at 82. Hempel ('99) reports D. biraphis Gosse and D. catellina Ehrbg. in waters immediately tributary to the river. All members of the 199 genus belong to the littoral fauna among vegetation, and are adven- titious in the plankton of open water. Euchlanis pyriformis Gosse. Recorded April 12, 1898, at 52. Hempel ('99) reports it from June to October in collections in the river in 1894 and 1895. Euchlanis triquetra Ehrbg. Average number, 19. Found irregu- larly from July to November at 84 to 41. Hempel ( '99) reports it also in June. It is probably adventitious. Hempel ('99) also reports E. dilatata Ehrbg. in, the river from July to September, and E. deflexa Gosse in tributary waters. Gastropus stylifer Imhof. A rotifer doubtfully referred to this species was found sporadically in the plankton of the river. It was recorded in June, 1894, and July, 1896, at temperatures above 75. It was almost continuously present in 1896 from February 20 to April 10, and again on November 17 and December 3. It did not reappear until January 31, 1899, from which time it continued present until the close of operations in March. Most of these oc- currences are at minimum temperatures and all of them below 45. I have followed Weber ('98) and Jennings ('00) in using Imhof 's name Gastropus stylifer instead of Hudsonella picta Zach. or Notops pygmcEUs Caiman, by which names the species has been frequently designated. The evidence from our records indicates that it is a somewhat sporadic winter planktont in our waters. Lauterborn ('93) finds it to be a perennial planktont in the Rhine, with its largest numbers in summer. Hydatina senta Ehrbg. was found September 20 at 73. Hempel ('99) also reports it in towings from the river in March and July, 1895. This species is very common in European waters, but has as yet been found in America only in the Illinois River and, by Kel- licott ('88), at Corunna, Mich. Mastigocerca bicornis Ehrbg. Average number, 42. Found irregularly and in small numbers from June 28 to September 13 above 63. Hempel ('99) reports it from Quiver Lake among vegetation, and it is evidently adventitious in the river plankton. Mastigocerca bicristata Gosse was found but once, late in Septem- ber, 1895, at 73, but it is more abundant in the backwaters. Mastigocerca carinata Ehrbg. Average number, 1,674. This species was present in the plankton from the middle of June till the 200 first of October, and at irregular intervals and in small numbers in fall and winter months. The distribution in years prior to 1898 falls within the limits shown in Table I. In this year the bulk of the occurrences lie between June 21 and August 4, and above 77 and 72. The optimum lies near the summer maximum, though occurrences at minimum temperatures in March and December reveal acclimatization to a wide range of temperatures. In this year there are several somewhat irregular pulses, the best-defined of which follow the pulses of chlorophyll-bearing organisms (cf. Table I. and PI. II.) at an interval of one or two weeks. The species was not recorded so frequently in previous years, in some of which also pulses are indicated. These pulses are not consequent upon floods, and the species is apparently not adventitious in the plankton but a normal constituent. Apstein ('96) reports M. capucina as abundant in Dobersdorfer Lake from June to October a seasonal distribution similar to that found in the Illinois River for M. carinata. Mastigocerca elongata Gosse was found once March 28, 1899, at 38. Hempel ('99) reports it in June in Quiver Lake. Mastigocerca mucosa Stokes was taken in August to October, 1898, at 82- 62, in small numbers. It is reported by Jennings ('00) as "one of the most abundant of the Rotifera among the vegetation of the shallow parts of Lake Erie," but it was not reported by Hempel ('99) in similar environment about Havana. Mastigocerca stylata Gosse was found -in the plankton in small numbers in June and July at temperatures approaching 80. Hempel ('99) reports it also in August. In addition to the species of this genus above listed, Hempel ('99) records M. lata Jennings. There are also in our records a considera- ble number of individuals referred to this genus but not specifically identified. Many of these belong to one, or possibly several, very small species. They are most abundant during the summer months, reaching a pulse of 16,800 on June 28. They occur in large numbers in the filter collections (average for 1898, 798 ; filter-paper, 145,384), and, it seems, must escape with ease through the silk net on account of their small size and their active movements. A number of species in this genus have been described of late from the fresh-water plankton, but in the present state of the litera- ture of the subject I am not certain to what species these forms 201 should be referred. The genus is sadly in need of critical revision. It includes a number of semi-limnetic species, whose importance in the plankton will probably be revealed by more perfect methods of collection. Metopidia lepadella Ehrbg. was found only in March and June at temperatures above 46. It is apparently adventitious. Metopidia oblonga Ehrbg. was found once July 29, 1895, at 75. Metopidia salpina Ehrbg. w r as recorded June 28, 1898, at 78. Metopidia solidus Gosse. Average number, 67. This is the most abundant representative of the genus in our plankton. It was recorded from March 15 to November 14, at temperatures above 45. Most of the occurrences are in the summer months (Table I.), at maximum temperatures. The numbers are small, the occurrences irregular, and the species evidently adventitious. M. rhomboides Gosse is recorded by Hempel ( '99) from the river plankton, as also M. acuminata Ehrbg., triptera Ehrbg., and bractea Ehrbg. from the backwaters. Monostyla bulla Gosse. Average number, 50. Present in small numbers and irregularly from April till the middle of October at temperatures above 50. It is evidently adventitious. Jennings ( '00) finds. this one of the most abundant rotifers among the aquatic vegetation in Lake Erie. It is in our waters the most abundant of the genus in the plankton, especially in the vegetation-rich back- waters. Monostyla lunaris Ehrbg. Average number, 37. Found in the extremes of the temperature range, but over 50 per cent, of the occurrences are in August-October. Its numbers are always small and its occurrences irregular. It is plainly adventitious. Monostyla quadridentata Ehrbg. Average number, 10. This species was found in the plankton irregularly in July-September, at maximum temperatures. It is abundant (Hempel, '99) in the backwaters, where vegetation is abundant, and is apparently adven- titious in the plankton. In addition to the species here recorded Hempel ('99) lists M. cornuta Ehrbg. and M. mollis Ehrbg. from collections in the river, and M. dosterocerca Schmarda from the back- waters. This is an exceedingly variable group, and will repay a thorough revision in the light of a study of the variation of its species. A considerable reduction in the number of these so-called species will doubtless result from such a study. 202 Noteus quadricornis Ehrbg. Average number, 19. This is a rare species in the plankton, being found in 1895 and 1896 in July at maximum temperatures, and in 1898, on April 12, at 52, and on November 8, at 46. Notholca longispina Kell. This species, which has been found in the summer plankton of many European and American waters, especially our Great Lakes, was noted but once in the Illinois in January, 1895 (Hempel, '99). It seems to prefer cooler and purer waters. Notholca striata Ehrbg. Average number, 437, including varie- ties. This is a winter planktont in our waters, appearing in 1897 on November 30, at 34, reaching a maximum of 10,840 March 22 (Table I.), at 51, and disappearing April 19; at 52. It reappears the following autumn on November 1, at 45, and attains a maxi- mum of 4,000 March 21, at 37. In previous years the occurrences all lie within the limits of November 1 and April 24 with the excep- tion of two records in 1895 September 5 and October 15, at 74 and 56. The spring maximum in 1896 (7,778) was on April 10, at 52, and in 1897 (4,260) on March 22, at 43. In each year but a single pulse, that of March-April, is indicated. Minor fluctuations during the winter (Table I.) are in some cases attributable to flood agencies. The temperature limits of this species are quite definitely estab- lished. The species reappears in autumn when 45 is reached, and declines rapidly in the spring after 50 is passed and is but rarely found above 60. It attains its greatest numbers late in winter or early in spring in the face of flood conditions, though the numbers attained in the channel waters are never very large. Empty loricas have been found in the plankton after the decline of the species in April, and females with a single egg were noted in small numbers in 1895 during the rise of the pulse. I follow the suggestion of Weber ('98) that N. striata should include as varieties the following: N. labis Gosse, N. jugosa Gosse, and N. acuminata Gosse. Examination of many individuals in the plankton proves beyond a doubt the great variability of the organ- ism whose seasonal occurrence we have traced. It varies in the length of the posterior spine, in the proportions of the lorica, and in the development of the striae and the anterior spines. Of a total of 81 ,227 of Notholca striata in this wider sense, 68,887 were referred 203 to var. acuminata, 3,852 to var. jugosa, 7,029 to N. striata in the narrower sense, and 1,469 to other varieties, including var.labis and var. scapha. The seasonal distribution of N '. striata (sensu strictu) and var. jugosa lies within the limits of that of var. acuminata, but occurrences are too few to trace their seasonal fluctuations. This species is reported by Lauterborn ('94) in the winter plankton of the Rhine. He also notes the connecting links between N. acuminata, N. striata, and N. labis, and regards them as belonging to the same " Formenkreis." Apstein ('96) reports N. acuminata, N. labis, and N. striata in lakes of northern Germany and indicates a seasonal distribution which coincides closely with that found for these forms in the waters of the Illinois. He also reports a March- April maximum and only isolated occurrences in midsummer. Forbes ('83) finds the species in the stomachs of young Coregonus feeding upon the March plankton of Lake Michigan. Seligo ('00) also finds it in the winter plankton of Prussian waters. Notommata cyrtopus Gosse was found in the plankton in April and September at temperatures above 50. Hempel ('99) reports A r . aurita Ehrbg. from the river, and N. tripus Ehrbg. and N. lacinu- lata Ehrbg. ( = Diaschiza lacinulata Ehrbg.) from the backwaters. Ploesoma lenticulare Herrick was found in the plankton of the river from September to December, 1896, throughout the whole range of temperatures from 75 to the winter minimum. Hempel ( '99) reports it from May to December, but principally in vegetation Polyarthra platyptera Ehrbg. Average number of individuals, 86,674; of eggs, 52,560. In 1897, 94,653 and 58,235 ; in 1896, 29,653 and 11, 138; in 1895, 28,947 and 20,074; in 1894, 743 and 217. The effect of the stable conditions of 1897 and of the recurrent floods of 1896 is seen in the larger averages in the former year and in the smaller ones in the latter. This is one of the most abundant rotifers in our plankton, includ- ing, as it does, one seventh of the total Rotifer a, and exceeding in numbers all other species of the group excepting only Synch&ta stylata. It is a perennial form, and was recorded in every plankton collection but two, and it may have been present then. The seasonal distribution of this abundant species is very char- acteristic of the form which most, though not all, plankton organ- isms exhibit. Two prominent features are (1) a limitation of large numbers to the warmer months and (2) a rhythmic occurrence of 204 recurrent pulses at approximately monthly intervals. In Plate V. I have plotted the seasonal distribution of this species for the years 1894-99. The plate will serve as one of the best illustrations of the nature of the data contained in my statistical records that could be chosen from them. It illustrates graphically the character of the seasonal distribution of this species and the nature of what I have called recurrent pulses. In the table which follows, as elsewhere in similar tables, these pulses are listed by the number of individuals attained at their maxima, and are located according to the dates of these maxima. PULSES OF POLYARTHRA PLATYPTERA. Year Date Temp. No. Date Temp. No. Date Temp. No. 1896 Jan. 6 " 25 32 33 5,406 2,736 Feb. 25 34 7.852 Mar. 24 41 57,267 Tan 9? Feb 22 39 1899 Jan. 17 33 20,800 Feb. 14 33 145,600 Mar. 7 33 71,200 Year Date * Temp. No. Date Temp. No. Date Temp. No. 1896 Apr. 24 72 233,436 May 8 76 54,365 June 1 11 69 73 18,000 35,200 1898 Apr. 26 57 696,000 May 17 64 195,200 June 14 82 432,800 Year Date Temp. No. Date Temp. No. Date Temp. No. 1894 1895 1896 July 30 July 6 July 10 " 28 82 81 80 82 1,908 231,504 90 , 000 71,000 Aug. 1 " 21 Aug. 8 79 82 86 6,350 117,513 39,200 Sept. 12 79 19,272 1897 1898 July 21 81 172,000 Aug. 24 Aug. 2 " 23 78 78 82 230,400 288,000 96,000 Sept. 14 Sept. 27 83 73 50,000 238,400 205 PULSES OF POLYARTHRA PLATYPTERA continued. Year Date Temp. No. Date Temp. No. Date Temp. No. 1894 1895 1896 1897 Oct. 17 Oct. 23 58 51 1,140 408 Nov. 27 33 74,942 Dec. 18 Dec. 29 Dec. 14 39 35 40 21,147 37,560 7,300 Oct. 5 71 816,000 Nov. 15 47 22,400 1898 Oct. 11 " 25 65 49 47,500 37,500 Nov. 22 40 6,000 Dec. 20 33 63,400 An examination of this table and the graphic presentation (PI. V.) of the seasonal distribution will show at once the uniformly small numbers attained at low temperatures. Between October 15 and April 15, that is below 60, no pulse exceeding 100,000 is reached save one of 122,400, February 21, 1899, at 33. Of all the records in this period only seven exceed 50,000. On the other hand, during the warmer months, above 60, the pulses have a much greater amplitude. Four of them exceed 400,000, and there are twenty-two records above 100,000. The summer pulses are often separated by minima which approach midwinter levels, but in spite of this the general level of summer occurrences is much higher than that of the colder season. In 1898 the average from April 15 to October 15 was 30,861 per m. 3 , and for the other months of the year, 15,813, or about half the number in the warmer season. From these facts of distribution it is apparent that though perennial the species finds its optimum conditions at temperatures above 60. The statement of Hempel ('99) that it thrives best in cold water is not borne out by the statistical examination in any of the years. The recurrent pulses of this species vary greatly in amplitude. The largest pulse recorded was that of 816,000, October 5, 1897, at 71. It appeared in a period of prolonged low water and at the close of one of high temperatures continued beyond the usual September limit (Pt. I., PL XL), in a very unusual development of Carteria and the smaller algae of the water-bloom (PL II.). Similar autumnal pulses do not appear in other years, the autumnal develop- ment as a rule not exceeding to any noticeable degree that of mid- summer. There has been in every fully tested spring a large vernal pulse, usually at the time of the spring volumetric maximum, or thereabouts. In 1896 and 1898 it was the largest pulse of the year. 206 This was not true in other years, but collections in those years were too infrequent to trace the seasonal distribution of the species with accuracy at that season. It is volumetrically of some importance in determining the quantitative fluctuations in the total plankton. Computations based on its average size indicate that approximately 600,000, including eggs, would be required to form 1 cm. 3 of plank- ton. On this basis, and allowing 10 per cent, for interstices, it constituted at the time of its vernal maximum in 1898 about 10 per cent, of the total volume of the plankton (silk-net catch). The table on pages 204 and 205 lists 43 pulses, of which 6 lie out- side of the period included in Plates I. and II. Of the 38 remaining pulses' 16 coincide in location with the whole or a part (in case of divided culminations) of the pulses of the chlorophyll-bearing organ- isms; 12 follow at the next collection, usually at intervals of one week ; and 6, after a fortnight. The remaining 4 do not bear this rela- tion, occurring in autumn or midwinter, when all pulses were feeble and ill-defined. A comparison of Plates I. and II. with V. will show that not all of the chlorophyll-bearing pulses are attended by pulses of Polyarthra; nor is there any constant relation, excepting the vernal pulse, between the size of the pulses of the two groups of planktonts in question. Nevertheless, the dependence of the recurrent periods of rapid multiplication of Polyarthra upon the rhythmic occurrences of the chlorophyll-bearing organisms upon which they largely depend for their food is strongly suggested by the data here offered. Food relations thus dominate the repro- ductive cycles. The pulses of Polyarthra form a considerable portion of many of the pulses of the total Ploima, and it is but natural that we should find a coincidence in their locations. This may be followed for 1898 in Table I . In a number of instances the culminations of the pulses are not exactly coincident, but separated by the interval between two collections. The association of the two pulses is, however, apparent in every case, and a similar relation may be traced in prior years. These recurrent pulses afford evidence for the polycyclic habit of this species. Additional proof of this phenomenon is found in the evidences of sexual reproduction either male or winter eggs attached to the female which have attended many of the pulses. The eggs of this species, both summer and winter forms, are very 207 readily detached in the manipulation of the plankton, so much so that in 1898 less than 6 per cent, remained attached. More or less uncertainty attends the determination of the parentage of detached winter and male eggs, so that decisive proof of sexual reproduction is best obtained from the attached eggs. In Table I. will be found the records of free and attached male and winter eggs recorded in 1898. Evidence will be found in this of sexual reproduction at- tending the pulses of March, April, May, September, and December. The presence of winter eggs at intervals throughoutrthe greater part of the year may be due either to their continual production or, as seems more probable, to their continuance in the plankton for some time after their formation. The presence of attached winter eggs, or of larger numbers of free winter eggs, seems to mark the culmina- tion and decline of the pulse. Male eggs, on the other hand, are more generally present during both the rise and decline of the pulses. Somewhat similar evidence of sexual cycles attends many of the larger pulses in years prior to 1898. This species affords a striking example of a perennial eulimnetic planktont. It is found in midwinter under the ice in water at the freezing point, and even under these conditions it multiplies, pro- ducing pulses whose amplitude surpasses that of many rotifers of the plankton, and runs a reproductive cycle similar to, though of less amplitude than, those at other seasons of the year. It shares with other organisms the vernal outburst, and repeats the process in summer months under maximum conditions of heat and in waters whose chemical condition is very different from that in which the hiemal and vernal pulses appeared. Successive generations of this species are thus adapted to widely different conditions. Through all the changes incident to ice, stagnation, flood, sewage pollution, changing temperature, the wax and wane and change of food, the constant and unceasing warfare of enemies which prey upon it and of parasites which plague it, and, above all and continuously, the removal of countless individuals from the place of their origin by the ceaseless current of the stream, this species lives on, holds its own in the plankton, and repeats year after year the same sequence of rhythmic pulses of occurrence in the river water. The secret of the process doubtless lies in its capacity to produce repeatedly these crops of winter eggs which serve to seed the environment and start 208 anew the cycle of growth and reproduction whenever the favorable conditions prevail. There is in this species no hard lorica whose variable processes might serve to demonstrate to every observer its capacity for varia- tion. This is doubtless one of the reasons why we do not find a host of new species and varieties oiPolyarthra as in the case of Brachionus. It is subject to considerable variation in size, and the swimming lamellae vary in length, width, and serrations. Hempel ( '99) records Wierzejski's var. euryptera in our plankton, and I have often observed it, but no record was kept of it since the characters which define it are not readily seen in plankton enumeration. Weber ( '98) has mentioned, without designating by name, a long-spined variety which I find very common among the individuals which occur in the Illinois. This planktont is subject to attacks of internal parasites (Sporo- zoaf) which infest it at the times of its maximum pulses, though never to the extent observed in the case of Bimcerium in Brachionus. It is very frequently loaded down by Colacium, and some of the smaller peritrichous Ciliata are often found upon it. The absence of a hard lorica has served to obscure somewhat its food relations to whatever animals prey upon it. Polyarthra platyptera is a cosmopolite, and is apparently found generally in the fresh-water plankton. Jennings ( '00) reports it as abundant in the waters of the Great Lakes, and it has been found generally in American waters. Zacharias ('98) and Marsson ('00) find it in pond and stream waters of Germany; Stenroos ('98) reports it as a predominant rotifer in the plankton and littoral regions of Finland waters ; and Borge ( '00) finds it in Swedish plankton. It has also been found to be an important constituent in the plankton of European streams. Skorikow ('96) finds that it is the most abundant rotifer in the summer plankton of the River Udy, consti- tuting almost a third of the total rotifers. There are indications in his records of recurrent pulses, and the largest numbers are found in September. Zimmer ('99) finds it perennial in the Oder, but never abundant. Schorler ( '00) finds it in the Elbe from April to September, with maximum in August. Lauterborn ('98a) lists this species among the perennial rotifers, and states that it is dicyclic in the Rhine and its adjacent waters, which he has examined quite thoroughly. The vernal sexual period begins with the appearance 209 of the male eggs in March, and winter eggs follow in April and May. The second sexual period extends from the end of July to the end of October, with a maximum in September-October. This bears some resemblance to the distribution in the Illinois, with the exception that the recurrent cycles which make the species poly- cyclic were not noted, and that male or winter eggs were not present in the colder months. It may be that the application of the quantitative statistical method with brief intervals of collection in the Rhine would reveal a still closer correspondence in the seasonal routine of Polyarthra in the two streams. Wesenburg-Lund ('98) finds that temperature has nothing to do with the appearance of the sexual cycle of this species in Danish waters. Males were found in December, as also (eggs only) in the Illinois. He also found differences in different bodies of water as to the times of the sexual cycles. Apstein ('96) has found this species perennial and one of the most abundant rotifers in plankton of the lakes near Plon, Germany, with maximum period from April to August, and in November in one lake, and in July-August in another. The sexual cycle was noted in May- June only. Seligo ('00) finds the species perennial in lakes near Danzig, with large numbers in April and July. His collections were too widely separated to trace fully the seasonal fluctuations. Burckhardt ('OOa) finds Polyarthra in small numbers in winter months in the plankton of Swiss lakes, and in larger numbers in the summer, but does not trace their seasonal fluctuations. Pterodina patina Ehrbg. Average number of females, 37. With two exceptions all the records of this species lie between the last of May and the first of October. There are but four records below 70. This indicates optimum conditions for the species during the period of maximum heat, and further evidence of this lies in the occurrence of the larger numbers during this period. Appearances in January-March suggest a perennial habit ; and small and irregular numbers, that the species is largely adventitions. Hempel ('99) also records P. valvata Hudson from Quiver Lake. Rattulus tigris O. F. Mull. Average number of females, 207. I have not found this species in any year later than October, though, as shown in Table I. , it appears in January at minimum temperatures, and continues in small numbers and somewhat irregularly until autumn. These conditions and the absence of pulses suggest that 210 the species is adventitious in the plankton. The greater part of the occurrences were recorded above 50 and the larger numbers above 60, indicating an optimum during summer months. The record in Table I. refers to the species figured by Jennings ('00) under this name. Rattulus sulcatus Jennings was found seven times in the plankton in July and August during maximum temperatures. It is probably adventitious in the plankton. Salpina brevispina Ehrbg. was found September 5, 1895, at 74, and April 29, 1896, at 70. Salpina eustala Gosse was found July 13, 1894, at 82. Salpina macracantha Gosse was found September 5, 1895, at 74. Salpina ventralis Ehrbg. was found July 29, 1895, at 75. In common with other species of the genus it is adventitious in the plankton. Schizocerca diversicornis v. Daday. Average number of females, 46. The earliest record of this species was June 1, 1896, at 70 ; and the latest, September 20, 1895, at 78. Most of the records and the larger numbers are in July-September during the period of maximum heat, in which its optimum conditions must be found. Egg-bearing females were also found in these months. This species is closely related to the Anuroza aculeata group, and like it is exceed- ingly variable, especially in degree of development of the various spines. Variety homoceros Wierz. was found in May, June, and August, 1896. Five sixths of all the individuals recorded were found in 1896, and the fact that this was a year of unusually dis- turbed hydrograph (Pt. I., PI. X.) suggests that this form may be to some extent adventitious in our plankton, but no direct relation to the access of flood waters can be traced. Lauterborn ('98a) lists this species among the summer planktonts of the Rhine, and Seligo ( '00) finds it in large numbers, with a maxi- mum in July, in lakes near Danzig. Zacharias ('98) reports it in German pond plankton, Zimmer ('99) finds it in the Oder, and Schorler ( '00) in the summer plankton of the Elbe. Synchazta pectinata Ehrbg. Average number of individuals, 3,950; of eggs, 13,823. It was much more abundant in previous years, averaging in 1897 23,227 and 28,230; in 1896, 7,064 and 7,927; in 1895, 13,071 and 4,730; in 1894, 7,520 and 1,659. The effect of the disturbed hydrograph of 1896 is seen in the smaller 211 numbers of that year, while the larger numbers in 1897 may be attributed to the more stable conditions. The small numbers in 1898 do not seem to be correlated with any feature of the environ- ment. This species has been found in every month of the year, and is thus perennial in our plankton. As will be seen, however, in Table I. , the most of the occurrences and a much greater proportion of the individuals are found between May and October, and thus above 60. The same limitations are found in the other years, with the exception that in 1896 there was a more continuous and larger de- velopment from the last of February. In the table which follows it may be noted that all of the pulses but four are at temperatures above 70, and of these four none exceeds 25,000, and two do not exceed 2,500. The optimum conditions for the species in our waters are therefore above 70. The average temperature at the time of the larger pulses is near 80. The vernal pulses are poorly defined, as are likewise the autumnal ones. It is a midsummer species in our waters, with its maximum in August. PULSES OF SYNCH^ETA PECTINATA. Year Date Temp. No. Date Temp. No. Date Temp. No. 1895 1896 Mar 3 35 6 360 1897 Year Date Temp. No. Date Temp. No. Date Temp. No. 1894 Tiilv 1 3 83 74 606 1895 1896 July 23 Tnlv in 80 80 1,749 22 200 Aug. 12 85 7ro 175,230 Sept. 12 79 27,740 " 28 82 38,000 1898 1898 July 19 84 350 20,800 " 24 Aug. 2 " 23 78 78 82 264,000 12,000 3,200 Sept. 27 73 30,400 (IS) 212 Of the 18 pulses listed in the preceding table 17, fall within the limits of periods included in Plates I. and II. Of these 17 there are 7 which coincide with, and 9 which follow shortly after, -the culmina- tion of the pulses of the chlorophyll-bearing organisms, while 1, a small one in March, 1896, shows no such correlation. Food is thus a primary factor in the production of these recurrent pulses. -As will be seen in Table I., these pulses uniformly coincide with those of the total Ploima, and a similar relation may be followed in pri< >r years. The eggs of this species are not usually carried by the female for any length of time, and are rarely found attached in preserved material. For this reason the sexual cycles are not easily followed with accuracy in the statistical data. It may be seen in Table I. that the free winter eggs belonging to both species of SynchcBta are most numerous in the period of the larger pulses, and that their occur- rences show some tendency to coincide with these pulses. Proof that these pulses terminate in sexual reproduction is thus lacking, though it seems probable from some of the evidence. Synch&ta pectinata has not been widely reported from American waters. Jennings ('94) finds it in Michigan and Kellicott ('97) in Lake Erie, but it has not been elsewhere reported in American plankton. It appears, however, in many European records. Skori- kow ('96) finds it in the summer plankton of the River Udy, in Russia ; Zimmer ( '99) finds it in common with 5. tremula in the Oder throughout the year. He makes the statements that it is never rare, is somewhat more abundant in the spring, and is, at other times, present "in relativ gleichmassiger Haufigkeit." In the light of our results it seems probable that the data at Zimmer 's disposal were insufficient to justify his conclusions as to the uniform- ity of its seasonal distribution. Schorler ('00) finds it in the Elbe in April, May, and October, with a maximum in May. Lauterborn ( '98a) finds it perennial in the plankton of the Rhine, and lists it among the dicyclic species with two periods of sexual reproduction, one in April and one from the end of July to October. Judging from the character of the statistical data which have been presented for this and other species in the Illinois it seems probable that the later period noted by Lauterborn may include several cycles, and that the species is usually a polycyclic one. Seligo ('00) reports it perennial in waters near Danzig, with largest numbers in April and 213 September. Apstein ('96) finds that this species (including 5. tremula and 5. grandis] is one of the most abundant in lakes near Plon, with variable maxima in different bodies of water. He finds it perennial in one case, and reports vernal maxima. Winter eggs were found in March and April. Synch&ta stylata Wierz. Average number of individuals, 120,391 ; of eggs, 17,797. In 1897, 42,577 and 9,127; in 1896, 24,099 and 5,125; in 1895, 155,880 and 2,418; in 1894, 8,582 and 132. This species affords an exception to the general rule hitherto observed among the rotifers of our plankton in that it is more abundant in 1898 than in the previous year. As will be seen in the following table both the vernal and autumnal pulses are unusually large in 1898, while in the previous year the vernal pulse is only moderate and the autumnal pulse is scarcely to be detected. For some reason the prolonged low water and sewage contamination of the autumn of 1897 was not favorable to the usual grow r th of this species. It may be that it was crowded out by the unusual develop- ment of Polyarthra at that season (PL V.). PULSES OF SYNCH^ETA STYLATA. Year Date Temp. No. Date Temp. No. Date Temp. No. " 25 33 3,648 330 1899 Jan. 14 34 12,000 Feb. 14 32 19,200 " 22 Mar. 21 51 37 58,000 5,600 Year Date Temp. No. Date Temp. No. Date Temp. No. 1896 Apr. 29 70 380,586 May 25 75 10,800 June 17 76 79,200 60 1 139 000 June 21 77 795 200 " 31 70 61,600 214 PULSES OF SYNCH/ETA STYLATA continued. Year Date Temp. No. Date Temp. No. Date Temp. No. 1894 1895 1896 1897 1898 Aug. 1 Aug. 8 79 86 10,287 8,400 Sept. 27 73 12,225 July 21 July 19 81 84 103,200 64,800 Sept. 7 Sept. 27 80 73 28,000 265,600 Aug. 2 " 23 79 82 170,400 24,800 Year Date Temp. No. Date Temp. No. Date Temp. No. 1 894 1895 1896 1897 Oct. 17 58 63,935 Nov. 27 Nov. 17 Nov. 9 " 30 33 44 50 34.5 901,901 114,000 26,400 87,200 Dec. 11 32 1,121,056 Oct. 5 " 19 71 65 12,000 15,800 Dec. 14 36 72,200 1898 Oct. 25 49 824,500 Nov. 15 41 110,000 Dec. 6 20 34 33 42,500 59,200 This is the most abundant of all the rotifers in our plankton, exceeding by 30 per cent. Polyarthra, the next in abundance. It constituted one fifth of the total Ploima in 1898, and is accordingly a large factor quantitatively and ecologically in the economy of the plankton of the Illinois River. It is a perennial planktont, occurring in six sevenths of our collections and usually in considerable numbers. The distribution in 1898 (Table I.) is a fair index of the usual seasonal routine, with the exception that in all prior years the July-August minimum is more pronounced and better sustained. The development in January-February is never large, rarely exceeding 20,000. In March, numbers rise rapidly, usually with a minor pulse, the re- covery from which in April culminates in a vernal pulse, which in three of the six years was the largest of the year. Following this vernal pulse there is a series of smaller pulses throughout the sum- mer. The decline of the June flood, when this occurs, seems to offer favorable conditions (cf. foregoing table and Pt. I., PI. IX.-XII.) for the development of a pulse which is but little smaller than the vernal one. It may be of some significance that this pulse and the 215 vernal one both occur on the decline of the major floods of the year, and that the relative proportions of the two floods are to some degree paralleled by the amplitude of the pulses of Synch&'ta which attend their decline. The effect of the impounding backwaters as reservoirs for the greater development of the plankton is suggested by these data. Following the midsummer minimum is an autumnal pulse whose amplitude and location alike are subj ect to much variation. As will be seen in the table on pages 213 and 214, the maximum jmtumnal pulse is located twice in October, twice in November, and once in Decem- ber. This may be due to the fact that the collections are insufficient in some of the years, or to the probability that any one of several recurrent autumnal pulses may be the major pulse of that season. An examination of the seasonal distribution. in 1898 (Table I.) and of the location and temperatures of the pulses recorded in the table on pages 213 and 214 will suffice to demonstrate the capacity of this species to develop at all temperatures within the seasonal range. The largest pulse (1,139,000 on May 3, 1898) is at 60, and the next in size (1,121,056 on December 11, 1895) is at 32. It will, however, be seen in the two tables that the pulses and the numbers in general during the periods of maximum heat and cold are not so large as in the intervals of more moderate temperatures. The impetus of the autumnal development may carry some of the pulses over in to minimum temperatures, but the level of development declines thereafter. There is thus something of a tendency for the average temperature of the larger occurrences to approach the average temperature of the year. The number of pulses listed in the table on pages 213and214is38. Of these, 34 fall within the period included in Plates I. and 1 1. of the pulses of chlorophyll-bearing organisms. Of the 34 there are 18 which coincide in location with these plant pulses, 12 which follow at a brief interval, and 4 which bear no such relation, three of the last being minor winter pulses. The dependence of the recurrent periods of rapid multiplication of Synchata the most abundant rotifer of the plankton upon the rhythmic increase of the food supply is thus fairly demonstrated. The coincidence of the pulses of Synch&ta with those of the total Ploima is readily seen in Table I., and is equally apparent in prior vears. 216 Eggs of this species are not carried by the parent for any length of time, so that reproductive cycles are not easily traced. The total number of the summer eggs of Synchceta will be found (Table I.) to fluctuate somewhat with the pulses of the species. The free winter eggs, belonging probably to both species of Synchatta, also show some tendency to predominate at and after the culmination (Table I.) of the pulses. A female carrying a male egg w T as recorded during the rise of the spring pulse in 1898, and attached winter eggs were noted at the vernal pulse in 1895 and 1897. The evidence points toward the culmination of these pulses in a sexual cycle. The soft and flexible nature of this rotifer and the absence of spinous outgrowths have made whatever variability the species possesses less evident than it is in such a genus as Brachionus. There is considerable variation in size possibly due to age even in the same collection. The determination of preserved material of this genus is fraught with insuperable difficulty. The separation of pectinata and stylata in our records is at the best only probable. It may be that other species of Synchceta have been included w r ith the individuals referred to stylata. In any event the result of the division has led to symmetrical results comparable with those of other planktonts. Synch&ta is often parasitized at the times of the larger pulses by some sporozoan (?). At the maximum of the vernal pulse in 1898 over 4 per cent, of the individuals were thus affected, the infestation continuing through the decline of the pulse. External parasites, Colacium and Rhabdostyla, are rare. This species has not been found widely in the plankton, possibly because of the confusion of stylata, tremula, and pectinata in identifi- cation. From the large numbers reported in almost every instance where it has been found, the expectation of its wide-spread occur- rence is at least raised, waiving in this connection the possibility of specific confusion. Jennings ( '94) found it to be very abundant in towings in Lake St. Clair, and ( '96) in Lake Michigan near Charle- voix. He finds it less abundant in the summer plankton of Lake Erie ('00). Stenroos ('98) reports it as one of the most abundant limnetic rotifers in Lake Nurmijarvi in Finland in the summer, and Skorikow ( '97) finds that next to Polyarthra it is the most abundant rotifer in summer months in the River Udy near Charkow, Russia. His figures of occurrence show some traces of recurrent cycles in these months, with maximum numbers at the first of August. Lau- 217 terborn ('98a) lists it among the summer rotifers of the plankton of the Rhine. The genus is in need of a thorough revision in the light of possible variation.* Taphrocampa annulosa Gosse. Average number, 71. Found in September, at 73. Evidently adventitious. Triarthra longiseta Ehrbg. Average number of individuals, 3,147; of eggs, 293. This species was about twice as abundant in the stable conditions of 1897, and was present in less than half these numbers in the recurrent floods of 1896. It is a perennial species, having occurred in every month of the year. The continuous occurrences and the larger numbers lie in all years between May and October and above 60. In 1898, only about 3 per cent, of the total individuals were found below this temperature. With the exception of the vernal pulse of 1898 all of the larger numbers were found in the period of maximum heat. The optimum conditions for this species are thus found within that period and above 70. The seasonal routine of the species is varied somewhat from year to year. There is usually a slight vernal pulse larger than usual in 1898 and this is followed by recurrent pulses throughout the summer. The season closes without a predominant autumnal pulse, and after September the numbers fall and the occurrences become sporadic until the following April. The pulses of this species are listed in the following table, which gives their locations and temperatures. Of the 21 pulses recorded, 18 are within the periods of the plant pulses shown in Plates I. and II. Of these 18 there are 8 which coincide with these plant pulses, 9 which follow after a short interval, and 1 which shows no such relation. The dependence of the pulses of Triarthra upon food conditions is suggested. The pulses of Triarthra will be found on examination of Table I. to coincide in 1898 in the main with those of the total Plointa. The pulses are never very large, and the evidences of reproduc- tion are not well defined. Attached summer eggs attend the larger pulses, and free winter eggs of the species were found in October- November in 1898. In previous years free or attached eggs attended vernal or summer pulses at times. The evidence indicates a poly- cyclic habit. * See Rousselet, '02. 218 PULSES OF TRIARTHRA LONGISETA. Year Date Temp. No. Date Temp. No. Date Temp. No. 7n 73 4 000 " 27 80 6,000 31 70 1,000 Year Date Temp. No. Date Temp. No. Date Temp. No. 1895 July 18 80 19,080 Aug. 21 82 10,683 Sept. 12 79 2,336 1897 July 21 81 49 , 600 Aug. 17 79 9,600 Sept. 7 80 70,000 1898 July 26 89 28,000 Aug. 30 83 6,400 Sept. 27 73 14,400 This is an exceedingly variable species. It varies in the relative length of the three long setae, in their spinosity, and in the location of the posterior one. Many of the individuals in our waters resemble the form described by Plate ( '85) as T. terminates. The long-spined form described by Zacharias ( '94) as var. limnetica is also abundant. It is doubtful if either form is worthy even of varietal distinction. This species has been reported only from Lake Erie and the Illinois River in this country, and seems to be rare in the former. Weber ('98) finds it abundant in the plankton of Lake Leman; Burckhardt ('00 and 'OOa) reports it as wide-spread and almost perennial in Swiss lakes, but with its maximum in December- February, and slight development during warmer months. Borge ( '00) finds it to be one of the common rotifers in the summer plank- ton in Sweden ; Marsson ( '00) reports its perennial seasonal range in several German waters, with greater numbers during the warmer season. Apstein ('96) gives it a perennial distribution in Lake Plon, with larger numbers in June-November, and maximum in June- July or August. According to Seligo ( '00) the species is per- 219 ennial in lakes near Danzig, rivaling Polyarthra in abundance, and exhibiting maxima in the warmer months from April to October. It is also a member of the potamoplankton of European streams. Skorikow ('97) finds it in summer months in the Udy, and Zimmer ('99) reports if as present in small numbers and irregularly in the Oder from April to November. Schorler ('00) finds it in the Elbe in May-October with maxima in May and September, and Lauter- born ( '98a) includes it in his list of perennial rotifers in the plankton of the Rhine. It has two sexual periods, the first in March-May and the second in July-October, and he suggests the probability of a poly cyclic habit in some waters. Trochosphczra solstitialis Thorpe was found June 27, July 2, and August 15, in 1896 ; in 1897, on May 25 and July 14-30. Free winter eggs were taken August 15, 1896. All occurrences were above 66. These records were all from plankton taken in mid-channel of the main stream. Trochosph&ra was found in greatest abundance at the outlet of Flag Lake (Pt. I., PI. II.) in July, reaching 9,664 per m. 3 at 72. It was also found in August in the weedy backwaters of Dogfish Lake. Both of these backwaters connect with the river (Pt. I., PI. II.) below the point at which our collections were made. It was either introduced from some similar backwater higher up the stream than our plankton station, or developed in the river itself. SCIRTOPODA. This order is represented in the plankton by a single species, whose discussion will suffice for the order. Pedalion mirum Huds. Average number, 4,524. This is a summer planktont of somewhat definite temperature limits. The following table combined with the data in Table I. will suffice to characterize its seasonal fluctuations. Its limitation to temperatures above 60, indeed almost 70, is apparent. There are in all but two records below 60, and but four below 70. It is a typical midsummer planktont, with several recurrent pulses during the period of maximum temperatures. The location of these pulses with reference to those of the chlorophyll-bearing organisms is significant. As shown in Table I., they follow immediately, or coincide with, those of the synthetic organisms. For example, the apices of the pulses of Mastigophora, 220 Year First record First maximum Date Temp. Date Temp. No. 1894 June 29 July 6 July 28 July 21 July 26 83 80 80 84 89 2,592 330,932 20,000 80 , 000 99,600 1895 1896 May 25 June 28 June 21 70 75 77 1897 1898 Year Second maximum Last record Date Temp. No. Date Temp. 1894 Sept. 17 Oct. 2 Sept. 16 Sept. 14 Nov. 1 72 63 71 73 45 1895 Aug. 21 Aug. 15 Aug. 17 Aug. 16 81 81 79 77 3,561 77,600 79,200 22,400 1896 1897 1898 Bactilariacea, and Chlorophycecs in the period in question in 1898 are (PI. II.) July 19, August 9, August 30, and September 27. The apices of the Pedalion pulses are July 26, August 16, and September 27, the last coinciding with the pulse of chlorophyll-bearing organ- isms. In 1897, the intercalation of the two pulses is apparent, and in 1896, two out of three pulses are intercalated and a third is coincident. As will be seen in Table I., these pulses of 1898 are approximately coincident in many cases with those of other roti- fers Synchceta, Polyarthra, Triarthra, and Brachionus. The sig- nificance of this intercalation lies probably in the food relations of the two groups of organisms. Females with a single egg attached to the body have been noted at the times of the maxima of the pulses, or immediately thereafter, 221 in five instances. On the pulse of July 26, 1898, a female with four male eggs was found. This species was not reported by Apstein ('96) from the lakes of Holstein, but was found by Lauterborn ( '98a) in the Rhine and its backwaters. Here also it was a summer form, appearing about the middle of June, with a maximum in August or September and disappearing late in October, conditions of distribution much re- sembling those in the Illinois. It is regarded, along with other summer forms, as monocyclic. The appearance in- our waters of male eggs July 26, at the height of the first pulse, leads to the in- ference that there may be several cycles; for example, three in 1898, with the recurrent pulses, in a single summer season. Weber ( '98) gives it as a summer rotifer in Switzerland, and Skorikow ( '97) finds it in July-September in the Udy River, in Russia ; but it is not reported from the Oder by Zimmer ('99), nor from the Elbe by Schorler ('00). Kellicott ('97) finds it in Lake Erie in small numbers in the summer. In addition to the species of rotifers noticed above, Hempel ('99) has reported the following in the Illinois River or its backwaters: Flosculana ornata Ehrbg., Limnias ceratophylli Schrank, Cephalosi- phon limnias Ehrbg., CEcistes intermedius Davis, 0. mucicola Kell., Pedetes saltator Gosse, Furcularia forficula Ehrbg., F. longiseta Ehrbg., Eosphora aurita Ehrbg., Diglena grandis Ehrbg., D. catellina Ehrbg., D. biraphis Gosse, Ccelopus tenuior Gosse, Scaridium longi- caudum Ehrbg., Distyla gissensis Eckstein, D. okioensis Herrick, D. stokesi Pell, and D. hornemanni Ehrbg. GASTROTRICHA. Ch&tonotus sp. occurred singly in the plankton August 29, 1896, July 30, 1897, and February 15, 1898, with a temperature range of 32.5 to 84. ENTOMOSTRACA. Average number, 47,042. In 1897, a more stable year, 91,050; in 1896, a year of disturbed hydrograph, 50,158; in 1895, in more stable conditions, 148,348. The Entomostraca appear in every collec- tion at all seasons of the year. The decline to the winter mini- 222 mum occurs in November-December. Numbers are at a minimum (generally less than 5, 000 per m. 3 ) in midwinter (January-February) ; rise in March to about 25,000 per m. 3 ; and attain the maximum for the year in a vernal pulse of 200,000 to 1,500,000 in April-May. Following this, there is frequently a second pulse of large proportions in June, which in 1898 exceeds (Table I.) that of May. During the remainder of the year there is usually a series of recurrent pulses, of declining amplitude in 1896 and 1898, but rising to unusual heights (618,750 on September 9) in the stable conditions of 1897. In the main the pulses of Entomostraca coincide with or approximate to the location of those of the other organisms of the plankton, and often show correlations in amplitude. BRANCHIOPODA. Eubranchipus serratus Forbes. Young branchiopod larvae questionably referred to this species appeared in the plankton in January-March, 1899, in small numbers at minimum tempera- tures. CLADOCERA. Average number, 6,068 per m. 3 In 1897 they were more abun- dant, averaging 17,863 per m. 3 in the more stable conditions of that year. In 1896, a year of recurrent floods, numbers fell to 7,7 19, while in 1895, a year of low water in spring, when many of the Cladocera attain their maximum, the greatest average, 31,937, was recorded. The phenomenal number of 443,716 per m. 3 appeared on June 19 in the stable low water (1.80 ft.) then prevailing. In 1894, another year of low levels, the annual average was also large (23,952), though probably enhanced by the fact that collections were not made in flood waters in this year. The Cladocera appear in all but 10 of the 182 collections enu- merated, the ten exceptions falling in November (1), January (2), February (6), and April (1), and usually in flood waters or, as in 1895, in stagnation conditions under the ice. Although the Cladocera occur in all months of the year, they nevertheless, as a group, exhibit decided temperature adaptations, as appears from the fact that all records in excess of 4,000 per m. 3 fall between May 1 and September 1 with but 6 exceptions, 4 in the phenomenally 223 early spring of 1896, and 2 in the delayed high temperature of October, 1897. The minimum records (less than 500 per m. 3 ) are found during minimum temperatures. The numbers increase slightly (generally less than 2,000) as temperatures rise in March- April, rise abruptly, as they approach or pass 70, to a vernal maximum in May- June, and decline during midsummer excepting when unusual pulses of Moina or Diaphanosoma raise the level of the pulse maxima above 25,000. This decline continues in channel plankton through the autumn until the low level of approximately 2,000 per m. 3 , at the most, is again attained in October, and falls irregularly to 500, or less, as minimum winter temperatures arrive in December. Ex- ceptions appear in 1897, when a well-defined autumnal pulse of large amplitude (193,500) is found on September 14, and is followed by others of declining amplitudes (137,600, October 5; 5,520, No- vember 15 ; 4,240, December 14) during stable autumnal conditions. All of the records above 4,000 per m. 3 , with one exception, are found at temperatures above 45, and all in excess of 8,000, with 4 exceptions, after the vernal rise in temperature passes 70 in April- May, and before the autumnal decline reaches this point in Septem- ber. The Cladocera are thus planktonts of the warmer channel- waters. The relation which hydrographic conditions bear to the seasonal occurrences of Cladocera is apparent in the yearly averages above quoted, and appears still more clearly in a comparison of the cladoceran population and movement in river levels in July- December, 1897 and 1898, as given below. Average No. Cladocera per m. 3 July August Sept. Oct. Nov. Dec. 1897 1898 1897 1898 1897 1898 1897 1898 1897 1898 1897 1898 12720 3050 13960 3756 70675 1700 40350 1615 2532 620 1945 236 Total movement in river levels, in ft. 5.2 7.4 2.6 7.5 0.6 6.2 0.6 3.9 2.2 2.6 0.5 2.4 224 Hydrographic changes affect the Cladocera by increasing the amount of silt and flocculent debris in suspension, which, by ad- herence to the swimming antennas and flotation processes of the animal, tend to impede its movements and sink it to the bottom, where it is removed from its normal feeding area and readily becomes the prey of the larger organisms of the bottom fauna. Barren flood waters also tend to displace and wash away in the increased current the Cladocera which have developed in the stream, and to afford both less food and less time for their further development. The occurrences of the total Cladocera fall into the type of recurrent pulses, though with slightly less distinctness than in the case of individual species of the group. Such pulses can be traced in all seasons in which records were made at short intervals, and suggestions of their occurrence appear in the less frequent records of other seasons. Thus in July-December, 1897, (PI. IV.), there are 6 well-defined pulses culminating at intervals of 3(1), 4(2), 5(1), and 6(1) weeks. In 1898 (Table I.) the pulses are less regular in the flood waters of the disturbed year. In 1896, when records were frequent, we can trace pulses in March, May, June, July, August, and September. The character of these pulses is well illustrated in the vernal pulse of 1898 (Table I. and PI. IV.), culminating June 7 at 136,000. The species which share in this pulse are Alona affinis, A. costata, A. quadrangularis , Bosmina longirostris*, Ceriodaphnia scitula*, Chydorus sphcericus* , Daphnia hyalina*, D. cucullata*, Diaphanosoma brachyurum, Leptodora hyalina, Macrothrix laticornis, Moina micrura, Pleuroxus denticulate s, Scapholeberis mucronata, and Simocephalus serrulatus. Of these, only the five marked by the asterisk occur in numbers sufficient by our methods to delineate a pulse. The other species are accordingly of little consequence in modifying the form or location of the pulse. The June volumetric pulse (Part I., PI. XII.) culminates June 14 at 6.99 cm. 3 per m. 3 , though the record for June 7 is also high (5.28). The cladoceran pulse culminates June 7 at 136,000. On this same day four of the dominant species also reach their culmination, viz. : Bosmina longirostris (62,800), Ceriodaphnia scitula (55,800), Daphnia cucul- lata (3,400), and D. hyalina (11,600), the remaining 2,400 being contributed by other species. Chydorus spharicus, which appears this spring only in small numbers, attains its maximum (7,880) on May 24, two weeks earlier, though the record for May 31 is also high 225 (5,040), indicating a probable maximum between these dates. In other seasons, for example in 1896 and 1897, the maxima of this species coincide generally with those of other Cladocera, so that this divergence seems to be anomalous. An inspection of the table of records for 1898 gives a remarkably uniform and coincident rise and decline of the pulses of the several species which constitute this characteristic vernal pulse. No effort has been made by me to determine the total cladoceran fauna of the Illinois River. Only those species are here given which have appeared in our plankton enumeration. A number of others are known to occur in the littoral fauna, and a few scattering indi- viduals found in the plankton were not identified. Of the 25 forms here listed, only 10 named in the sequence of their relative numbers as shown in grand totals may be regarded as typical planktonts, autolimnetic in channel plankton-, viz. : Moina micrura, Bosmina longirostris, Daphnia cucullata and vars. upicata and kahlbergiensis, D. hyalina, Ceriodaphnia scitula, Chydorus sphcericus, Diaphanosoma brachyurum, and Leptodora hyalina. Of the ten, the last named and the varieties of D. cucullata appear to be of little quantitative importance in the channel plankton, though it may be that our methods of collection fail adequately to represent Leptodora. Of the remaining 15 species, Alona a~ffinis, Ceriodaphnia reticulata and C. rotunda, Scapholeberis mucronata, and the two species of Simocephalus are the only adventitious Cladocera of quantitative importance, and this only to a relatively small extent. DISCUSSION OF SPECIES OF CLADOCERA. Alona affinis Leydig. Average number, 36. This species has a well-defined seasonal distribution. It appears in autumn in the last of October, as temperatures approach 40, and remains until the end of June, when the summer maximum of 80 is re-established. The numbers are too small (Table I.) and irregular to define its seasonal fluctuations, though there are suggestions in the records of late autumnal and of vernal pulses. Egg-bearing females were recorded in January-February at minimum temperatures. No close dependence on hydrographic fluctuations is apparent to account for their occurrence in the plankton. Alona costata Sars. Average number, 11. Only a few scattered occurrences of small numbers. Earliest autumnal record, Novem- ber 22, at 40; latest vernal, May 24, at 73. 226 Alona quadrangularis O. F. Miill. Average number, 5. A few scattered occurrences in March-May. Alona spp. It is probable that some of the foregoing species of Alona are here included. There are 16 occurrences, scattered through all months but January, April, and November, with no large numbers and no marked seasonal distribution. Bosmina longirostris O. F. Mull. Average number, 2,441, of which 1,527 are adult females without large embryos, 390 with them, and 524 immature. I include in this species B. cornuta Jurine, for I am unable to find any constant line of demarcation between these forms. The longirostris form is the dominant one in the channel plankton, the cornuta form being relatively rare. Bosmina is a perennial planktont in our channel plankton, but occurs in small numbers only in October-May, no record in this period with the exception of that of October 5, 1897 (20,400), at 71, exceeding 5,000 per m. 3 , and most of them falling below r 2,000. The records in November-March, with the exception of November- December, 1897, all fall below 1,000 per m. 3 In like manner the percentage of collections containing Bosmina in December-April is lower than that in the summer, the percentages being 64, 16, 26, 47,. and 55 per cent, respectively for these colder months, and averaging 82 per cent, for the rest of the year. The percentage of occurrences in October-November remains high (82 and 81 per cent.), though the numbers per m. 3 fall off greatly. The usual seasonal distribution is as follows : In January-March the occurrences are scattered and irregular and the numbers very small less than 500 per m. 3 Toward the close of April the vernal increase makes its appearance, continues slowly through May, rarely attaining more than 5,000 per m. 3 , and at the end of this month or early in June reaches the maximum development of the year in a vernal pulse of 40,320 (1896) or 62,800 (1898) per m. 3 From this summit there is an abrupt descent in a period of exhaustion to a level of less than 2,000 per m. 3 in the last fortnight of June. During the remainder of the year there appears a series of recurrent pulses of less magnitude, exceeding 10,000 per m. 3 in but three instances. These follow at intervals of four to six weeks. In July-September the amplitude of these pulses exceeds in all cases 5,000 per m. 3 In October (with the exception of 1897, when temperatures were un- 227 usually high), they decline in amplitude, and in November-Decem- ber often fail to appear in the small numbers recorded. In 1894, records are too scanty to be of significance. In 1895 there are three well-defined pulses, and traces of a fourth in August-Novem- ber. In 1896 there are five in May-September. In 1897 there are six in July-December, data during the remainder of the year being insufficient to define the pulses. In 1898 the vernal pulse in June and a feeble one in October are the only ones which appear. The pulses of Bosmina are best defined in the stable low water of the last six months of 1897. During that period they closely approxi- mate in location of maxima and minima the quantitative pulses and those of the chlorophyll-bearing organisms and of the rotifers. (Compare on this point the plates for 1897 in Part I. Kofoid, '03 and PI. III. and IV.) . The slopes of the pulses indicate that Bosmina is capable of very rapid multiplication; and their coincidence with other pulses just noted, taken in conjunction with the fact that males and ephippial eggs appear but rarely, suggests that these pulses of Bosmina are immediately dependent, in large part, upon fluctuations in the food supply for their origin and for the varying courses which they run. The relations of Bosmina to temperature appear in the facts that all pulses exceeding 5,000 per m. 3 in amplitude occur at tem- peratures above 70, that the vernal rise does not proceed with any rapidity until this temperature is attained, and that the depressing effect of the autumnal decline below 70 is at once apparent in the reduced numbers per m. 3 No constant relation between the pulses of Bosmina and the midsummer heat pulses such as appears in the records of Diaphanosoma can be traced in the occurrences of Bosmina. An inspection of the accompanying table, in which the mean monthly Bosmina population per m. 3 of channel water in July-De- cember, 1897 and 1898, is given, and also the total + and - move- ment in river levels for these months in each year, will suggest an intimate connection between stability of hydrographic conditions and the increase of Bosmina. In 1897 the total movement for these months is from five sevenths to one tenth of that in 1898, and in every instance the Bosmina population is also greater by from 7.5 to nearly 400-fold in 1897, the more stable year. The means of the six months are 2.03 ft. and a population of 3,691 in 1897 to 5.3 ft. and 228 BOSMINA AND HYDROGRAPHIC FLUCTUATIONS.* Year July August September Total movement, in feet Bosmina per m. 3 Total . movement, in feet Bosmina per m. 3 Total movement, in feet Bosmina per m.^ 1897 f -3.9 5 I +1.1 6,213 r -2.6 2.6\ ( + o 3,973 r - -2 .6 I + -4 3,022 1898 r -6.9 7 U, 140 r -3.3 7.7 I +4.4 10 r-2.6 6 [ +3.4 IS Year October November December Total movement, in feet Bosmina per m. 3 Total movement, in feet Bosmina per m. 3 Total movement, in feet Bosmina per m 3 1897 ,(-' I + .5 5,875 2.2\ [ +1.5 1,680 f - .6 1.2 I + .6 1,585 1898 r -1.1 3.9 1 +2.8 780 f - .6 3.2 I +2.6 32 f -2.8 3.8] [ +1.0 60 * + = rising levels; = falling levels. 173 Bosmina in 1898. It is also true that months in which the disparity in stability is greatest are those in which the Bosmina ratios are greatest, and vice versa. It seems very probable that the increased current, the lessened time for breeding, and the greater burden of silt in flood conditions, especially rising waters, do not conduce to the rapid increase of Bosmina in channel plankton. The effect of the high temperatures of the late autumn of 1897 is apparent in the amplitude of the October, November, and De- cember pulses (20,400, 3,440, and 3,440, respectively), which exceed those of all other years at this season. Temperature thus plays perhaps by virtue of its relation to the food supply an important 229 part in the seasonal delimitation of the amplitude of Bosmina pulses. The Bosmina population in the plankton consists largely of parthenogenetic females. Males and females with ephippial eggs, were recorded only in October-December, 1897, and then only in small numbers and isolated occurrences. Females with eggs or embryos and the free young were found at all seasons of the year and at all temperatures, but most abundantly at the time of the pulses. Parasitized or fungused individuals are also found occasionally at these seasons of greatest numbers, and the high mortality following a pulse is evidenced by the large number of dead occurring in the plankton. The proportions of females, females with eggs or em- bryos, young, and* dead during the May- June pulse of 1898, may be traced in the following records. BOSMINA PER M. 3 , MAY-JUNE, 1898. Date Females Females with eggs Young Total living Dead Apr. 26 800 800 May 3 1,600 400 800 2,800 " 10 1,600 1,000 1,000 3,600 400 " 17 1,300 1,100 1,100 3,500 100 " 24 3,280 1,400 1,240 5,920 920 " 31 25,120 2,000 6,800 33,920 1,280 June 7 38,800 9,200 14,800 62 , 800 9,200 " 14 2,200 3,000 800 6,000 1,400 " 21 1,000 500 1,500 100 " 28 300 200 200 700 100 Bosmina longirostris has been frequently reported in the plankton of European lakes. Apstein ('96) finds it perennial in Plonersee with larger numbers in June-September and a maximum in July. No pulse-like recurrence is noted, parthenogenesis prevails, and males and ephippia are rare. His results, save in the matter of pulses, are thus in general accord with ours. Stingelin ('97) notes 230 great seasonal polymorphism in B. cornuta near Basel. Zacharias ('97a and '98b) records it in the plankton of German carp ponds. Stenroos ('97 and '98) finds it in waters of Finland and Karelia, where the cornuta type is littoral, and a limnetic form, distinguished by him as forma vernalis, is abundant in the plankton in May. Scour- field ('98) finds it common in the waters of Epping Forest, where it is perennial, males and ephippia appearing only in September- November. According to Scott ('99) it appears at various seasons in the lochs of Scotland in both the littoral and limnetic fauna. Burckhardt ( 'OOa) gives an extensive revision of the genus Bosmina, and includes in the B. longirostris group nine other so-called species, among which are B. cornuta Jur. The species is "pelagic or hemi- pelagic" in various Swiss lakes, though apparently not in num- bers. The genus is there represented in the plankton princi- pally by the B. coregoni group. Amberg ( '00) lists it from Katzen- see, near Zurich, as a perennial planktont with large numbers in May, August, and February, but gives no statistical data. Fuhr- mann ('00) finds Bosmina perennial in Neuenburgersee, and B. longirostris with a maximum in May. Marsson ('00) finds B. " longirostris-cornuta " in lakes about Berlin throughout the year, with larger numbers in some lakes during the warmer months and in others in November-December. In Barlewitzersee, near Danzig, Seligo ('00) reports B. cornuta as perennial, with maxima in June and in October-November, the latter being the greater. Larger numbers appear in summer than in winter. Cohn ('03), in waters near Konigsberg, finds B. longirostris only sparingly present, appearing in May-September with a maximum in July. In European streams, also, B. longirostris is widely distributed. Lauterborn ( '94) finds it abundant in the winter fauna of the Rhine. He also states that it is not acyclic in the backwaters, where he has found in three successive years both males and ephippia in May- June and again in November. There is thus a suggestion of a vernal and an autumnal pulse in these waters. Zimmer ( '99) finds it through- out the whole year in the Oder. Schorler ( '00) reports it from the Elbe at Dresden in May-October, with larger numbers in May- June and September, while Fric and Vavra ('01) find it in the same stream near Podiebrad. They state that B. cornuta is found in great numbers in 1 m.- surf ace in summer months, and B. longiros- tris sparingly in the littoral fauna. Steuer ('01) finds B, ' 'longirostris- 231 cornuta" m the backwaters of the Danube at Vienna in April-January. It exhibits a distinct seasonal polymorphism, with a large winter form and a smaller summer one. Data as to relative numbers during the year are not given. Skorikow ('02), in reviewing the investigations on the plankton of Russian waters, reports B. cornuta from the summer plankton of several streams, but expresses doubts as to whether "sie als autopotamische Planktonorganismen anzu- sehen sind oder nicht." Meissner ('03) finds B. cornuta generally in the Volga and its adjacent waters in the summer plankton, with largest numbers in August; and Zykoff ('03) reports it in small numbers from the same stream in May- July. It is not listed by Volk ('03) in the Elbe at Hamburg. B. longirostris occurs generally in American waters, though apparently, often in small numbers. Thus Forbes ('82 and '90) reports it in the plankton of Lake Michigan and Lake Superior, and it appears generally in lists of Cladocera from many widely separated smaller bodies of water in this country. Birge ('95 and '97) finds only a few Bosmina (species not stated) in Lake Mendota, but Marsh ('97) reports it (species not given) as perennial in Green Lake, with a maximum in November. His records have also a suggestion of an earlier pulse, in June, in which month there is a sudden rise from a previous minimum. This partial survey of the literature of the records of Bosmina in the plankton shows its wide distribution, suggests the probability of great variation, necessitating caution in the description of new species in this genus, and indicates a wide diversity in its seasonal career even in waters with somewhat closely similar environmental conditions. Ceriodaphnia me gaps Sars was found singly but once July 25, 1896, at 80. Ceriodaphnia reticulata Jurine was found in the plankton occa- sionally, and always in small numbers, in April-September. All occurrences appear at temperatures above 66, and the earliest is on April 1 7 , and the latest is September 2 1 . Females with summer eggs were found in June-September. Ceriodaphnia rotunda Straus was recorded in 1894-1895, but not' thereafter. Its identification is somewhat questionable, and if correct, this is apparently the first record of this species in North American waters, unless it should appear that C. alabamensis 232 Herrick or C. acanthinus Ross, which appear to resemble C. rotunda in some particulars, should be included here as forms or synonyms. The genus is sadly in need of revision. The forms referred to C. rotunda were found in August, 1894, and July-August, 1895, 16,536 per m. 3 appearing in the plankton on July 18 of the latter year. Ceriodaphnia scitula Herrick. Average number, 1,539. This species is closely related to the European C. quadrangula O. F. Mull., if, indeed, it is not identical with it. It is not impossible that it is the form imperfectly described by Say ('18) as Daphnia angulata. In the absence of a critical monograph of the genus I use the name applied in current American literature to this form. This is the most abundant species of the genus in our waters, outnumbering all others by over sixfold in the totals of our records. It is also one of the most important members of the Entomostraca in the channel plankton (total of all records, 156,119), being ex- ceeded in numbers only by Moina micrura (1,121,808), Bosmina longirostris (381,598), Daphnia cucullata (237,444), and D. hyalina (231,746). It occurs in all months of the year except January and February, but in larger numbers and in more of the collections in May-Septem- ber. Thus less than 6 per cent, (reduced to 2 per cent, if one col- lection in the warm autumn of 1897 is omitted) of the individuals and only 20 of the 79 occurrences are found outside of the May- September period. Ceriodaphnia scitula is accordingly a summer planktont in channel waters. It is found in each year, though in varying numbers according to hydrographic and other conditions. Thus in 1898 the vernal pulse in June attains the unsurpassed amplitude of 55,800 per m. 3 , but declines in a fortnight and makes no recovery during the disturbed hydrographic conditions of the summer. In 1897, on the other hand, our records were too meager to delineate fully the vernal pulse, and in the stable conditions of the summer and autumn the species continued in numbers whose totals exceed those of 1898 by 81 -fold. Similarly in 1896 the more gradual changes in levels which attended the floods of that year permitted a considerable development of Ceriodaphnia throughout the summer. Stable hydrographic conditions thus conduce to increase in Ceriodaphnia. The relations which I have shown to exist between Bosmina and movement in river levels (see table on 233 page 228) exist also in the case of Ceriodaphnia and in much the same form. The relation of temperature to Ceriodaphnia is evident in its seasonal distribution. It does not advance rapidly in its vernal increase until after the water warms to 70, and drops suddenly in numbers when the autumnal decline passes this point. Moreover, seasonal variations in temperature are accompanied by correspond- ing shiftings of the pulses of Ceriodaphnia. Thus in 1898 the water did not reach 70 until about May 20, reaching 73 on May 24, and the vernal pulse of Ceriodaphnia began at once its rise to the maxi- mum of June 7. In 1896 spring was early, 72 being recorded in surface waters on April 24, and we find a vernal pulse rising to a maximum on May 8. So also in 1897, when high temperatures continued into the autumn, the decline passing 71 on October 5, instead of in the first half of September as in other years, we find the pulses of Ceriodaphnia extending into October with unusual amplitude, reaching 5,200 per m. 3 October 5, while the highest record in this month, or later, in other years was 280 per m. 3 Tem- perature rather than season is thus the dominant factor in the seasonal curve of occurrence of Ceriodaphnia. The form of this seasonal curve is typically that of a series of recurrent pulses of varying magnitude tending to reach the maxi- mum height in the vernal pulse of May- June, attaining often lower levels in July and rising again in August-September, and falling to a minimum, or even to disappearance, in October. These later pulses do not appear in the disturbed hydrographic conditions of 1898 (Table I.), but are clearly delineated in the summer records of other years, especially in the stable conditions of 1897, where well-defined pulses appear in July, August, September, and October, at intervals of approximately four weeks, culminating July 14, August 10, September 14, and October 5. Their maxima attain respectively 5,600, 2,720, 6,000, and 5,200 perm. 3 , and the pulses are delimited in each case by minima of less than 500 per m. 3 They tend to coincide with those of other Entomostraca and to approach those of the Rotifer a. The Ceriodaphnia population in channel waters is almost ex- clusively made up of parthenogenetic females. Males were not recorded at any time, though females with ephippial eggs appeared after the October pulse of 1897 and the vernal one of 1898. 234 Ceriodaphnia scitula appears but once in the records of European plankton, Scourfield ( '98) finding it in the waters of Epping Forest in September. The closely related C. quadrangula as well as the other species have been frequently recorded by European investi- gators both in the littoral and the limnetic fauna, but they appear to be less generally found there than the other dominant Cladocera of our waters. It does not appear in the plankton of our Great Lakes (Forbes '82 and '90, Birge '95), or in that of Lake Mendota (Birge '95 and '97), or .Green Lake (Marsh '97), but Herrick ('84) reports it as the most abundant species in Minnesota, and Fordyce ('00) finds it in Nebraska in shallow waters. A revision of the genus is needed before the seasonal distribution of the various species can be worked out on a basis that will make satisfactory discussions of the literature possible. Chydorus sphcericus O. F. Mull. Average number, 422, of which 26 are egg-bearing females, and 6 are immature, the remainder, 390, being females in which the ova were not prominent. The identification of species of Chydorus is attended by consider- able uncertainty. Comparison with named specimens from Europe supplied by Prof. G. O. Sars, leaves no doubt that C. sph&ricus is common in our waters, and it is apparently the dominant species. It is probable that several other species, as, for example, C. globosus Baird and C. c&latus Schoedler, occur sparingly in our waters and have been included with C. sphczricus in my enumerations. The difficulties which attend the attempt to assign every individual to one of the several species of Chydorus can be appreciated only by one who makes the effort. The problem of their specific validity should be solved by a statistical analysis of the range of varia- tion. The seasonal distribution of Chydorus sphcericus in channel waters is in its general outlines very characteristic and well defined. The following table, which gives the average number of Chydorus per m. 3 for each month of our collections, shows clearly that it is a vernal planktont, and that there is a slight tendency toward an autumnal pulse in September, when vernal temperatures return. The number for November (222) would probably be considerably reduced if more than one collection had been taken in that month in 1896. Omitting this year, the average for November falls to 235 78, and a secondary, hiemal rise becomes apparent in December. This December pulse of Chydorus is one of the elements in the upward movement of production in this month (see Part I.), and fuller data may serve to connect it fully with the September-October pulse, especially in more stable conditions. Both of these autumnal- hiemal movements have less than one tenth of the development that the vernal pulse exhibits. The number and percentage of occurrences also confirm the conclusions drawn from the numbers per m. 3 Percentages run higher in the spring, in March-May, and in September-October and in December, and lower in June- August, November, and January-February. Chydorus occurred in all March collections, and in only one third of the August collections. The analysis of the data in this table indicates the presence of Chydorus in the plankton practically throughout the whole year in the whole seasonal range in temperatures, with the larger develop- ments following shortly after the thermograph passes the yearly mean (57 average of monthly means of surface waters) in vernal rise and autumnal decline, the maximum development in April-May SEASONAL DISTRIBUTION OF CHYDORUS. AVERAGE NUMBER PER M. 3 Year Jan. Feb. March April May June 1 804. 234 1895 11 2 044 1896 304 167 1,682 10,271 5,701 448 1897 20 540 320 32 800 900 1898 160 256 300 3,364 356 1 S99 36 65 193 Average 167 53 668 3,235 13,955 388 No of occurrences 9 6 15 9 9 10 Percentage of occur- rences 75 40 100 82 90 72 236 SEASONAL DISTRIBUTION OF CHYDORUS. AVERAGE NUMBER PER M. 3 continued. Year July Aug. Sept. Oct. Nov. Dec. 1894 95 461 100 16 56 1895 91 103 164 38 203 448 1896 64 104 78 160 800 277 1897 213 40 407 650 64 115 1898 50 30 60 28 172 1899 Average 103 49 228 202 222 214 No. of occurrences 11 7 13 12 10 14 Percentage of occur- rences 61 33 81 71 63 82 occurring in average temperatures, for these months, of 60.5 and 68.3, while the minor autumnal development appears in September- October at 74.2 and 57.6 respectively, and the December pulse, if indeed it be a separate and independent pulse, is at the low tempera- ture of 35.2. The December movement may be simply the result of the more stable conditions which attend the appearance of the ice-sheet on the approach of winter. An analysis of the course of the seasonal distribution of Chydorus in channel waters, as given in Table I. and in statistics of other years, indicates the following seasonal regimen. In January-Feb- ruary, at minimum temperatures, the occurrences are irregular (75 and 40 per cent.) and the numbers small (average, 167 and 53 per m. 3 ), while in March, with rising temperatures, occurrences are more numerous (100 per cent.) and numbers rise to 668 per m. 3 In April-May a high percentage of occurrences (82 and 90 per cent.) continues, and they mount rapidly to the maximum record of the year, which in our statistics varies from 4,088 in 1895 to 32,800 in 1897. This vernal pulse reaches its maximum in our records on April 29 in 1895, at 64, and in 1896 on the same day, at 70; on 237 May 25 in 1897, at 66. 3; and on May 24, in 1895, at 73. From this maximum the pulse declines abruptly in a fortnight to a midsummer minimum during maximum temperatures, which continues until September. During this period the numbers are small, rarely rising above 400 per m. 3 (average, 388, 103, and 49), and the occurrences are also less numerous (72, 61, and 33 per cent.). With the decline of temperatures which begins in September the percentage of occur- rences mounts to 81, and the average per m. 3 to 228, and remains near this level during the remainder of the year. An analysis of the full statistical data, of which the records for 1898 are fairly typical, confirms the conclusions drawn from these averages. Chydorus in channel waters is monocyclic, with a well- defined vernal pulse in March- June which includes 95 per cent, of the total annual Chydorus population. There are suggestions of an autumnal pulse, but the data are not sufficient to delimit it. There is no satisfactory evidence that there are recurrent cycles or pulses at briefer intervals during the year. The dominating effect of temperature as a regulating factor in delimiting the seasonal distribution of Chydorus is very evident. This, in addition to its appearance in the annual curve of occurrences, is also exhibited most clearly in a comparison of the vernal pulses in the two years of fullest representation in our records, 1896 and 1898. The following table gives the data of dates, temperatures of surface waters, and numbers of Chydorus. From these facts it appears that the late spring of 1898 delayed the vernal pulse of Chydorus, and that the early spring of 1896 accelerated it in that year so that their apices (April 29 and May 24) are four weeks removed from each other in seasonal location. In both years the rapid rise in the pulse appears after 60 is passed, the culmination occurs at about 70, and the decline, in temperatures above 70. Egg-bearing females were more abundant during the rise of the pulse, and less numerous during its decline. Evidence of great mortality during the decline of the pulses is to be found in great increase in the relative numbers of empty carapaces. Thus, during the decline of the vernal pulse in 1896 there were on the day of culmination, April 29, 2,780 dead to 18,904 living, on May 1, 3,570 to 14,875, and on May 8, 1,578 to 6,706. From 14 to 24 per cent, of the Chydorus population had thus recently perished. Parasitized 238 1896 1898 Date Tempera- ture No. of Chydorus Date Tempera- ture No. of Chydorus Mar. 17 42 256 Mar 15 46 440 " 24 40 7 610 " 22 51 480 " 30 48.1 6 405 " 29 49 5 240 Apr. 10 46 4 1 666 Apr 5 48 3 200 " 17 66 3 4 515 12 52 200 " 24 72 1 S 000 " 1 56 " 29 68 18 904 " 26 57 800 May 1 68 8 14 875 60 8 76 6 706 " 10 62 600 " 18 71 2 1 143 " 17 64 3 300 . " 25 75 3 80 " 24 73 7 880 June 6 79 320 " 31 June 7 70 78 5,040 600 " 11 73 320 14 82 3 200 and fungused individuals were also noted in these periods of decline. Males were recorded in September, December, and February. Chydorus is not given as a constituent of the plankton of Nor- wegian lakes by Huitfeldt-Kaas ( '98) or of Swiss lakes by Fuhrmann ('00), Amberg ('00), or Burckhardt ('00 and 'OOa). Its absence from these cooler waters stands in sharp contrast with its abundance in warm and shallow European lakes. It is reported as abundant in Chroococcacea-rich lakes of North Germany by Apstein ('96), where it is acyclic, with larger development in April-October, and maximum in August or in May- June. According to Weismann (79) Chydorus in some waters is polycyclic. It is also reported by Zacharias ('97a and '98b) from the pond fauna of Trachenberg and many other German localities, where it forms '"em notorisches Mitglied des Teichplanktons." He also lists it ('98b)- from some German streams. Marsson ( '00) found it in some waters near Berlin in April- August, noting a great abundance in one instance in May. Seligo ('00) gives a few statistical data indicating the occurrence of Chydorus in the plankton of Hintersee near Danzig in April- December, with a maximum in August and a secondary one in October. It was, however, sparingly present in adjacent waters. Cohn ('03) finds a like irregularity in its occurrence in waters near Konigsberg. Stenroos ('97) finds it to be one of the most abundant Entomos- traca in the waters of northern Russia and ( '98) a littoral and bottom species near Helsingfors. Scourfield ('98) finds it to be one of the most abundant Cladocera in the waters of Epping Forest, occurring from March to December, with maxima of sexual reproduction in April and November. Scott ('99) reports it as abundant in the littoral fauna of Scottish waters, but rare in tow-net collections in open water. It also occurs in the potamoplankton of European streams, Zacharias ('98b) listing it from a few minor streams, but without seasonal, statistical, or temperature data. It was not separately listed by Skorikow ('97) in the summer plankton of the Udy at Charkow, or by Lauterborn ('94) in the winter plankton of the Rhine. Zimmer ( '99) found it from February to July in the Oder, and Schorler ( '00) finds it abundant in the plankton of the Elbe in April. Steuer ('01) finds it at all seasons in the backwaters of the Danube at Vienna, and in the plankton from March to November "oft in grossern Mengen," but gives no statistics of its seasonal distribution. Fric and Vavra {'01) find it in the channel and backwaters of the Elbe near Podiebrad, but more' abundant in the littoral fauna, though no quantitative or statistical data of its occurrence are given. Zykoff ('03) reports it as present in the plankton of the Volga at all times in small numbers, and suggests a predominance in May- July. Meissner ( '03) also reports it for the Volga, but states that it is predominantly a member of the littoral fauna though present in the plankton of the stream in restricted numbers. No statistical data are given by him. Volk ('03) reports it in the Elbe at Hamburg, but without any details. This species is reported generally from American waters. Forbes C90) reports it in the summer plankton of Lakes Superior and 240 Michigamme in small numbers, and ( '93) in that of the Alpine waters of Wyoming and Montana, where it is, however, more abundant in smaller pools. Birge ('94) finds it generally distributed in collec- tions, including plankton, in Lake St. Clair and ('97) a member of the plankton of Lake Mendota, where its abundance is dependent on the supply of Anab&na. Its maximum only a single well-defined one occurring in each year was found in July-October. Birge regards it as an accidental member of the limnetic fauna, maintained there as long as suitable food is present. Its mode of occurrence does not, however, differ from that of typical plankton organisms, which would doubtless likewise disappear from the plankton if their food should be lacking. It is noteworthy in this connection that it was only sparingly present in the channel of the Illinois in the midsummer-autumn plankton, when as, for example, in 1897 Anab&na and its allies were abundant. It seems "not improbable that temperature even more than food is an important factor in controlling its seasonal and local distribution. It is unquestionably a member of the plankton in our waters, though also abundant here, as elsewhere, in the littoral fauna. In our locality in channel plankton it shows distinctly seasonal limitations which suggest the operation of tem- perature rather than food. Its occurrence in large numbers in Wisconsin lakes in midsummer and its absence in the Illinois at that time may also be correlated in part with the contrasted tem- perature conditions in the two localities. Its occurrence in our littoral fauna may also in part be due to the lower temperatures consequent upon spring-fed areas and the shade of aquatic vegeta- tion. Chydorus is one of those organisms capable of both the littoral and limnetic habit under suitable conditions of food and temperature. In our waters, at least, and, as it seems from the data of distribu- tion, elsewhere, temperature, rather than food directly, appears to be the factor controlling the occurrence of Chydorus in the plankton. Daphnia cucullata G. O. Sars. Average number, 181. In 1897, very much greater, 5,483 per m. 3 For the reasons given by Burckhardt ('00) I use Sars's name cucullata rather than jardinei of Richard to designate those forms of the subgenus Hyalodaphnia in our plankton. In channel waters this species varies considerably, but not to the extent that it does 241 where its numbers are greater. The forms known as apicata Kurz and kahlbergiensis Schoed. appear in small numbers in some years. This species appears in our collections in April-December only, with the exception of one occurrence in January and two in March. Its occurrences and numbers vary greatly in different years. In 1894-95 its numbers were small and occurrences scattering, it being most abundant in November-December. In 1896 there was a large vernal development in April- June, and a series of diminishing pulses in July-September. In 1897 no vernal development appeared in our scattered collections, but in the stable conditions of late summer and autumn occurred the largest development recorded in any year, with a maximum record of 72,760 per m. 3 on October 5. In 1898 there was a small vernal development (3,400) in May- June and a still smaller one (600) in October. A well-defined seasonal routine is thus not demonstrable from our data, though the fact that both the percentage of occurrences and the numbers are highest in May- June and September-October suggests a tendency toward vernal and autumnal pulses separated by a period of less develop- ment in midsummer and of autumnal decline followed by a period of almost complete extinction in midwinter. The statistics of the D. cucullata population in all years in which weekly collections were made, exhibit very clearly the phenomenon of recurrent pulses of 3 to 5 weeks ' duration, with maxima of varying amplitude and minima of less than 400 per m. 3 in all cases but those which mark the September pulse of 1897. There are in 1896 pulses culminating April 24 (2,544 per m. 3 ), May 8 (11,965), June 11 (12,000), July 18 (1,040), August 8 (800), and September 16 (507). In 1897, vernal records are incomplete. Pulses appear July 14 (800), August 17 (1,680), September 14 (57,000), October 5 (72,760), and November 15 (2,040). These pulses coincide exactly or approxi- mately with those of the other Entomostraca which exhibit the same phenomenon, and approximate also those of the Rotifera. A typical pulse, that of October, 1897, is shown in the following table. It is a noticeable fact that the proportion of immature forms is often greater at and after the period of maximum development than at other times, as appears in the table. The relations of temperature to the development of D. cucullata in channel waters appear in the fact that all occurrences in excess of 242 Date Females Females with eggs Young Total Percentage of young Sept 27 160 320 640 1 120 57 ." 29 7,520 4,000 12,800 24,320 52 Oct. 5 3,560 10,800 58 400 72 760 82 12 1 600 7 600 9 200 83 " 19 560 840 4 440 5 840 76 600 per m. 3 are found after the temperatures pass 70, with the[single exception of the decline of the October pulse and the rise of the November pulse to 2,040 per m. 3 at 47, following the high tempera- tures in the late autumn and stable conditions of 1897. From the depression in numbers during the period of maximum heat in mid- summer and the occurrence of the major vernal and autumnal pulses before and after its reign it appears that the temperature optimum for D. cucullata in channel waters lies below this level, that is, below 80. D. cucullata is evidently very easily affected by the changes in hydrographic conditions. Thus, in July-December, 1897 and 1898, the total movement in river levels was 12.4 and 31.4 ft., respectively, while the total cucullata population for these months was 186,420 and 1,140 164-fold greater in the more stable year. D. cucullata thus exhibits the maximum sensitiveness among the Entomostraca to these environmental factors. The D. cucullata population in the plankton consists almost entirely of parthenogenetic females and young. The immature stages form about 60 per cent, and the egg-bearing females 16 per cent, of the total individuals. Dead, parasitized, or fungused indi- viduals were found at times of the maxima or shortly thereafter, but never in very large numbers. Males were found once in December, 1896, and ephippial females also but once, on October 19, 1897, during the decline of the maximum pulse in our records. Daphnia cucullata var. apicata Kurz, in well-developed condi- tion, was found in relatively small numbers during the vernal pulses of 1895 and 1896 and the autumnal pulse of the former year. 243 Incipient stages of this variety appeared also at other times. Burck- hardt ('OOa) does not even concede varietal standing to apicata, regarding it merely as a form of seasonal or local value. Its occur- rence in our plankton when reproduction and growth are most active suggests that it may have a growth value, and be in some way correlated with the factors involved in its cyclic production. Daphnia cucullata var. kahlbergiensis Schoed. appears but once in our records in the plankton of June 11, 1896. The D. cucullata group is a cosmopolitan constituent of the fresh-w r ater plankton, appearing frequently in the records of Euro- pean plankton. Apstein ( '96) finds it in lakes in northern Germany in April-October with maximum numbers in July. The seasonal limits thus resemble those in the Illinois, but the maximum falls at the time of our midsummer decline. Temperatures in these German lakes (16.3 C.) do not, however, reach the high levels attained in our waters in midsummer. Stenroos ('98) records it in several varieties in the plankton of Nurmijarvi See, the helmeted varieties being found in midsummer. Zacharias records it from the plankton of German ponds. Scourfield ('98) finds it in small numbers in Epping Forest interruptedly in April-November, a season coinciding with that in the Illinois. Burckhardt ( '00) finds it represented by five different "forms" in Mauensee in the June plankton. Marsson ('00) finds representatives of Hyalodaphnia (species not given) in the April- June plankton near Berlin. Am- berg ( '00) states that this species appears in April, increasing to a maximum in July- August, and disappears again at the end of November, a seasonal course similar in limits but not in maximum to that in the Illinois. His data are too scattered to trace the course of production with completeness. Seligo ('00), in waters near Danzig, finds the species present in June- January, with maxima in June- July and October. In the period of maximum summer temperatures (16-21 C.) the numbers decline as in this period in the Illinois. In Seligo 's infrequent (two to three weeks' interval) data there are suggestions of minor recurrent pulses in other months. Cohn ('03) finds in Lowentin a Daphnia w r hich he calls D. galeata with vars. kahlbergiensis and cederstromii, and includes all three in his enumeration. His investigation covers the months of May- September, throughout which these forms appear, rising in a series of recurrent maxima on June 26, August 4, and September 2 and 29. (17) 244 Cohn seems not to have called attention to these clearly denned recurrent pulses. In European streams D. cuciillata also forms an important part of the plankton. Lauterborn ('93) states that, with its varieties kahlbergiensis and cederstromii, it appears abundantly in the plank- ton of the Rhine in summer, but is not found in it in winter. Zimmer ( '99) states that D. kahlbergiensis was found constantly in the plank- ton of the Oder in July-September, and Schorler ( '00) also finds it in the Elbe at Dresden in May- August, with larger numbers in June and August. Steuer ('01) reports it, in small numbers only, in August in the backwaters of the Danube at Vienna. Fric and Vavra ('01) reportD. kahlbergiensis as rare in the Elbe. Sowinski ( '88) finds it in several varieties in plankton of the Dnieper and its tributaries, Rossinski ('92) finds it in the summer plankton of the Moskwa, and Zernow ('01) in the June-July plankton of the Schoschma and Wjatka. Meissner ('02 and '03) finds it in several varieties in the May- August plankton of the Volga. D. cucullata in some of its various forms or varieties appears to be widely distributed in American waters. It was reported by Forbes ('82), as D. retrocurva, from the plankton of Lake Michigan, and also ('90) from Lake Superior and adjacent waters. Birge ('91 and '94) also finds it abundantly in Wisconsin waters and in Lake St. Clair. Herrick ('84) and Ross ('97) report it from Min- nesota and Iowa. Careful studies of its seasonal and vertical distribution in Wisconsin waters have been made by Marsh ('97) in Green Lake, and by Birge (95 and '97) in Lake Mendota. In Green Lake D. kahlbergiensis is reduced to a minimum or even extinction in December- April, rises in a late vernal maximum in June- July, falls again to a lower level in August-September, and then rises to a second and sometimes higher autumnal pulse in October. In its main outlines this conforms to the seasonal course of the cucullata form in our channel plankton. Our vernal maxi- mum appears somewhat earlier, as a result probably of an earlier warming up of the water. According to Birge ('97) this species is more definitely periodic in its occurrence in Lake Mendota, being confined entirely to July-December. Here also the largest numbers are found in October, and the individuals gather in lower levels as temperatures decline. 245 Daphnia hyalina Leydig. Average number, 417. In channel waters this species has appeared in but two years, in 1895 in April-July, attaining on June 19 a maximum of 166,208 per m. 3 , of which 150,626 were immature. The collections were too infrequent in these months to trace the course of this vernal pulse. D. hyalina did not reappear until the spring of 1898, on May 24, in a single vernal pulse culminating at 1 1,600 per m. 3 on June 7, and disappear- ing a fortnight later. Its occurrences with one exception were all at temperatures above 70. There is no apparent - cause for its absence in later months or in other years. Males and ephippial eggs were not found. Daphnia hyalina is an exceedingly variable species, and a large number of forms have been described which belong to the hyaUna group. Burckhardt ('00), for example, recognizes 26 such forms as varieties of this cosmopolitan planktont. This variability and the difficulties attending the resulting synonymy cause any discus- sion of the species in other waters to be attended by much uncer- tainty. I shall therefore not attempt to distinguish in my dis- cussion between the various varieties included by Burckhardt in the hyalina group. In lakes of northern Germany, Apstein ( '96) finds that D. hyalina is essentially a winter planktont with a seasonal range of September- July, and with maximum numbers in November- January. The maximum thus appears there at the time of complete extinction in our waters. Stenroos ('97) records it (as D. galeata) in the summer plankton of Karelia, Huitfeldt-Kaas ('98) finds it in Norwegian lakes in July and September in considerable numbers, and Scour- field 's careful studies ( '98) of its seasonal occurrence in waters of Epping Forest reveal an interrupted distribution in April-Novem- ber. vScott ( '99) finds it in numbers in Scottish lochs in the plankton examined at long intervals in March-January. Fuhrmann ('00) reports it as perennial in Neuenbergersee, with a maximum in June followed by a midsummer minimum. Burckhardt ('OOa) finds great diversity in different Swiss lakes and in different years in the relative numbers present. His intervals of collection were too great to detect any pulse-like movement in the production, and it may be that the diversity is due in part to the incompleteness of his records. He concludes that D. hyalina is at a minimum in March-May, increases in numbers slowly (with a preponderance of young indi- 246 viduals) in May-October to a maximum in November-January, which is followed by a rapid decline (with preponderance of adults) to the minimum. His results agree with those of Apstein ( '96) in the main rather than with ours in the Illinois. Seligo ('00) finds D. hyalina in Hintersee, though it is apparently absent from the adjacent Barlewitzersee. In the former lake it appears in May, rising to the year's maximum early in June, continuing throughout the summer in diminished numbers, and disappearing in October. In his infrequent records there are suggestions of several recurrent minor pulses during the summer. Cohn ('03) reports D. galeata regarded by Burckhardt ( 'OOa) as a form of D. hyalina from the region of Konigsberg, but refers it rather to the cucullata group. I shall therefore consider his results only in connection with D. cucullata. D. hyalina appears but rarely in the records of European potamo- plankton. Steuer ('01) reports it, in small numbers only, in May from the 'backwaters of the Danube at Vienna. Fric and Vavra ('01) state that D. microcephala regarded by Burckhardt ('OOa) as a form of D. hyalina is abundant in the plankton at a depth of 0-1 m. in April-November in the Elbe and its backwaters at Podiebrad. It is also reported by Zykoff ('00 and '03) in the late vernal (June- July) plankton of the Volga at Saratoff, and by Meissner ('02 and '03) in the same stream in May- June. The examination of the plankton of the Volga made by these authors is far less extensive than that made of the Illinois River plankton, but as far as it goes it indicates a similar distribution of D. hyalina in the two streams. Volk ('03) reports it from the Elbe at Hamburg without data. The species appears to be widely distributed in American waters, being reported, in some of its various varieties or synonyms, especially from lakes and ponds. Smith ( '74) finds it in the plankton of Lake Superior, Forbes ('82) in that of Lake Michigan, and Birge ('94) in Lake St. Clair. It was also found in the Illinois by Forbes ('78) and in the backwaters of the Ohio River by Herrick ('84), who reports it also from Minnesota waters. Birge ('91) finds it in lakes about Madison, Wis., and Fordyce ('00) in deep pools in western Nebraska. The only investigation of its seasonal distribution in American waters is that of Birge ('95 and '97) in Lake Mendota, where it forms about 3 per cent, of all the Crustacea. It is perennial 247 in this lake but exhibits great differences in its seasonal course from year to year. The vernal development in May-June (the only one in our channel plankton) is relatively large in each year, but is sometimes exceeded by an autumnal one in October. A midsummer minimum sometimes appears between these pulses, and a winter minimum in December- April is always present. From the data here reviewed it seems probable that the very limited seasonal distribution and irregular annual recurrence of D. hyalina in our channel plankton is in a measure indicated in streams elsewhere, and may have its cause in the instability of the fluviatile environment as compared with the lacustrine, where the species evidently finds its environmental optimum. Diaphanosoma brachyurum (Lievin). Average number, 479, of which 154 are females, 49 females with eggs, and 276 immature. This species in our waters is monocyclic, with sharply defined seasonal distribution. With the exception of two records of young individuals in March- April, 1895 (and the identification of these individuals is questionable), all our records of occurrence in 1894-1899 fall between May 25 and October 19, the first vernal records appearing at temperatures of 55.8 to 72.3, and the last autumnal at 52.5 to 65. The one pulse in each year except in 1894, when none was recorded falls in a period of 3-6 weeks in July-September, the first record above 2,000 per m. 3 appearing July 26, and the latest (with one exception, 2,175 on September 27, 1895) on September 7. The pulse varies in duration in different years from 3 to 6 weeks, and attains a maximum on dates ranging from July 26 to August 31, and varying in amplitude from 8,580 to 19,602 per m. 3 An analysis of the distribution of 61 recorded occurrences in channel plankton shows that of these only 13, or 21 per cent., occur outside of July-September, and that the records outside of the seven weeks of the pulse include less than 12 per cent, of the total individuals. A comparison of the seasonal curve of distribution with the annual thermograph reveals the fact that the pulse occurs toward the close of the period of maximum summer heat, and in every case at a temperature of 78 or above, and that the decline of the pulse often begins with declining temperatures, and is always accom- plished during the autumnal decline. The effect of summer heat pulses upon the Diaphanosoma curve is strongly suggested by the 248 data of the appended table, which gives the statistics of temperature, river level, and Diaphanosoma population during the periods of maximum development in 1895-1898. All these data except those of Diaphanosoma are shown graphically in Part I., Plates IX.-XII. The data for Diaphanosoma are less complete than the others, since all of the collections were not counted. In 1895 the Diaphanosoma pulse culminates at 19,602 on August 21, following immediately upon a heat pulse which culminates August 15 at 85.3. The decline of the pulse occurs with a decline of temperature to 72 on September 7. The declines, both of Diaphanosoma and temperature, are hastened after September 3 by 1895 1896 Date River gage Temp. No. of Diaph- anosoma Date River gage Temp. No. of Diaph- anosoma July 2 " 1 n 5.15 4nn 80.8 7r> e 40 cnn " 1 Q 2 en ic\ July 23 5.20 80 424 " 23 4.20 80 120 " 29 5.38 75.5 240 " 28 6.40 82 7,440 Aug. 1 4.20 78.5 1,088 Aug. 3 8.50 80.3 160 8 2.63 79 988 8 8.40 86 14,260 " 12 2.40 84.8 9,801 " 15 7.40 82 2,240 " 21 2.08 81.5 19,662 " 21 7.10 79 880 " 29 2.58 80 7,950 " 26 ' Of) 6.50 77.5 7/1 1 600 Sept. 5 5.70 74 189 Sept. 16 4.10 73.5 663 " 12 3.90 79 1,053 " 30 4.30 58 80 " 20 7 ?n 7Q 468 " 27 3.23 73 2,175 1 249 1897 1898 Date River gage Temp. No. of Diaph- anosoma Date River gage Temp. No. of Diaph- anosoma July 14 6.30 79 160 July 12 7.00 78 60 " 21 5.20 81.1 960 " 19 4.70 84 40 " 30 4.60 84 4,720 " 26 2.90 89 8,580 Aug. 10 2.30 80.8 7,600 Aug. 2 2.70 78.3 6,960 " 17 1.90 79 7,120 9 3.20 83 360 " 24 1.80 77.5 5,120 " 16 3.70 77 60 " 31 1.80 80 1 1 , 000 " 23 4.20 82 1,020 * 3O 7 on 09 cr 7 con Sept. 7 1.80 80 7,600 Sept. 6 4.70 79 240 " 14 2.00 83 1,500 " 13 4.20 62.5 1,800 " 21 2.00 71 240 " 20 4.20 73 960 " 27 4.90 73 400 the rise in river levels. Prior to that date hydrographic changes are slight. With falling levels and higher temperatures after September 7 there is a slight recovery in Diaphanosoma from 189 per m. 3 on the 5th, to 1,053 on the 12th. In 1896 a well-defined heat pulse culminates August 10 at 86.5; and Diaphanosoma, on August 8 at 14,260, with an abrupt depression from 7,440, on July 28, to 160, on August 3, in flood waters. The decline of this pulse from the maximum on the 8th to 440 on the 29th is attended by a uniform decline in temperatures from 86 to 74.3 in fairly stable hydrographic conditions, that is, declining river levels. In 1897 there are two well-defined summer heat pulses, one culminating August 3 at 89, and the other September 14 at 83, separated by a depression to 77.5 on August 24. The crest of the Diaphanosoma pulse likewise has two apices, the first culminating at 7,600 on August 10, followed, during the decline in temperatures, 250 by a fall to 5,120 on the 24th, and, in the rising temperatures which then ensue, by a recovery to a second maximum of 11,000 on the 31st. Diaphanosoma then declines though temperatures continue to rise. These fluctuations all take place in comparatively stable hydrographic conditions. There is a suggestion in the records of this year that rising temperatures in midsummer conditions tend to accelerate, and falling temperatures to depress, development of the Diaphanosoma pulse, and also that after the pulse has continued for some time (six weeks in this instance) rise in temperature ceases to be effective. The autumnal decline in Diaphanosoma may therefore not always of necessity be due to temperature decline alone. In 1898 there are also two midsummer heat pulses, culminating on July 26 at 89, and August 30 at 82.5, separated by a depression which reaches 77 on August 16. The depression to 78.3 on August 2, with the consequent appearance of a third summit at 83 on August 9, is due mainly to the fact that the temperature was taken at 9:15 a. m., while all the others were in the late afternoon. The seasonal curve of Diaphanosoma shows likewise two apices, the first at 8,580 on July 26, and the second at 2,520 on August 30, separated by a depression to 60 per m. 3 on August 16, when temperatures are lowest. In this year the flood of the middle of August doubtless plays a large part in depressing alike the thermograph and the seasonal curve of Diaphanosoma, but in the light of the evidence from 1897 in stable hydrographic conditions the direct influence of temperature is also possible in this instance. Diaphanosoma is thus a late summer planktont w r hich in develop- ment is very responsive to changes in temperature. It appears in the plankton in small numbers shortly after the establishment of summer temperatures in May- June, but does not begin its maximum development until maximum summer temperatures have existed for six to eight weeks, and is apparently incited to this by a summer heat pulse. Males were recorded on July 18 and August 1, and ephippial females on August 1 and September 5. Dead individuals were most numerous during or subsequent to the maximum of the pulse. This species is reported by Apstein ('96) in the plankton of Dobersdorfersee, where it is also monocyclic, first appearing in May, and attaining its maximum in September, when the males first appear. In contrast with conditions in our waters the maxima 251 appear after the period of maximum summer heat. Zacharias ('97a) reports it from German carp ponds in July, and Stenroos ('97) lists it as a littoral species in midsummer in northern Russia. Scott ( '99) finds it rarely in lakes of Scotland in August, and then only in the plankton, though many shore collections were examined. Burck- hardt ( '00) reports it from the smaller and shallower Swiss lakes in isolated records ranging from May to November, and regards its absence from the deeper lakes as due to the low temperatures which at all seasons would surround its winter eggs, which sink to the low r er levels. In Vierwaldstattersee ('OOa) he finds this species in the plankton only in September-November, and then more abundantly near shore than in the middle of the lake. In Alpnachersee the period of occurrence extends from June to November with a maxi- mum in July. Fuhrmann ('00) gives the seasonal distribution in Neuenburgersee as extending from May to November, with a maxi- mum in September. Marsson ('00) finds a seasonal distribution from July to October in small lakes near Berlin. Seligo ( '00) finds in Hintersee, near Danzig, a seasonal distribution in 1898 extending from June 6 to October 18, with a maximum of 225,000 under 1 sq. m., depth, 24 m. (?) on August 9. Fric and Vavra ( '01 ) state that this species is very abundant in summer months in the plank- ton of the backwaters of the Elbe, especially in levels at depths of 0-1 meter. Cohn ('03), on the other hand, finds in waters near Konigsberg that Diaphanosoma is present in greatest abundance in depths of 20-30 meters. It occurs in summer months, with large numbers in July-September and a maximum in August-September. It was not found in shallow waters. As a constituent of the potamoplankton Diaphanosoma has been reported by Schorler ('00) in the Elbe at Dresden as abundant in June-September. Steuer ('01) finds it in the backwaters of the Danube at Vienna in June-September, with a maximum in August, but never in great numbers. Meissner ('03) reports it sparingly from the Volga in July. In American waters Diaphanosoma is widely distributed. Forbes ('90) found it abundant below surface levels in Lake Michigamme in August. Birge ( '94) reports it in the plankton of western Lake Erie but not in that of Lake St. Clair in September. In Lake Mendota, Wis., he ('95 and '97) has worked out its seasonal and vertical distribution with a fulness and care not equaled by any 252 European author previously quoted. Our results in Illinois waters are in striking confirmation of his conclusions. He finds the first scattering individuals in the plankton late in May, but numbers do not rise until late in July or early in August, increasing rapidly through August or even into September, then declining rapidly, and disappearing entirely before November 1 . The active period is thus at a time when a considerable part of the lake is at or above 68. In our waters these temperature limits are 78 or above, but the sea- sonal distribution is almost identical with that in Lake Mendota. He finds it more abundant in the upper strata, 0-2 meters, than in the deeper ones just the opposite of Cohn's ('03) results. Marsh ('97) has also determined its seasonal and vertical distribution in Green Lake, Wis., with considerable care. Occurrences from the last of October to the last of June are very few, and maximum numbers appear from the middle of August to the middle of Septem- ber, when surface waters have a temperature of 65-80. It occurs in all depths (0-40 m.), but 70 to 80 per cent, of the individuals were taken within 10 to 15 m.of the surface, the upper 5 meters being more densely populated by night than. by day and in September- October than in August. Diaphanosoma is a typical planktont, with strong antennae, and an active swimmer. Examination of the literature indicates its wide distribution in the plankton of lakes and streams, and its very marked seasonal limitation to seasons of higher temperature. It is thus, as Birge ('97) has stated, markedly stenothermous. The divergent conclusions concerning its limnetic habit and its vertical distribution will doubtless be found to rest in some cases upon insufficient data, and in others, upon its reactions to varying condi- tions of light and temperature. Eurycercus lamellatus O. F. Mull. This species occurred spar- ingly and irregularly in the winter plankton at minimum tempera- tures from November 30 to March 28. It is evidently adventitious. Ilyocryptus spinifer Herrick. Average number, 4. This species occurred sparingly and irregularly in the plankton during the warmer months. The earliest record was on July 23, and the latest October 11 at 65. This species is evidently adventitious in the plankton. I have doubtfully referred our examples to Herrick 's species /. spinifer, for the reasons given by Herrick and Turner ('95), rather than to /. longiremis, to which Birge ('91) would refer our 253 American form described by Herrick as /. spinifer. A larger amount of material exhibiting a fuller range of variation may, however, serve to connect the two. Leptodora hyalina Lilljeborg. Average number, 3. This species occurred in small numbers and somewhat irregularly in our collec- tions of channel plankton in summer months. Our earliest record was June 28 ; and the latest, August 30. It is our largest crustacean planktont and a fairly active swimmer, and was often taken in our tow-nets, which had a larger mouth and coarser mesh (No. 12) than our plankton net. I took this species in great numbers in the upper meter of water at midday in May- June in Lake Meredosia with a seine of No. 000 silk. It may be that it is less abundant in the channel than in the backwaters, and the small number in the plank- ton collections from the channel may also be accounted for in part by the escape of Leptodora from the small orifice (10 cm.) of the plankton net, or to its negative rheotropism when stimulated by the currents of the plankton pump. Macroihrix laticornis Jurine was found in the plankton in May at 64-73, adventitious in flood waters. Moina micrura Kurz. Average number, 261 per m. 3 In 1897 it was much more abundant, averaging 5,106 in the more stable conditions of that year. This is the most abundant of all our Cladocera, appearing in great numbers in periods of stable low water during maximum temperatures. It is exceedingly irregular in the extent of its devel- opment in different years, the average numbers per m. 3 in 1894-1898 being respectively 21,844, 22,842, 188, 5,106, and 261. After mak- ing allowances for the irregularity in the number and distribution of the collections in the several years, it still remains apparent that Moina is very uneven in its distribution. The seasonal distribution of Moina in channel plankton is con- fined to July-September with the exception of 9 occurrences in small mmibers in the last days of June and the early part of October. The earliest record is June 19, in 1895, w r hen the very large number of 329,448 per m. 3 were found, a degree of development which implies a previous period of multiplication. The first records in subsequent years were all later than this date in June or early in July. After several recurrent pulses, each of 3 to 5 weeks ' duration, the numbers decline to a very low level, and the species disappears 254 from the plankton in September-October. In 1898 (Table I.) the last record was made October 11 the latest in any year with the exception of an isolated record October 26, 1897. Moina micrura is thus distinctly a summer planktont. It appears in the plankton only after maximum summer tem- peratures of approximately 80 have been reached, and decreases rapidly as soon as the autumnal decline passes this point, and soon thereafter vanishes from the plankton. Its optimum temperature in channel waters is thus near 80. The relation which hydrographic conditions bear to the ap- pearance of Moina in channel plankton appears upon a comparison of the Moina population and the movement in river levels in differ- ent years, as shown in the following table. MOINA AND HYDROGRAPHIC CHANGES. June July August September October S ^ s ^ B ^ 8 ^ s d d a S S d V S B 0) ** B O E ^j E E O E o E B s B B Year . E > O E O ** . E > o . E o G g S3 E ^ OJ E & E Z CD p. I a !* & , i p, ,, P, ^_i M +3 M 5 H S M a bo $ > ?> O t> ^ o j> o ^ ^ h ^ H ^ H ^ 1894 192 3.4 40,415 2.1 129,880 2.6 3,677 4.7 3.1 1895 329,448 2.7 91,318 7.3 2,597 3.5 87 8.8 10 2.7 1896 3.4 152 7.8 1,220 4.3 3.7 4.6 1897 6.3 1,373 5.2 1,280 2.6 70,040 0.6 605 0.6 1898 75 4.0 660 7.4 1,496 7.5 770 6.2 40 3.9 While the correlation is not proportionate between the extent of movement in levels and the Moina per m. 3 , it is still very evident that in years of continued and more stable low water Moina is found in much greater numbers, as appears on a comparison of 1897 and 1898. It is also confined largely to the more stable part of the year, appearing in 1895 in June- July in large numbers, but falling off when the minor floods of August-September occur, while in 1897 the large numbers are found in the stable levels of August. 255 The cause of this limitation of Moina to periods of low levels in maximum temperatures appears to lie in the food relations of the species. Moina abounds in waters approaching stagnation. The slackened current, increased sewage contamination, and excessive growth of the smaller algas and chlorophyll-bearing flagellates at such seasons in the channel of the Illinois furnish an environment favorable to the great increase in Moina, such as was recorded in the low water of July- August, 1894, of June- July, 1895, and of September, 1897, exceeding in each instance that of any other species of Entomostraca in the plankton. The relatively smaller numbers of Moina at the same seasons in the less contaminated backwaters lends additional support to the view that these condi- tions approaching stagnation are in a measure responsible for its unusual development in channel plankton. Of the total Moina population, over 65 per cent, are young or immature, 7 per cent, are egg-bearing females, embryos are often freed from the parent on application of the preserving fluid, 11 per cent, are males, and the remainder, females without eggs. Males appeared with the maximum or decline of the major pulse for the year in 1894 (August), 1895 (July), 1897 (July and September), and 1898 (September), but ephippial females were recorded only in June-July, 1895. The seasonal distribution of Moina conforms to the type of a series of recurrent pulses wherever the numbers are considerable and the collections sufficiently frequent to delineate their courses. Even in the small numbers of 1898 (Table I.) there are suggestions of such pulses. Moina micrura seems to be a species characteristic of the pota- moplankton. It is not mentioned as a constituent of the plankton or littoral fauna by any of the various investigators quoted else- where in this paper who deal with lakes or ponds in Europe or North America; nor does it appear as a frequent constituent of the potamoplankton elsewhere. Skorikow ('02), indeed, makes the statement, " Bemerkenswert ist fur die Fliisse vollstandiges Fehlen der Gattung Moina." This, however, is hardly the case, for Sowinski ("88) finds it in the plankton of the Teterew, a tributary of the Dnieper, and Fric and Vavra ('01) report it from the Elbe in 0-1 m. strata in July-September, males appearing in the latter month. Meissner ( '02 and '03) also finds it in the Volga at Saratoff, 256 where it " appears almost constantly in the plankton." His investi- gations, however, appear to cover only the months of May-August. Maximum numbers appeared in July, and considerable differences were noted in two successive years. I find no previous record of the occurrence of Moina nticrura in American waters. Pleuroxus denticulatus Birge. Average number, 5. Occurs in small numbers and irregularly during the autumn and spring months during declining or rising temperatures. The earliest autumnal record is November 2, and the latest, December 15; the earliest vernal is March 8, and the latest is May 31. Egg-bearing females appear in the earlier occurrences in each season. It is evidently adventitious. Pleuroxus hamatus Birge was found once March 29, 1898. Scapholeberis mucronata O. F. Mull, was recorded in small num- bers in May and August-December through the seasonal range of temperatures. It is apparently adventitious in channel plankton, though not attending flood invasions. Sida crystallina O. F. Mull, is rare in the summer plankton. Simocephalus serrulatus Koch. Average number, 261. This species appears irregularly in the plankton, generally in small numbers and in isolated occurrences. An exception to this is found in May- June, 1898 (Table I.), when it is found continuously May 10- June 14 in numbers which furnish 61 per cent, of the total for all years. There is a slight preponderance of occurrences in May and September, 12 of the 26 recorded appearing in these months. Their irregular appearance in the plankton in general suggests that they are adventitious from the littoral area, especially at times of their maximum development there. The period of their occur- rence in the channel plankton in 1898 was one of rising water, 10 to 14 feet above low-water mark a stage permitting free communica- tion between the channel and large areas of slightly submerged bottom-lands. Simocephalus vetulus O. F. Mull, appeared irregularly and in small numbers in the plankton in April- June (4 occurrences) and September-December (5 occurrences). It is evidently adventitious in the plankton, coming from the littoral area, though not confined to flood waters. 257 i OSTRACODA. The species of this order are in the main, during adult life, limicolous forms found in the littoral or bottom ooze or amid the decaying organic matter which accumulates in these regions. The current, the movements of fish and other large aquatic organisms, the action of waves along shore and in shoal regions, all tend to bring these animals into the limnetic fauna. Their centers of distribution are thus in littoral or bottom regions, and in the adult stage they are almost wholly adventitious in the plankton of our waters. In 1898 the average number per m. 3 was 191, but in 1897, a more stable year, only 97. The seasonal distribution of their occurrences in the plankton indicates a decided predominance in March-October, in which months all but 6 of the 73 records were made. In these months from 23 to 82 per cent, of the collections contained Ostracoda, while in December-February only 8 to 20 per cent. The percentages in April-September are all above 45 per cent., and the numbers per m. 3 are also larger in this period (see Table I.). The tendency toward a vernal increase is apparent in the records of each year in much the form in which it occurs in 1898 (Table I.). The numbers are always small at all seasons, not exceeding 1,600 per m. 3 even in the vernal season. The seasonal distribution is such that the greater part of the occurrences and the greater number of individuals appear in the plankton during the warm season, that is, above 50. Thus, in 1898 all but 4 of the 24 occurrences and 99.5 per cent, of the indivi- duals appear after the vernal rise passes 50 and before the autumnal decline reaches that point. The Ostracoda are plank- tonts of the warmer season. It is significant that the Ostracoda in our plankton collections are largely young or immature individuals. In 1898, 'for example, 74 per cent, of individuals observed were not adult, and most of these appeared in April-June. Their occurrence in the plankton can not be traced to the action of flood waters. It thus seems probable that the young Ostracoda may temporarily adopt more of a limnetic habit than the adults. No attempt was made to systematically identify the Ostracoda of the plankton catches. The list of species and the notes thereon 258 which follow, are drawn in the main from Sharpe ('97), to whom I am also indebted for assistance in identifications which I have made. A few supplementary notes are based on my plankton .records. DISCUSSION OF SPECIES OF OSTRACODA. Candona sigmoides Sharpe. is rare in shore collections below the plankton station. Candona reftexa Sharpe was taken but once in the river on November 11. Candona simpsoni Sharpe appears commonly in April-May, and again, in smaller number, in October-November in shore collections on the west side of the river at the plankton station. It is occa- sionally adventitious in the plankton at these seasons. Cypria exsculpta Fischer appears rarely in the channel plankton and in shore collections in April-October. Cypria ophthalmica Jurine is found frequently in the plankton throughout the year, but more abundantly in May-September, and especially in late summer and early autumn. Cypria pustulosa Sharpe was taken rarely in channel plankton in July and September. Cypridopsis vidua O. F. Mull, was perennial in the plankton, though present in greater numbers in May-October. It is the commonest of the Ostracoda in the plankton, and it seems probable that many, though not all, of the young and immature forms belong to this species. Limnicythere illinoisensis Sharpe was taken in the plankton in March, August, and November in 1898, in two instances in flood waters. COPEPODA. This is the most abundantly represented order of the Entomos- traca in channel plankton. Though the species number but 12 to the 25 Cladocera, the individuals among the Copepoda outnumber the Cladocera over fivefold in the grand totals, the ratio varying in individual years from twofold in 1894 to almost sevenfold in 1898. The average number in 1898 was 40,608 per m. 3 ; in 1897, in more stable conditions, 80,632; in 1896, a year of recurrent floods, 43,764 approximately the number in 1898; in 1895, a year of low w r ater in spring, 116,264 the highest average of any year; and in 259 1894, 53,149. On June 19, 1895, the Copepoda attained a vernal maximum of 1,022,476 per m. 3 more than twice the maximum record for any other year. The Copepoda occur in every collection examined, and throughout the whole seasonal range in temperatures. As shown in Table I., the copepodan population during minimum temperatures in De- cember-February is at a minimum, the number per m. 3 rising above 10,000 per m. 3 in but 6 instances in 44 collections in these months, and falling below 1,000 in but 5. In March- April, as temperatures rise, the numbers increase rapidly, especially after 50 is passed, to a vernal maximum in the last days of April or early in May, usually at the time of the vernal volumetric maximum or very shortly thereafter. In fact, volumetric maxima are generally accompanied by copepodan maxima culminating at the same time or a week later, as in May, 1898, when the volumetric is on May 3 and the copepodan on May 10. Numbers continue to be large during the period of summer heat, declining somewhat tardily with the autumnal decline in temperatures. In midsummer in 1898 numbers fall below 20,000 in 9 instances in disturbed hydrographic conditions, but in all previous years in April-September there are only 9 such records in a total of 63. The decline to the winter minimum is usually com- pleted in November, though in 1897, 20,000 is not permanently passed until December 21, at 32. The Copepoda are thus perennial in the plankton, and the fact that they exhibit a larger winter population than the Cladocera is due to the fact that a number of species, the Harpacticida, Cyclops bicuspidatus, C. prasinus, C. serrulatus, and C. modestus appear to be planktonts belonging to the colder part of the year. As a whole, however, the Copepoda reach their greatest quantitative develop- ment in the warmer part of the year, with a major pulse in April- May and an occasional autumnal pulse, as in 1897, of equal or greater proportions. The whole course of the seasonal occurrence of the Copepoda as revealed by collections at frequent intervals, exhibits the phenome- non of recurrent pulses at intervals of 3 to 6 weeks, and more clearly denned in stable conditions. Owing to their relatively smaller numbers the adult Copepoda do not show the pulse phenomenon ( 18) 260 as clearly as the nauplii and immature forms. In 1898 the adults form only 10 per cent, of the total. The relation which hydrographic conditions bear to the copepodan population may be inferred in part from the comparison of years given above, and from the following table, in which are given the average number of Copepoda per m. 3 and the total monthly movement in river levels in July-December, 1897 and 1898. July August September 1897 1898 1897 1898 1897 1898 Average Copepoda per m. 3 81,543* 5.2 7,720 7.4 121,070 2.6 11,080 7.5 261,387 0.6 36,920 6.2 Total movement in levels in ft October November December 1897 1898 1897 1898 1897 1898 Average Copepoda per m. 3 128,093 0.6 28,285 3.9 49,240 2.2 10,692 2.6 15,740 0.5 7,908 2.4 Total movement in levels, in ft. ... With a total movement of 1 1.7 ft. in July-December in 1897 and nearly three times as much (30 ft.) in 1898, we find copepodan population falling off to less than one sixth that of the more stable year. Of the total Copepoda in our records for 1894-1899, 78 per cent, are nauplii of Cyclops and Diaptomus, 13 per cent, are immature Cyclops, and the remaining 9 per cent, are HarpacticidcB, Diapto- mus, and adult Cyclops. Of the twelve forms, Cyclops viridis var. insectus is the most important quantitatively, and includes one fourth of the total adult copepodan population, exceeding the next in importance, C. viridis var. brevispinosus, by over threefold. 261 The following forms are of numerical importance in the order named : C. bicuspidatus , young Diaptomus, Cyclops edax, Diaptomus sicilbides, D. pallidus, Canthocamptus spp., and Cyclops albidus. Cyclops prasinus, C. modestus, C. phaleratus, and C. serrulatus are also found, but in such small numbers as to be of no quantita- tive consequence. DISCUSSION OF SPECIES OF COPEPODA. Argulus sp. A small and apparently young argulid was found in the plankton on August 10, 1897. Members of this genus are abundant upon Amia calva and both species of Lepisosteus, all very common fish in channel waters. Canthocamptus spp., including C. illinoisensis Forbes. Average number, 78. Canthocamptus was found in the plankton in every month of the year but June. The percentage of collections contain- ing Canthocamptus is greatest (44 to 63 per cent.) in March-May and November, and the numbers per m. 3 are highest in March-May, when females, females with eggs, and nauplii all occur in their maximum numbers. All records of totals in excess of 400 fall in this vernal period with the single exception of one collection in August, 1897. The largest number, 3,058 per m. 3 , was found April 29, 1896. Canthocamptus occurs throughout the whole seasonal range in temperatures, with smallest numbers and least regularity during maximum summer heat in June- August. It is thus a planktont of the colder rather than the warmer part of the year. The relations which hydrographic conditions bear to the occur- rence of Canthocamptus in the plankton may be inferred from the fact that of the 48 records in 1894-1899, 24 were made in rising flood waters, 14 in falling flood stages within a few days after the culmination of the rise, and but 10 in stable conditions or in declining levels when flood waters of recent origin did not fill the channel. From these facts it seems probable that Canthocamptus is in the main adventitious in the plankton from its normal habitat in the slime at the bottom and margins of the river and its backwaters. Over 88 per cent, of the total Canthocamptus recorded in the plankton consists of nauplii. It may be that as is the case with the young Ostracoda they enter the area of the plankton more readily than the adults. Adults were found in the plankton only in 262 November-May; females with eggs, only in February- April ; and a female with attached spermatophore, in March. Nauplii appear in greatest numbers in April-May, attaining 2,862 perm. 3 April 24, 1896, but they rarely rise* above 400 per m. 3 outside of this vernal period, and are found only in very small numbers in December- March. It appears from our data that the breeding season is prin- cipally in April-May. Cyclops albidus Jurine. Average number, 113; in 1897, 136; in 1896, 33; and in 1894, but 10. A discussion of the variation and synonymy of this species has been published by E. B. Forbes ('97). The species is numerically least important of the dominant members of the genus in our plankton. It was recorded in all months but December and February, but its season is practically confined to April-October, the only exceptions being three records in small numbers in January, March, and November, and two of larger numbers (300 and 200) in the higher temperatures of the delayed autumn of 1897. There is a tendency toward a summer minimum in June- July, with pulses of greater amplitude in April-May and again in August-October. In these months the percentage of collections containing C. albidus is highest, being respectively 55, 50, 38, 56, and 53 per cent., and these are the only months in which the numbers per m. 3 rise above 600. The highest numbers recorded, 2,862 and 2,400, occurred respectively on April 24, 1896, and October 5, 1897. Although C. albidus is found in the extremes of temperatures, it shows a decided increase after temperatures pass 60 in the vernal rise, and falls off immediately after the autumnal decline passes this point. With high temperatures continued into October, in 1897 we find it continuing in larger numbers. On the other hand, during maximum summer heat (about 80) numbers, as a rule, fall below 300 per m. 3 The temperature optimum thus appears to be in the neighborhood of 70. The three greatest pulses re- corded, occur respectively on April 24, 1896, at 72; on April 26, 1898, at 57-, and on October 5, 1897, at 71. The numbers are too small to exhibit very clearly the phenome- non of recurrent pulses, though the vernal and autumnal pulses are usually well defined, and in the stable conditions of 1897, August, September, October, and November pulses may be traced. 263 Hydrographic conditions appear to affect C. albidus as they do other Entomostraca. In July-December, 1897, in stable low water the C. albidus population exceeds by over threefold that of these months in 1898. Of the totals of all records in 1894-1899, 74 per cent, are fe- males, 4 per cent, with eggs and 70 per cent, without, and the remaining *26 per cent, are males. Immature forms and nauplii were not distinguished from those of other species. Egg-bearing females were recorded only in May and August-October, at times of maximum pulses. Over 82 per cent, of the males were found in August-October a period of declining temperatures and decreas- ing food supply. This is a widely distributed species, though it seems generally to be present in relatively small numbers in the plankton. It occurs in many European lakes. Stenroos ( '98) finds that it is the most abundant species of Cyclops in Nurmijarvi See, occurring in both the plankton and littoral fauna throughout the summer. Scourfield ('98) finds it common in the waters of Epping Forest, where it is perennial in ponds and small lakes ; and Burckhardt ( '00) also finds it in the smaller lakes of Switzerland. It appears to be more generally reported from European streams. Thus, Schorler ( '00) finds it to be rare in the plankton of the Elbe at Dresden. in May; and Fric and Vavra ('01), perennial in the littoral fauna of the same stream at Podiebrad, while Volk ('03) reports it in the plankton at four of seven localities examined at Hamburg. Meissner ('02 and '03) finds it in May-August in the Volga at Saratoff, where it is abundant in the littoral zone or among vegetation and in quiet backwaters. Under a variety of synonyms this common and variable species has been reported from many American waters by Herrick ('84) and others. It was described by Professor S. A. Forbes ('90) as C. gyrinus, from the plankton of Lake Superior. With the exception of Marsh's record ('95) from Lake St. Clair, it does not elsewhere appear to have been found in the plankton of the Great Lakes. Marsh ('93 and '95) finds it generally in the plankton of smaller bodies of water in Wisconsin and Michigan, and E. B. Forbes ('97) reports it as generally distributed in American waters of a permanent character. Brewer ('98) reports it (as C. signatus) in the vernal plankton of deep pools near Lincoln, Neb. No statistical 264 data on its seasonal distribution are given by any of the authors cited. C. albidus appears thus to be adapted to both the littoral and limnetic areas, but seems never to attain great numbers in the latter. Cyclops bicuspidatus Claus. Average number, 373; in 1897, 206; in 1896, 145; in 1895, 312; and in 1894, only 2. A full dis- cussion of the variation and synonymy of this species has been published by E. B. Forbes ('97). This species shows sharply marked seasonal limitations. Every one of the 68 records, with the exception of one of a single female found September 30, falls within November-May, and all of the May records were made in the delayed low temperatures of the spring of 1898. The general distribution of this species during this period is indicated by the high percentage of collections in which it was found, viz., 63, 71, 67, 73, 93, 53, and 40, respectively, for November- May. The numbers per m. 3 are, however, high only in November and April-May, reaching 8,000 in 1895 and 1898 in this vernal pulse, and 3,560 in November, 1897, in the autumnal pulse. In Decem- ber-March numbers do not rise above 500 per m. 3 save once in December and on March 24-30, 1896. C. bicuspidatus is thus a winter and early spring planktont in channel waters of the Illinois. The temperature adaptations are exhibited by the fact that only 13 of the 68 occurrences are in temperatures above 50, only 5 above 60, and but 1 above 70 that of May 24, 1898, at 73. On the other hand, the greater developments in numbers take place during these higher temperatures of 50-70, the only rises above 1,000 per m. 3 at temperatures below 50 being those of March 30 and April 10, 1896, at 48 and 46.4, and of November 15, 1897, at 47. Mini- mum numbers thus prevail below 45, and the temperature opti- mum in channel waters of the Illinois appears to lie near 60. The seasonal routine in channel waters begins with the appear- ance of small numbers about November 1 , with an occasional pulse of some amplitude in that month followed by a continuance of small numbers through the minimum temperatures of December-Feb- ruary, and a rise with the temperatures in March to a maximum vernal pulse toward the end of April or the first of May, and a complete disappearance of adult individuals after temperatures pass 70 during May-October. 265 Stable hydrographic conditions appear to favor the increase in C. bicuspidatus, as is seen in the large pulse of November 15, 1897 (3 ,560) , and the slight pulse (240) during declining levels in February, 1899. The vernal development of 1898 (Table I.) is distinctly pulse-like, and there are traces elsewhere of similar phenomena, but in general the numbers of C. bicuspidatus are too small to exhibit clearly the phenomenon of recurrent pulses. Of the totals of all individuals recorded in 1894-1899 I find that 37 per cent, are males, 16 per cent, egg-bearing females, and 47 per cent, females without eggs. Immature forms and nauplii were not distinguished from those of other species. With the exception of a few stragglers, the egg-bearing females were limited principally to March-May. In exceptional cases the males greatly outnum- bered the females, as on November 15, 1897, when the ratio was 2,820 to 680. Though apparently widely distributed, this species does not appear frequently among the planktonts reported from European lakes. Scourfield ( '98) reports it as a common species in the waters of Epping Forest throughout the year with the exception of a period of absence or depression in July-August, and Scott ( '99) finds it in shore collections made in various months of the year in Scottish lakes, and more abundantly in the warmer months. It has been reported in the potamoplankton in Europe only by Rossinski ('92) from the Moskwa, by Zernow ('01) from the Schoschma, and by Volk ('03) from but one of seven localities in the Elbe at Ham- burg. In American waters, on the other hand, C. bicuspidatus is more abundant, and in the Great Lakes it forms a very important part of the plankton. Forbes ('82) finds it (as C. thomasi] to be the dominant Cyclops in the summer plankton of Lake Michigan and ('90) also abundant in that of Lake Superior. Marsh ('93 and '95) finds it in the summer plankton of the Great Lakes, near Charlevoix, in Lake St. Clair, the Detroit River, and Lake Erie, but only rarely and in small numbers in the smaller bodies of water in Wisconsin and Michigan. E. B. Forbes ('97) extends its recorded range to Massachusetts and to the lakes and rivers of Wyoming, and states that it is widely distributed in America and occurs in large ponds and rivers. Brewer ( '98) reports it in the vernal plankton of deep 266 pools near Lincoln, Neb. None of the investigators quoted give statistical data of the seasonal limitations of C. bicuspidatus. The absence of this species from the summer plankton of the Illinois River and its abundance in that of the Great Lakes is perhaps explained by the temperature conditions. Surface waters in Lake Michigan are reported by Ward ('96) to range from 62 to 67 August 11-29, while deeper waters at and below the thermocline reach a minimum of 42. The warmest waters there (62-67) are thus considerably cooler than the coolest in the waters examined by us (which are usually above 70 and often above 80) during the months in which C. bicuspidatus is not found in our plankton. That its absence is not due to sewage contamination in low water which usually prevails during the warmer months is shown by the prompt reappearance of the species in the autumn; as, for example, in 1897, when sewage was even more abundant than usual. It may be that temperature is also one of the factors limiting its distribution elsewhere. Cyclops edax Forbes. Average number, 49; in 1897, 194; in 1896, 159; in 1895, 321 ; and in 1894, 187. This is the third species of Cyclops in numerical importance in channel plankton of the Illinois. With the exception of a single record on November 2, 1897, all occurrences of this species in channel plankton are confined to April-October, and all but 9 of the 48 occurrences are in July- October, and 32 of them in July- September the period of maxi- mum summer heat. During these three months the percentage of collections containing C. edax is highest (44 to 75 per cent.), and they are the only months in which the C. edax population rises above 1,200 per m. 3 in channel waters excepting a single instance on October 5, 1897, in the high temperatures of that delayed autumn. In other months the records are all below 800 and generally below 400 per m. 3 The highest number recorded was 3,600 on October 5, 1897. The seasonal distribution, with maximum numbers in July- September, exhibits a temperature adaptation on the part of C. edax to maximum summer temperatures (70 to 80) in channel waters. An examination of the records shows that only 13 of the 48 records of this species fall in temperatures below 70, and these were all in the months of April, May, September, October, and November, at 267 times when occurrences were scattering and numbers few; that is, during the rise or decline of the species to or from the summer maximum. Of the 13 records below 70, there were 5 between 60 and 70, 7 between 50 and 60, and but 1 below 50. Cyclops edax in channel waters of the Illinois is thus stenothermic in narrow limits near the maximum temperatures of the year. The relation which hydrographic conditions bear to the seasonal development of C. edax may be inferred from the fact that the July-October population of this species in the disturbed waters of 1898 was only 35 per cent, of that in the more stable months of the preceding year. The occurrences of C. edax take the form of pulses, though less distinctly recurrent and less clearly denned than in species present in larger numbers. Such pulses appear in July, August, and September, 1895, and in August and October, 1897. In 1898 (Table I.) the numbers present are too small to clearly indicate recurrent pulses, though suggestions of the phenomenon appear in the records. In general these pulses tend to coincide with those of other Entomostraca. Of the totals of all our records of C. edax in 1894-1899, 60 per cent, are females without eggs ; 1 1 per cent., females with eggs ; and 29 per cent., males. Young and nauplii were not distinguished from those of other species. Egg-bearing females were found in April and in June-October, but in greatest numbers in July- August. Males occur in June-November, with no marked predominance in any period. This species has not been separated from C. leuckarti by other investigators of the plankton, though E. B. Forbes ('97), after a careful comparison of American forms with C. leuckarti of Europe, concludes that edax is specifically distinct, and that leuckarti also occurs in American waters, though apparently not in numbers com- parable with those in European waters. C. edax appears in a measure to replace it in our plankton. He reports it as widely distributed in American lakes and streams and in the plankton of our Great Lakes. Cyclops leuckarti Claus. A single dead specimen was recorded in channel plankton August 26, 1898. E. B. Forbes ('97) records it from the Fox and Sangamon (tributaries of the Illinois) , from the Illinois and Mississippi rivers, and from Quiver, Flag, and Dogfish 268 lakes, backwaters of the Illinois at Havana. It is not, however, at any time a factor of any importance in channel plankton of the Illinois at Havana, being confined to the spring-fed lakes or those shaded by vegetation, where regions of lower temperatures may be found. This is a widely distributed form in the plankton of European waters. Stenroos ('98) finds it abundant in the plankton of Nur- mijarvi See, Scourfield ( '98) reports it as common in the waters of Epping Forest in February-October, and Scott ( '99) as rare in that of Scottish lakes. Fuhrmann ( '00) states that it is always rare in Neuenburger See except in April, and is absent in November- December, while Burckhardt ('OOa) finds it to be perennial in Vierwaldstatter See, with breeding season in May-September and maximum in August or September. It has been generally reported from European streams. Schorler ( '00) finds it in the Elbe at Dresden in May-October, with greatest numbers in July-September, and Volk ('03) reports it from four of seven localities in the same stream at Hamburg, though Fric and Vavra ( '0 1 ) do not find it at Podiebrad. Zykoff ( '03 ) , Zernow ('01), and Meissner ('02 and '03) find it in the plankton of Russian rivers. The last author states that it occurs in both channel plankton and littoral fauna among vegetation where breeding females abound during the maximum in May. The young only appear in the chan- nel plankton. In American waters this species has often been held to include C. edax, and the data here quoted from Birge and Marsh refer to the combined species. Marsh ( '93 and '95) finds it generally distributed in the lakes of Michigan and Wisconsin, and in the plankton of lakes Erie, Michigan, and St. Clair. Birge ('97) finds it in the summer plankton of Lake Mendota, where it is even more abundant than C. viridis var. brevispinosus. Cyclops modestus Herrick was recorded in channel plankton only in November, December, and March, in small numbers and isolated occurrences at temperatures of 41 and below. E. B. Forbes ('97) states that this species lives in shallow, weedy water, and has never been found in large numbers, though widely dis- tributed. On account of its relative rarity it may have been over- looked by me and have a wider seasonal distribution than my scanty data indicate. 269 Cyclops phaleratus Koch was recorded in channel plankton only in small numbers in November-December, 1897, at minimum tem- peratures. E. B. Forbes ('97) states that it is a littoral form, confined to marginal vegetation. Cyclops prasinus Fischer. Average number, 2. This species occurs sparingly and irregularly in September-March in channel plankton, appearing in largest numbers in the early autumn of 1895 and most continuously in the winter of 1898-99. The numbers are always small, never reaching 400 per m. 3 , and in 12 of the 17 records falling below 100 per m. 3 The percentage of collections containing C. prasinus in the totals rises above 20 per cent, only in December (24 per cent.). The seasonal distribution in channel plankton indicates a limitation to the colder part of the year, all records but 5 being below 40. Nevertheless, in September-October, 1895, the species was recorded in 56-79. This fact and its relatively small numbers generally, make it probable that inferences from our scanty data concerning its seasonal distribution can not be con- clusive. Of the totals in all years, 86 per cent, are females without eggs, 6 per cent, females with eggs (found in February and November), and 8 per cent, males. E. B. Forbes ('97) finds the species widely distributed in American waters from the Great Lakes to roadside pools. Marsh ('93 and '95) finds it (as C. fluviatilis} in the larger bodies of water in Wisconsin and Michigan, and in lakes Erie, Michigan, and St. Clair. In Green Lake he ('97) finds it to be the most abundant species of Cyclops, and perennial, with maxima in September- November. His statistical data exhibit somewhat irregular numbers which contain suggestions of recurrent pulses such as appear in our records of other species of Cyclops. Brewer ('98) finds the species in the plankton of pools near Lincoln, Neb. Cyclops serrulatus Fischer. Average number, 3. This species was taken sparingly in channel plankton, exhibiting only isolated occurrences in December, January, March, and May, in flood waters at temperatures of 32-75. It is much more abundant in Spoon River, where it is sometimes the dominant species of the genus, appearing in May-September, and in small numbers in colder months. It appears to be adventitious in channel plankton of the Illinois River. 270 This widely distributed Cyclops appears but rarely in the records of the plankton of European lakes, and then only in the smaller ones. Stenroos ('98) reports it as abundant in the littoral zone of Nurmijarvi See; and Scourfield ('98) finds it perennial and the most abundant species of Cyclops in the waters of Epping Forest. On the other hand it has been found generally in the plankton of European streams. Zimmer ('99) finds it in the Oder, and Schorler ( '00) states that it is abundant in April- June in the plank- ton of the Elbe at Dresden ; Fric and Vavra ( '01) find it only in the littoral fauna at Podiebrad; and Volk ('03) in the plankton in four of seven localities in the Elbe at Hamburg. Sowinski ( '88) found it in the plankton of the Dnieper, Rossinski ('92) in that of the Moskwa, Zykoff ('00) in the summer plankton of the Volga, and Zernow ('01) in the winter plankton of the Schoschma. Meissner ( '02 and '03) reports it in May- August as not abundant in the back- waters and vegetation of the Volga at Saratoff. In American waters Marsh ('93 and '95) finds it in smaller lakes of Wisconsin and Michigan but not in the Great Lakes, and E. B. Forbes ('97) states that it is one of the most common and widely distributed species in American waters. It appears, however, not to be quantitatively an important element in lake or river plankton. Brewer ('98) finds it to be the most abundant vernal Cyclops in the small bodies of water near Lincoln, Neb. Cyclops viridis Jurine. A synonymy and a discussion of varia- tions in this the dominant and most variable of all the Cyclops in our channel plankton, has been given by E. B. Forbes ( '97). I have grouped the individuals in our plankton under two varieties, brevispinosus Herrick and insectus Forbes. The two varieties inter- grade, and in my separation I have followed only a single character readily visible without dissection or manipulation, namely, the outer terminal spine of the stylet, which is short, broad, and lance- shaped in brevispinosus, and more spine-like in insectus. Judging from the results of this method of separation, it appears that this lance-shaped spine is a character of the male in many instances, though not found in all males or limited to this sex. Cyclops viridis var. brevispinosus Herrick. Average number, 124; in 1897, 447; in 1896, 622; in 1895, 850; and in 1894,68. This form occurred in all months but January, but predominantly from the last days of April to the first week in October, the percentage 271 of collections containing brevispinosus in these months being 27, 80,62,67,48,75, and 5 9 per cent , respectively , while in other months it does not rise above 20 per cent. The number of individuals is also greater during the warmer season. No record between October 15 and April 20 exceeds 200 per m. 3 , while between April 20 and October 15 the pulses often culminate at 3,000-5,000 per m. 3 , and over 98 per cent, of the total individuals were recorded. This variety appears throughout the whole seasonal range of temperatures from summer's maximum to winter's minimum, but predominantly during the warmer season. Only 15 of the 71 occurrences and 2 per cent, of the individuals w r ere recorded at temperatures below 60. As soon as the vernal rise in temperatures passes 50-60, the minimum numbers and scattered occurrences of the winter months give way to a vernal pulse of considerable mag- nitude in April-May, attaining 4,452 on April 25, 1895, and 4,960 on May 25, 1897, but only 2,600 on June 7, 1898. This is followed by a period of depression in July, when the summits of the pulses did not often surpass 1,000 per m. 3 In the late summer and autumn of 1895 and 1897, and to a less extent in 1896 .and 1898, a second period of maximum pulses appears, attaining 9,711 September 12, 1895, and 4,800 October 5, 1898. When temperatures decline in September-October below 50, this variety falls at once to minimum numbers. The records of brevispinosus in channel plankton exhibit some- what clearly the phenomenon of recurrent pulses whenever collec- tions at brief intervals make it possible to delimit the pulses. Thus, in 1895 there are pulses culminating in July, August, September, and October; in 1896, in April, May, June, July, August, and September; in 1898, in July, August, and October; but in 1898 (Table I.) the numbers are too small to exhibit fully the phenomenon of recurrent pulses. The relation to hydrographic conditions may be inferred from the fact that while in the stable conditions of July-October, 1897, pulses culminated at 800-4,800 per m. 3 , in the same period in the disturbed hydrographic conditions of 1898 no pulse rose above 200 per m. 3 , and the total of all records in those months is only 8 per cent, of that in 1897. Evidently brevispinosus does not thrive in flood waters. 272 The surprising fact derived from the examination of our records of this variety of C. viridis, is that the individuals referred to it are predominantly of the male sex. Out of a total of 74,308, 64,883, or 88 per cent., are males, 8,542, or 11 per cent., females without eggs, and only 883, or one per cent., egg-bearing females. In so far as these data go, they indicate that this so-called species, or even variety, of C. viridis, in so far as it is based on the lance-like spine of the stylet, is not well founded. This is, it seems, predominantly a male character, though not exclusively so, since females, and even egg-bearing females, are found which exhibit this structure. C. viridis var. brevispinosus appears to be confined to American waters. Marsh ('93 and '95) reports it from the larger lakes of Wisconsin and Michigan, and from the Great Lakes, except Lake Michigan. Birge ('95 and '97) finds that it is the most abundant species of Cyclops (except in summer, when C. leuckarti abounds) in Lake Mendota, and the only one reproducing under the ice. His data exhibit a major pulse in May, and a second one, of less ampli- tude, in October, with slight indications of recurrent minor pulses in midsummer, obscured possibly by the massing of his data in fortnightly averages. The seasonal distribution in Lake Mendota is thus much like that in the Illinois River. Marsh ('97) finds the maximum in Green Lake in June at 68-69, and only scattering occurrences at other seasons. E. B. Forbes finds this variety widely distributed in American waters, but never especially abun- dant. Cyclops viridis var. insectus Forbes. Average number, 539; in 1897, 2,115; in 1896, 949; in 1895, 2,966; and in 1894, 905. It is thus more abundant by two- to threefold in the stable years of 1895 and 1897 than in the flood-swept years of 1896 and 1898. This variety was found in every month of the year, though predominantly in April-October, when the percentages of the collections containing it were respectively 64, 100, 85, 100, 100, 87, and 76 per cent. In November-March the percentages were only 44, 6, 17, 7, and 13. The numbers of individuals are very small, however, from October 1 to April 20, excepting in the autumn of 1897, when, with the delayed high temperatures and the great impetus given to plankton development in the stable conditions of low water, the maximum pulse of all our records, 30,800 per m. 3 , was reached on October 5, a pulse of 1,200 following in November. With 273 these exceptions no record exceeding 600 per m. 3 was made between the dates named. Between April 20 and October 1 the minimum records rarely fall below 600 per m. 3 , except in 1898, and the pulses often culminate at 2,000-8,000. C. viridis var. insectus is thus a planktont of the warmer season, and its seasonal distribution is strikingly similar to that of the so-called var. brevispinosus. This form occurs in our plankton throughout the whole seasonal range in temperatures, but only in small numbers and irregularly below 60. Only 21 per cent, of the collections containing insectus were made at temperatures below 60, and these contained less than 3 per cent, of the total individuals. With the exceptions of the pulses culminating at 43 November 23, 1897, at 1,200 per m. 3 , and at 57 April 26, 1898, at 4,160 per m. 3 , no development of this species exceeding 600 per m. 3 occurs below 60. All pulses of more than 3,000 per m. 3 , excepting only the April pulse of 1898, occur at temperatures above 70. The species reaches its greatest development in channel waters during the period of maximum temperatures, 70-80. The seasonal distribution of this form shows a few straggling individuals in November-March during temperatures below 50, and a meteoric rise to a vernal pulse in April-May as this tempera- ture is passed and 60-70 arrives. This is followed by a series of recurrent pulses, often of considerable amplitude, through Septem- ber or until temperatures fall below 60, as in October, 1897. With falling temperatures the drop in numbers to the winter minimum is quickly accomplished. A comparison of the distribution in 1897 and in other years, shows a close correlation between the decline in temperatures and the falling off in numbers of insectus. The relations which hydrographic conditions bear to the develop- ment of insectus in channel plankton may be inferred from the hydrographs on Plates IX.-XII, Part I., and from the data sum- marized in the following table, 1894 being omitted because of the incompleteness of the seasonal representation. In 1895 levels were low, unusually so in the spring, and the flood-free intervals of the year were of more than the usual extent. About 10 feet of the total movement in levels (51.9 ft.) is found in the late December rise. If this is excluded, the total movement falls to 42 feet, and the range in levels to 6.5 feet. Under conditions, 274 Year Range in levels, in ft. Total movement, in ft. Average height, in ft., of stage of river Average number of insectus per m. 3 1895 12.2 51.9 3.61 2,966 1896 10.1 45.7 6.98 949 1897 14 3 44.8 6 90 2, 115 1898 15.5 67.2 8.02 539 then, of lowest levels, least range, and total movement, we find the largest development (2,966) of insectus in channel plankton. In 1896 the average river level is much higher, affording in- creased current and more silt. A series of recurrent floods also flush the channel, though the total movement and range in levels within the limits of the year are not greatly increased. Neverthe- less, the changes, which appear mainly below bank-height, affect channel plankton profoundly, and the production of insectus falls to 949 per m. 3 In 1897 the population rises to 2,115 per m. 3 , largely as a result of the stable conditions of flood-free waters at low levels and with slight current in the last half of the year. In 1898 the total movement (67.2), range in levels (15.5), and average stage (8.02) reach the extremes in the four years under comparison, and the insectus population falls to the lowest level 539 per m. 3 A detailed comparison of the July-November period of the two years follows. Month July August September Year 1897 1898 1897 1898 1897 1898 Total movement 5.2 6.05 5,093 7.4 5.70 210 2.6 2.29 2,030 7.5 3.66 304 0.6 2.01 2,275 6.2 4.44 325 Average stage Average number of C. viridis var. insectus. . . 275 Month October November Average Year 1897 1898 1897 1898 1897 1898 Total movement 0.6 2.01 8,625 3.9 4.86 200 2.2 2.82 520 2.6 7.44 68 2.2 3.04 T.709 5.5 5.22 221 Average stage Average number of C. viridis var. insectus. . . In 1898, with two and a half times the movement in. levels found in 1897, the development of insectus attains less than 6 per cent, of the numbers reached in the latter year. The occurrences of insectus in channel plankton exhibit the phenomenon of recurrent pulses during the season of its occurrence in large numbers whenever collections are sufficiently frequent to delimit the pulses. Thus, in 1895 there are. such pulses in July, August, September, and October; in 1896, in April, June, July, August, and September; in 1897, in July, August, September, October, and November; and in 1898, in April, May, June, July, August, and September, though of slight amplitude in the last three months. Some of the seeming gaps and irregularities in the series of pulses of brevispinosus and insectus will be eliminated if the statistics of the two forms are combined in a single series, a fact which lends support to the view that the two forms belong to the same species, and are parts of a common group of variable organisms. Steuer ('01) concludes from his examination of the plankton of the Danube at Vienna, based on 19 (?) collections in 15 months, that Cyclops has usually two maxima and two minima in each year, and that in the same body of water, owing to various meteorological influences, the two maxima do not in any year fall near each other. The more extensive data at my command show the limitations of such a general conclusion. An examination of the records of indi- vidual species of Cyclops and of the total Cydopida in our waters, make it clear that the major pulses may follow each other at about a monthly interval. For example, in 1897, the total Cyclopida (19) 276 have their major occurrences in our records as follows, the pulses appearing September 14 and October 5 : July 30 8,080 Sept. 14 117,000 Aug. 10 49,360 Sept. 21 15,260 Aug. 17 17,120 Sept. 29 14,400 Aug. 24 20,320 Oct. 5 101,600 Aug. 31 67,200 Oct. 12 3,400 Sept. 7 107,200 Again, in 1896, the two major pulses of the year are on June 19 (928,984) and July 18 (563,815). Steuer's conclusion seems to be founded upon insufficient data, and can not have general applica- tion. Of the total 240,830 individuals of C. viridis var. insectus in our records in 1894-1899, 117,166, or 49 per cent., are males; 109,460, or 45 per cent., females without eggs; and 14,204, or 6 per cent., females carrying egg-sacs. If the brevispinosus totals are included, the percentages change to 42 per cent, of females of which 37 per cent, and 5 per cent., respectively, are without and with egg-sacs and 58 per cent., males. The apparently high proportion of males may be due to the fact that in the enumeration more young females than males were. included in the "young" Cyclops. The egg-bearing females were generally more numerous in April- July. No marked predominance in the proportion of males appears at any season in our records. Cyclops viridis does not appear extensively in the plankton literature of European lakes. Stenroos ( '98) finds it not rare in the littoral fauna of Nurmijarvi See. Scourfield ('98) reports it as next in abundance to C. serrulatus in waters of Epping Forest, w r here it is perennial. Scott ('99) finds it at all seasons in both littoral and pelagic collections in Scottish lakes, and Amberg ('00) lists it for Katzensee. It appears but infrequently in the investigations of European streams. Neither Schorler ('00) nor Fric and Vavra ('01) report it from the Elbe, though Volk ( '03) lists it from six of seven localities in this stream at Hamburg. Sowinski ( '88) finds it in the littoral fauna of the Dnieper, and Zykoff ('03) in the summer plankton of the Volga, though Meissner ( '03) states that it is never found in the plankton of that stream at Saratoff, being confined to the littoral 277 zone and to vegetation. No statistical data concerning its seasonal distribution are given by any of these authors, though Meissner states that it reaches its maximum in May in the Volga. In addition to the species of Cyclops here listed for the channel plankton of the Illinois, E. B. Forbes ('97) records in May-Septem- ber, 1896, C. varicans Sars as common, and C. fimbriatus var. poppei Rehberg and C. bicolor Sars as rare. Owing to the impossibility of separating with certainty the nauplii and young of the various species of Cyclops they were all recorded together under the head of " nauplii " and " young Cyclops." The former includes also the nauplii of the two species of Diaptomus occurring in our plankton. Young Cyclops. Average number, 4,780; in 1897, 16,035; in 1896, 10,196; in 1895, 21,960; and in 1894, 5,960. With two ex- ceptions in January and February they occur in every collection examined. Numbers are, however, at a minimum in November- March, only 9 instances of more than 1,500 per m. 3 appearing in our records in this season. With the exception of two pulses in the autumn of 1897, and two in this season in 1895, all pulses of an amplitude exceeding 8,000 per m. 3 are confined to the interval between April 20 and October 1, practically to temperatures above 70. They also exhibit relations to hydrographic conditions of the same nature as those found in case of the adults of the various species of Cyclops, and manifest likewise the phenomenon of re- current pulses (Table I.). The totals of all young Cyclops in 1894 1899 are almost five times those of all adults of the genus. This ratio gives an index of the extent of the decimation by enemies and inimical factors of the environment which exists after the nauplius stage has passed and before that of the adult is reached. Nauplii of the Copepoda (excluding the Harpacticidce) . Average number, 36,707; in 1897, 53,786; in 1896, 24,560; in 1895, 88,442; and in 1894, 45,648. Nauplii were recorded in all collections ex- amined with but two exceptions. As in the case of the adults and young, the large numbers are, however, confined to the warmer season between April 1 5 and October 1 . During the colder months the pulses rarely rise above 20,000 per m. 3 , and those in excess of 35,000 during these months are with one exception confined to the delayed high temperatures of the stable autumn of 1897. During 278 the warmer season, on the other hand, the pulses frequently attain 100,000 or over. The maximum record of 928,984 was made in the stable low water of June 19, 1895. All large developments thus lie at tem- peratures above 70. The nauplii bear much the same relation to hydrographic condi- tions as that found in the adults; for example, in Cyclops viridis. This is seen in the fact that in unstable years such as 1896 and 1898 the numbers are on the average only 28 and 68 per cent, of what they were in the more stable conditions of 1895 and 1897, and the average monthly population in July-December in the unstable conditions of 1898 is only 18 per cent, of that in the same months of the previous year. The relative numbers of adult, young, and larval stages of the Cydopida are given in the accompanying table. Year Nauplii Young Cyclops Adult Cyclops No. Ratio No. Ratio No. Ratio 1894 456,483 2,741,718 1,451,524 1,828,720 1,908,780 121,345 38 19 17 18 30 61 59,598 680,749 428,211 545,200 248,576 5,422 5 5 5 5 4 3 11,726 140,779 84,786 102,730 62,735 1 1 1 1 1 1895 1896 1897 1898 1899 Totals 8,508,570 21 1,967,756 5 404,749 1 The ratios between total adult and young, 1 to 5, are fairly constant in the different years, falling to 1 to 3 in January-March, 1899, and to 1 to 4 in 1898, a year in which the colder part of the year was most fully represented. This ratio probably represents more truly the relationship of young and adult in the total yearly production. The ratios of adults to nauplii in the several years vary considerably from the totals of all years (1 to 21), rising to 1 to 279 61 in winter conditions of 1899 (January-March), and falling as low as 1 to 17 in 1896. This was a year of recurrent floods, but its ratio is in sharp contrast with that of 1898 (1 to 30), also a year of con- siderable hydrographic disturbances during the summer. The adult population was reduced during this year, and especially during the summer floods, but the nauplii do not fall conspicuously below those of other years. It would therefore seem that the deleterious action of flood conditions operates more effectively upon the adult and young than upon the nauplii. This fact may be due to the relative absence of spines and hairs on the nauplii, structures which gather silt and load down the larger forms in the flood, waters. The greater number of young and adults in 1896 as compared with 1898 may be due to the more gradual rise of the floods of the former year (see PI. X. and XII., Pt. I.) jand the proportionally greater amount of silt in the more sudden floods of the latter. The ratios given in the table are of course subject to the error arising from the uneven seasonal distribution of the collections in some years, and to that arising from varying location of the collec- tions on the pulses, especially on those of greatest amplitude. An ad- ditional error arises from the leakage of the smaller nauplii through the meshes of the silk net. I have found on experiment that they will thus escape under pressure of a column of water only 3-4 cm. in height. Their dimensions are such that the smaller individuals can pass through the meshes of even the No. 20 silk. It seems probable that ratios of nauplii to adults are actually greater than our records indicate. The relationship which the pulses of nauplii bear to those of the adult Cydopidcz may be inferred from an examination of the data of Table I. An analysis of the seasonal distribution of the total young and adult Cydopidce and of the nauplii reveals the fact that in all seasons in which collections at approximately weekly intervals were made, their pulses coincide in a majority of cases in their maxima, and when the coincidences do not occur the maximum of the nauplius pulse appears in the collection of the week following that of the young and adult Cydopidcs. This appears less constantly and clearly in the disturbed hydrographic conditions of 1898 (Table I.) than in the records of more stable years. Apstein ('96) finds that nauplii of Copepoda are most abundant w r hen eggs are most common, and that this bears no constant relation 280 to the abundance of adults. Our collections, extending over longer periods and being at briefer intervals, indicate, however, that this relation does exist. As above stated, the larvae are most abundant at or shortly after the times of greatest abundance of adults that is, the maxima of the recurrent pulses. Apstein also states that reproduction is periodic and development rapid. Maximum numbers are reported by him in May and September. Cohn ( '03), on the other hand, maintains that the " innere Logik" and his data show him that the nauplii reach their greatest numbers just prior to the appearance of largest numbers of young and adult Copepoda. His data are from 12 collections between May 1 and October 1 , and favor his contention in 2 out of 3 cases (of maxima) , and both of these lie in collections at intervals of 15 to 16 days. In the light of our data obtained at briefer intervals and the conclusions therefrom that the pulses of larvae tend to coincide or follow at a brief interval those of the adults, it becomes questionable whether his data are sufficient for his conclusion. His logic also overlooks the fact, apparently, that smaller numbers of larvce might lead to coincident maxima of grown forms during a period of abundant food, on which all pulses must be based, since the larval stage may be at such times a brief one and the adult a relatively longer one, and the cumulative effect of this relationship would make the conditions shown in our data logically possible. Furthermore, Cohn used a No. 12 silk in his plankton net, and this allows many nauplii to escape, and probably accounts for the fact that the ratio of larvas to grown forms in his figures is only 1.3 to 1, while in our records it is 3.5 to 1. The discrepancy arising from this leakage may further tend to weaken his data for his conclusions concerning the relations of larvas and adults. Steuer ('01) finds that the nauplii in the Danube at Vienna reach maxima in June and in August, but his data are too scattered to fully delineate their fluctuations. Two out of three of his max- ima coincide with those of all Cyclops, and the third antedates it (monthly intervals of collection), as in Cohn's data. Diaptomus pallidus Herrick. Average number per m. 3 , 11; in 1897, 367; in 1896, 87; in 1895, 152; and in 1894, 146. This species was recorded in all months of the year but February, though in a larger percentage of the collections and in larger numbers in July-December. Prior to this season the percentage does not 281 rise above 31 per cent., the occurrences are irregular, and the num- bers are small. Thus in 1896 and 1898, years of numerous winter and vernal collections, there were but 4 occurrences in each prior to July 1 , and all but one of these was of numbers less than 100 per m. 3 Only 12 of the 72 occurrences and 8 per cent, of the total individuals were recorded in the first and less stable half of the years. In July-December numbers rise in feebly outlined pulses which attain at the most 800-2,400 per m. 3 The percentage of collections con- taining the species rises to 33-75 per cent., and in stable autumns such as 1895 and 1897 the occurrences are but little interrupted. In its seasonal distribution in channel waters it is thus largely confined to the last and more stable half of the year. Its relationship to hydrographic conditions here suggested also appears in a comparison of the yearly averages given above. The average numbers per m. 3 in 1896 and 1898, 87 and 11, are greatly exceeded by those of 1895 (152) and 1897 (367). The total number recorded in July-December in 1897 is 29 times that in 1898. This well-defined predominance in stable seasons, which appears also in the case of the closely related D. siciloides, exceeds that of the other Entomostraca, and indicates a greater sensitiveness on the part of these species to the deleterious effects of flood waters. The long antennas and great development of the feathering of the caudal stylets afford a large area for the attachment of the silt and debris of flood waters, and accordingly facilitate the destruction or removal of Diaptomus from the plankton more quickly than in the case of Entomostraca in which these processes are less developed as in Cyclops or Bosmina. The numbers of individuals are too small to delineate accurately the recurrent pulses which are suggested in the data of distribution. In the autumns of 1895 and 1897, when the occurrences are most continuous, the larger numbers tend to fall at the times of the maxima of pulses of other Entomostraca. There is no marked limitation placed upon this species by the seasonal changes in tem- perature. It is found throughout the seasonal range in tempera- tures, though numbers are slightly smaller in channel waters in November-December. Nevertheless it occurs in considerable num- bers in the backwaters in breeding activity under the ice at mini- mum temperatures in December. 282 Of the total individuals, 40 per cent, were males; 45 per cent., females without eggs; and 15 per cent., females with eggs. The sexes show no marked or constant seasonal differences in distribu- tion. Females with eggs are more abundant in August-October, and with spermatophores in the same months. Detached sperma- tophores were found until December. This species is stated by Herrick ('84) to be distributed in the entire Mississippi Valley. Marsh ('93) finds it in Wisconsin, but it appears nowhere in the plankton of the Great Lakes. Brewer ('98) reports it in the backwaters of the Platte in Nebraska, and Schacht ('97) states that it is an exceedingly common species in central Illinois, and that it has been reported from Wisconsin, Ohio, and Minnesota. It thus appears to be limited to the shallow and relatively warm waters of the prairie regions of the Mississippi basin. Diaptomus siciloides Lilljeborg. Average number, 10; in 1897, 350; in 1896, 56; in 1895, 282; and in 1894, 23. As will be seen on comparison, these yearly averages are very similar to those of the preceding species with the exception that the development of D. siciloides is about twice that of D. pallidus in 1895. In other particulars its seasonal data so resemble those of D. pallidus as to make their discussion in large part a repetition. Its seasonal- distribution relations to temperature and hydrographic conditions, breeding season, and its tendency toward a pulse-like recurrence in coincidence with other Entomostraca are all very similar to these features in D. pallidus. The proportions of the sexes differ slightly, the males being less numerous (3 1 per cent.) and egg-bearing females more abundant (18 per cent.) than in the previous species. This is also an American species, reported thus far only from Lake Tulare, Calif., the Illinois River, and waters of Indiana and Iowa (Schacht, '97), and by Brewer ('98) in lakes and pools of Nebraska. It is thus confined largely to shoal and warm waters. Diaptomus spp., immature. Average number, 19; in 1897, 560; in 1896, 158; in 1895, 336; and in 1894, 120. The immature individuals of D. pallidus and D. siciloides were not distinguished from each other in the records. Young Diaptomus presumably belonging to these two species occur in every month but March, though but 10 of the 74 records were made in January- June. The percentage of occurrences and the numbers per m. 3 are lowest in these months, not rising above 33 per cent, and 500 per 283 m. 3 save in two instances. Occurrences of small numbers continue through July, but from August 1 to October 15 appear the major pulses of the year, attaining an amplitude of 1,000 to 8,800 per m. 3 With the decline of temperatures in October, numbers fall to levels below 400 per m. 3 , with one exception (December 14, 1897) at 700. The percentage of occurrences is, however, high (41 to 44 per cent.) and declines only to 33 per cent, in January. The period of greatest numbers of young thus coincides with that of greatest abundance of adults, and lies at temperatures of 70Vand above, in channel waters. The effect of hydrographic changes upon the occurrence of young Diaptomus appears in striking form in the annual averages above quoted. In 1898, a year of sudden changes, the average per m. 3 is only 19, while in the stable conditions of the previous year it is 560. The July-December production in 1897 is 28 times greater than that of 1898. In 1896, a year of recurrent but less sudden floods, the average (158) is less than that of 1895 (336), a more stable year. The great reduction of adults noted in 1898 and 1896 is thus paralleled by an even greater reduction of the young. Osphranticum labronectum Forbes occurs in the plankton of Quiver Lake in small numbers (see Schacht, '98), and was found once in channel plankton in June, 1896. AMPHIPODA. Allorchestes dentata (Sm.) Faxon. This is an abundant littoral species found amid vegetation, especially in the vegetation-rich backwaters, such as Quiver Lake. It was not often found in channel plankton, being taken only in the summer of 1895, when the July- August floods carried away the vegetation which had accumulated during the antecedent low water. ARACHNIDA. ACARINA. In vegetation-rich backwaters members of the family Hydrach- nid& were frequently taken, along with other adventitious or- ganisms, with the plankton. In channel waters they are less frequent, and are represented principally by Atax, which is parasitic 284 in great numbers (see Kelly, '99) in the Unionida which are found in the bottom of the channel. Occurrences in the plankton were limited to the months of May-August, and may be due in part, especially in the warmer months, to the release of the parasites by the death and flotation of their hosts. Flood waters in warm months were often disastrous to the Unionidce because of the load of silt, sewage, and industrial wastes which they carry in channel confines at the lower river stages often prevailing in these months. Other small aquatic Acarina were also present, probably adven- titious from the littoral or bottom ooze. With two exceptions their occurrences in the plankton were all in warmer months, April- September, though not in flood waters. During the period of the migration of waterfowl, parasitic Acarina were noted in plankton collections in a few instances. TARDIGRADA. Macrobiotus macronyx Duj. Average number, 11. This species is found principally in the colder part of the year, from October to May. The earliest autumnal record was October 30, 1895, at 45, and the latest vernal one, May 1, 1896, at 68.8, and the maximum number (2,980 per m. 3 ) was recorded on April 10, 1&96, at 46.2. Of this number, one sixth were females with eggs. Females with eggs were also found in November, February, and March. Because of its seasonal distribution it is found principally, though not solely, in disturbed hydrographic conditions, and its occurrence in the plank- ton is largely adventitious. HEXAPODA. Owing to the shoal waters, relatively narrow confines, and the hydrographic fluctuations in our fluviatile environment, the aquatic insects, both larval and adult, have many points of contact with the plankton. They constitute a large element in the total volume of the animal population of shore and bottom, and are all connected by chains of food relations, more or less complex and remote, to the plankton organisms or their sources of food. With the single exception of the larvae of Corethra they are all in the main adventi- tious members of the plankton assemblage, and are much more abundant in the vegetation-rich backwaters than in the channel. 285 Since the aquatic insects of these collections are being studied by others, with reference to publication in this Bulletin (see Hart, '95, and Needham and Hart, '01), only passing notice of the more important representatives appears in this connection. EPHEMERIDA. Ephemerid larvae, as a rule in early stages, were found singly or in small numbers in the channel plankton in the warmer months, April-October, at temperatures above 56. Since these occurrences were with few exceptions in stable hydrographic conditions, it seems probable that the younger larvae of this order may adopt, at least temporarily, a limnetic habit. Specific identifications of these larvae were not made. HEMIPTERA. Corisa (?) sp. Average number, 37. A small hemipterous larva doubtfully referred by Mr. C. A. Hart to Corisa, was taken with some frequency but in relatively small numbers in the plankton during the summer months. Of the 36 occurrences 27 fall in June-August, 2 in May and 3 in September, 2 in January, and 1 each in October and November. It thus appears in the temperature extremes, but exhibits a great predominance in the season of maxi- mum heat. There is no marked increase in its frequency or numbers in years of more disturbed hydrographic conditions. Its numbers are always small and somewhat erratic. Adult Corisa, as well as many other aquatic Hemiptera, were found in plankton collections singly and infrequently. DIPTERA. This group of insects is abundantly represented in the plankton, but in all cases by larval or pupal stages. Chironomus spp., larval stages. Average number, 124. Larvae in various stages of development from that immediately after hatching to that approaching pupation were found in channel plankton. They occur in considerable numbers in the ooze in the river bottom, but appear to abandon the limicolous for the limnetic habit, temporarily at least, as a result of hydrographic or other disturbances. There is evidence from their relative numbers in 286 years of different hydrographic conditions that these have consider- able influence in bringing them into the plankton. Thus in 1897, in stable conditions, there were only 5 occurrences in 3 1 collections examined, averaging 88 per m. 3 , while in 1898, in more disturbed conditions, there were 29 occurrences in 52 collections, averaging 124 per m. 3 There is also a marked seasonal distribution. The larvae appear in the plankton in March-December through the seasonal extremes of temperature, but the numbers in March and November-December are always small. Only 15 per cent, of the occurrences and 5 per cent, of the individuals were found at tem- peratures below 45. The percentage of occurrences in the collec- tions is highest in March-September, the percentages being 53, 73, 80, 47, 78, 52, and 50, respectively, to 8 to 35 per cent, during the remaining months. Corethra sp., larval stages. Average number, 6. These semi- transparent and active larvae have the characteristics of limnetic organisms, and may be reckoned among the autolimnetic planktonts of our waters. Because of their activity , it seems probable that they escape the drawn net, especially the small model used by us, and also, because of their negative rheotaxis, elude the suction of the plankton pump to an even greater extent. Thus, in 1895, in net collections, there were 8 occurrences averaging 32 per m. 3 to 4 in 1898, in pump collections, averaging 8 per m. 3 Corethra larvas were never abundant in our plankton, probably in part for the reasons just cited. With two exceptions all the occurrences lie in the period of maximum temperatures in June-September, 7 of the 14 occurrences and one third of the individuals being recorded in August. Dixa sp., larval stages. Average number, 8. Larvas were recorded singly in scattered occurrences in all months but February and October-December, though most of them appear during maxi- mum temperatures. Larvae of Tanypus and Odontomyia were also recorded in May and June in isolated occurrences. In addition to the larval stages of these aquatic insects there occurred in the plankton a considerable number of insect eggs, principally those of Diptera and Ephemerida. These were generally isolated, though sometimes fragments of the egg-string of Chirono- 287 mus appeared. They were recorded in all months but February and December, though 20 of the 30 records and 81 per cent, of the individuals appeared in May- August. The numbers are never very large, the maximum record, 5,424 per m. 3 on June 29, 1894, being due to a number of fragments of egg-strings. MOLLUSCA. GASTROPODA. The adults and young of many of our aquatic gastropods have the habit of gliding on the under side of the surface film of water, and they are also frequently dislodged from their foothold on aquatic vegetation, and thus enter the habitat of the plankton temporarily. This is especially true in vegetation-rich backwaters. The smaller forms, such as Ancylus, Amnicola, and Planorbis parvus were occa- sionally taken in the summer plankton of the channel. LAMELLIBRANCHIATA. This group is represented in the plankton by the larval stages, or glochidia, of the Uniomdcz, which form an important part of the bottom fauna of the stream and its tributaries. Anodonta corpulenta Cooper. Average number of glochidia, 21. The seasonal distribution of the glochidia in the plankton is very well defined. With but two exceptions the 48 occurrences all fall in October- April, and 40 of them in November-March. The occur- rences are thus during the period of minimum temperatures ; indeed, 31 of the 48 are at temperatures not exceeding 35 in surface waters, and only 9 are above 45. The earliest autumnal record is Septem- ber 30, at 58, and the latest vernal one, June 6, at 79. Generally the earliest records are in the closing days of September or the early ones of October, and the latest records are about the first of April. The occurrences are more frequent in December-March, the glo- chidia appearing in 64, 50, 53, and 60 per cent, of the collections, respectively, in these months. Their numbers are also several fold greater at this season than in the earlier and later months of their occurrence. The period of minimum temperatures is thus the season of greatest discharge of glochidia. The numbers are always relatively small, 520 on December 28, 1897, being the maxi- 288 mum record. Their fluctuations are erratic, and show no apparent relation to hydrographic or other environmental changes. Lampsilus anodontoides (Lea) Baker. Glochidia referred with some uncertainty to this species appeared somewhat irregularly in the plankton in small numbers in September-December and again in June-July. The seasonal distribution in two periods suggests the inclusion of two species. Arcidens confragosus (Say) Simpson. Glochidia of the type referred by Lea to the old genus Margaritana, and presumably belonging to this the commonest member of this genus (as formerly understood) in our locality, were taken in the plankton December 18, 1895, in small numbers. BRYOZOA. This group is represented in our plankton by the floating stato- blasts, when these occur, as in Pectinatella and Plumatella, by detached and floating fragments, as in Urnatella, or by natant colonies, as in Lophopus and Cristatella. Genera such as Fredericella and Paludicella, whose statoblasts sink, fail to appear in the plank- ton, though in some cases they may be abundant in the bottom fauna. The Bryozoa are plankton feeders, and play an important role as plankton reducers in vegetation-rich backwaters. DISCUSSION OF SPECIES OF BRYOZOA. Cristatella mucedo Cuvier. This species was found in the back- waters in summer months, especially in Quiver Lake. Statoblasts probably referable to this species occurred sparingly in May and August. Lophopus cristallinus Pallas. This rare bryozoan occurred in the channel plankton, though not in our quantitative collections, in July, 1897, in that part of the channel containing the discharge from Quiver Lake. Small, free-swimming colonies of 5-50 zooids were taken in surface waters. Pectinatella magnified Leidy. Statoblasts of this superb bryo- zoan were not uncommon in the backwaters, and were seen several times in the vernal plankton of the channel. The large floating colonies are found near the surface in July-October in the open backwaters, and more rarely in the river itself. The translucent 289 gelatinous coenoecia are spherical, ellipsoidal, or often somewhat flattened. The longest diameter of these floating masses often exceeds 30 cm. Plumatella repens L. This is by far the most abundant bryozoan in our locality, being found everywhere on submerged vegetation in the backwaters. It often develops with surprising rapidity on the submerged stems of plants, where, as in 1896, summer floods reinvade the vegetation-covered margins of reservoir backw r aters. It is represented in the plankton by its floating statoblasts. Their seasonal distribution shows some correlations with temperature, hydrographic conditions, and the seasonal cycle of the parent organisms. During the period of minimum temperatures (Decem- ber-February, inclusive) they are relatively rare in the plankton, appearing in 30, 8, and 20 per cent., respectively, of the plankton catches. They are rare in high- as well as low-water conditions, as, for example, in the floods of 1895-96 and 1898, when they appear in but one of 15 collections. With the rise of temperature in March they occur more frequently, as, for instance, in 1898 (Table I.), and continue during the run-off of the spring flood. The occurrences rise in March-May to 60, 46, and 50 per cent, of our total collections in these months, and the numbers also are larger. For example, in 1898, 81 per cent, of the total individuals for the year were found in these months. The discharge from impounding backwaters, the principal breeding grounds of the parent organisms, doubtless tends to increase the numbers of statoblasts in channel plankton during this season. During the remainder of the year, June-November, the percentage of occurrences again falls to 30, 50, 24, 32, 18, and 44 per cent., respectively. The 50 per cent, in July is due to the summer flood of 1896. If this year is omitted the record falls to 33 per cent. The large percentage for November is probably due to the predominantly higher levels of this month, to the invasion of lake margins seeded with statoblasts, and to the increased activity in the fishing industry, which tends to disturb the summer's growth of vegetation in tributary backwaters. The relations to the seasonal cycle of the species are patent. The summer months, June- September, are the season of growth and spread of the parent organisms and of the formation of statoblasts, especially as receding levels expose the water margins. Hydrographic or other disturb- ances tend to increase the number of statoblasts in the plankton 290 until minimum temperatures are reached, when minimum numbers appear in the plankton. As temperatures rise, the statoblasts tend to float and become more abundant in the plankton, as a result, perhaps, of the physiological and accompanying physical changes in the contents of the statoblast. The declining phase of the major flood of the year is thus the period of greatest flotation and dispersal of the statoblasts. Urnatella graciUs Leidy. This unique species is found in some abundance on the projecting margins of the shells of the Unionida which line the river bottom in many reaches of the channel. Small fragments of the colonies containing only several polypides were found in the plankton in May-August and October. The earliest record was May 25, and the latest, October 25, at 48.5. THE PERIODICITY IN THE MULTIPLICATION OF THE ORGANISMS OF THE PLANKTON. One of the most obvious conclusions brought to light by the detailed study of the volumetric fluctuations of the plankton pub- lished in Part I. of this report, and most strongly reinforced by the statistical data showing the fluctuations in the numbers~of the indi- viduals of the various species and in the sums total of the various biological groups represented in the limnetic fauna and flora, is that plankton production is fundamentally rhythmic or periodic in character, viewed either in its constituent elements or as a whole. This total result is simply the sum of a like phenomenon pervading more or less completely and coincidently the reproductive cycles, the rise and decline in the numbers of the typical constituents of the plankton. The exceptions to this rhythm are usually found in those organisms which are adventitious in the plankton and have their centers of growth and distribution in other regions than the open water. Many illustrations of this periodic movement in the multiplica- tion of organisms of the plankton have been cited in the preceding pages and may be seen in the accompanying plates. As an illustra- tion for discussion in detail we may take the pulse of July, 1898, shown in the volumetric data of Table III. and Plate XII. of Part I. The fluctuations in the biological population during this period are also tabulated in Table I. of this paper, and graphically presented in Plates II. and IV., which exhibit the movement in the totals of the Chlorophycecz, Bacillariacece, and chlorophyll-bearing Masti- gophora, and of the Rotifera and Crustacea. In the volumetric data the pulse rises from a minimum of .14 cm. 3 per m. 3 on July 5 to a maximum of ,88 cm. 3 on the 19th, declining again on the 26th to the second minimum, of .67 cm. 3 Its duration is thus four weeks and its amplitude, in comparison with many other pulses in the records, relatively slight. It occurs in the more stable conditions of declining river levels and midsummer tempera- tures. The following list gives the names of the more or less typical planktonts considered in the discussion of this pulse. Others, largely adventitious or insignificant in numbers, might be added (20) 291 292 to the list. Forms whose antecedent minimum does not fall on June 28 or July 5 are designated by a superior 1 ; those whose maximum does not fall on July 19 or 26, by a superior 2 ; and those whose subsequent minimum is not on July 26 or August 2, by a superior 3.' The component forms and groups are Crenoihrix, etc. 1 , total Schizophyce, Microcystis ichthyoblabe 1 , total Chlorophycea, Actinas- trum hantzschii, Crucigenia rectangularis, Pediastrum boryanum 1 ' 2 ' 3 , P. pertusum 2 ' 3 , Raphidium polymorphum 1 , Scenedesmus genuinus, S. obliquus, S. quadricauda, Schroederia setigera, total Bacillariacece 1 , Cyclotella kuetzingiana, Diatoma elongatum 1 , Fragilaria virescens 2 , Melosira granulata var. spinosa 1 , M. varians 2 , Navicula spp., Synedra acus, total Conjugates 1 , Closterium acerosum, C. gracilis, total Protozoa, total Mastigophora, Eudorina elegans, Euglena acus, E. oxyuris, E. viridis, Glenodinium cinctum, Lepocindis ovum, Pandorina morum 3 , Phacus longicauda 1 - 2> 3 , P. pleuronectes 2 ' 3 , Platydorina caudata, Pleodorina calif ornica, Trachelomonas acuminata 1 , T. hispida 3 , T. volvocina, total Rhizopoda, Difflugia globulosa, total Ciliata 2 , Codonella crater a 2 , Halteria grandinella 2 * 3 , Tintinnidium fluviatile 2 ' 3 , total Rotifera, total Bdelloida 1 ' 2 , total Ploima, Anurcsa cochlearis and var. tecta, eggs of A. cochlearis and var. tecta, A. hypelasma, Asplanchna brightwellii 1 ' 2> 3 , Brachionus angularis and var. bidens, eggs of B. angularis and var. bidens, B. bakeri and vars. duniorbicularis 1 , melhemi, and tuber culus 1 ' 2 , total of all varieties of B. bakeri, B. budapestinensis, B. militaris 1 ' 2 , B. pala and var. amphiceros, B. urceolaris var. bursarius, B. variabilis 2 * 3 , Mastigocerca carinata 1 , Monostyla bulla, Polyarthra platyptera, eggs of P. platyp- tera 2 ' 3 , Rattulustigris 2 , Synch 3 , Bosmina longirostris 1 ' 3 , Ceriodaphnia scitula, Chydorus sphcericus,Diaphanosoma brachyurum 3 ,Moina micrura 1 ' 2 ' 3 , total Copepoda lj 2> 3 , Cyclops viridis var. brevispinosus and var. insectus, C. edax, young Cyclops 1 , nauplii of Copepoda 1 ' 3 . An examination of the preceding list and of the qualitative data of Table I., reveals the fact that 71 of the more typical planktonts are found in appreciable numbers in the plankton during this month. To this number we may add 6 immature forms separately listed in the table and 14 group totals, making in all 91 sets of statistical data bearing on the components of this pulse. An analy- 293 sis of the behavior of the constituent species shows that 43 of the 7 1 species (including varieties and forms), 4 of the 6 immature forms, and 10 of the 14 group totals reach their greatest amplitude on the 19th, coincidently with the volumetric maximum. Thus, in all, a total of 57 out of 91, or 63 per cent., of the sets of data are in pre- cise agreement as to the time of maximum development. Fur- thermore, of the remaining 35, there are 10 culminating in the collection prior to the 19th (on the 12th), and 16 on the next subse- quent one (on the 26th,) in all, 26 or 29 per cent, which culminate on immediately contiguous dates of examination. This leaves a residuum of only about 8 per cent, which do not exhibit precise or substantial agreement as to the time of maximum development. In the matter of the location of antecedent and subsequent minima the agreement is less pronounced, possibly because the enumeration error is relatively greater in the case of minimum numbers. We find, however, that 65, or 72 per cent., of the antecedent minima of the pulses occur on June 28 or July 5, and 71, or 79 per cent., of the subsequent minima are on July 26 or August 2. Nineteen, or 20 per cent., of the antecedent minima are on July 12; and 10, or 11 per cent., of the subsequent ones are on August 12. There is thus a residuum of not over 10 per cent, of instances where the data of species or group totals do not coincide or approximate to this pulse, as described, in position of maximum or one or both of the limiting minima. Considering the necessarily large error entering into our data, it is not surprising that exceptions should occur. Some exceptions as, for example, that of Pediastrum pertusum (Table I.) are plainly not due to insufficient data, but are appar- ently normal dislocations; that is, the rhythm of this species at this time is not in harmony with that of the majority of the components of the plankton. . But this is only a temporary derangement, and is not the habitual relationship which movement of production in Pediastrum bears to that of the plankton as a whole. So, also, many of the Entomostraca are much delayed in the culmination of their increase, running over to August 2 or 9, while the most of the other planktonts culminate on July 19 or 26. This lag on the part of the Entomostraca is not, however, habitual, as will be seen on examination of Plates II. and IV. This tendency toward a coincident rhythmic movement in production on the part of the constituent organisms of the plankton will be found throughout all 294 the data where collections are of sufficient frequency to adequately delineate the curve of production, that is from July, 1895, to Oc- tober, 1896, and from July, 1897, to March, 1899, a total of 37 months, and suggestions of a like phenomenon appear in the less complete data of other years. The degree of agreement indicated in the pulse of July, 1898, will be found, on examination of the data in Table I. and in the plates of this paper, to vary with the environ- mental conditions. Times of rapid change in hydrographic condi- tions or in temperature generally show less agreement, and more stable conditions will exhibit an equal or even greater uniformity in the prevalence of the pulse-like rise and decline of the component organisms. In order to show the course of these recurrent pulses in the chlorophyll-bearing planktonts, the total Chlorophycetz, Bacil- lariacecE, and chlorophyll-bearing Mastigophora on the one hand, and of the Rotifera and Entomostraca (''Crustacea" of the plates), I have presented the data graphically on Plates I.-IV., and in the table on pages 296-299 have drawn up a list of the pulses, indicating the dates of the collections which in the main enter into the respective pulses, and the dates of the maxima or culminations of the five groups named. Owing to the irregularities in the data, there are some instances in which several possible dates might have been chosen. Reasons for the choice are in several important instances given in the foot-notes 'to the table. It is evident from the data here presented in graphic and tabular form that the pulses of the five groups of organisms tend in the main to coincide. This is shown in Plates I.-IV., and in the fact that the average divergence of 175 group pulses listed in the table is 6.4 days, or, if 5 aberrant instances are omitted, only 4.8 days. In other words, the pulses of the totals of the 5 groups included in the table culminate on an average within an interval of 6.4 (4.8 in 170 cases) days. The average of the extreme limits between maxima of group pulses in the 36 periods of movement listed in the table is 11.7 days. It is apparent that the pulses would be more completely de- lineated by collections at daily intervals, but even in the somewhat irregular and at times chaotic data here presented, the evidence seems conclusive that the seasonal production of the dominant species and groups of planktonts tends to fall into coincident 295 recurrent pulses, which, in turn, are the cause of the similar and often coincident volumetric fluctuations. Attention should be directed to the fact that without any im- portant exceptions this recurrent movement pervades all the organisms of the plankton which are eulimnetic, such as Scenedes- mus, Melosira, Trachelomonas, Codonella, Synchtzta, Daphnia, and Cyclops, and often those which at certain seasons become tempo- rary planktonts, such as Difflugia and Hydra, but not with any regularity the tycholimnetic organisms, such as bdeltoid. rotifers or nematodes. It affects the more highly organized Rotifer a and Entomostraca with slower growth, longer life, and consequent greater cumulative function as well as the algae, diatoms, and flagellates, where rapid multiplication, brief existence, and non- cumulative (in the individual) function prevail. The large share which the young (eggs and immature stages) play in the pulses of Rotifera and Entomostraca will be seen in Table I., and repeated attention has been called to this in the discussion of species. The prevalence of breeding females and of eggs or young during the rise of the pulse, and of eggless, moribund, or dead individuals or their skeletons during the decline, is a common phenomenon in all well- defined pulses. No species of plankton organisms appears to escape the operation of this recurrent movement in production. The proportion of individuals surviving from one pulse to the next is subject to great variation, being often least when the ampli- tude of the pulses is greatest, and largest when the pulses culminate at slight amplitudes. As a result of periods of minimum develop- ment, it follows that the possible length of life of most plankton organisms, even of the Rotifera and Entomostraca, in the plankton must fall within rather narrow limits of a few days or a fortnight at the most. Since the contrasts between minimum and maximum numbers are relatively greater among the chlorophyll-bearing organisms, it follows that the survival proportion is less in these groups. The duration and amplitude of the plankton pulses will vary within certain limits according to the method of delineation. The volumetric minima and maxima present the total product in cubic centimeters, and the pulses thus marked cat have been described in Part I. They may also be delineated by statistical data of the total plankton or of its larger groups of organisms, or by the domi- O o VO vO to VO CN ni CN CN -^ -rt CN o" !H CN * 3r o ^ imum CN CN CN -H CN o 03 O A 5 5 5 ' 5 ro bo cS CN CN CN CN CN CO CN O O a 1 IO IO VO IO CD +i CJ O rt .p C -J o : : o o O ^ O CN bo CN Qv O ON X^ o" J CN CO N CD a IO CN CN CN O 10 Cfi a ,-H T-H ^-H CN "^ CD CO 'x ft ; ; .". ' 5 CD Cfi CN bo r^ i^ oo co t^ ,_T C8 ^ 1 ^H ^-1 CN i-3 CN * OO" ^ a N ^J '-I '-I CN CN '-I ". a CN CN -^ ^-l CN 1 1 ^ ON bo O CN O O CN CN ^ 03 CD 00 > ) a | \O oo vO ^O CO 1 1 03 i i CO C Q' *^H o o a 2J Ct3 W ni CD /j 8o a O *s >, ^ _J- , t t| t 43 .3 P- 03 w O CD o CO 1 03 u 03 I 1 I * 1 s t; r^ +-> rz o O fi 03 ^_i .^j ' Q cS Q (S r| efl oi Q cj .0 s %% e o *o CN bo cS *-< vo ^n vo vc CO CN CO CN C^ "- 1 u CN " 1* CN rt.8 . o oo o 'go o" 03 3 _a CN CN CN CN O Q. u - ^ S rt ^ 5 X a &,:::: S'S u m c rt CO bo CO 00 00 O g " 08 ^ i ^ i i 5s CN i-5 in ,c CO c.s c c V a) 6- oT CO CD PH mum CD C |S .J 3 VH' C f 11 4) bo \o oj 08 OS - - ^ C S ^ !2 < 00 * a -- 1 li 10 CN M S CN ^ CN _ (U CN O.S s*g o" d 3 i > imum 10 ^ O to CN *-H CN a 4/S ;3 IH CD PH M rt -2 : r : j rv, ^ o t.S bfl ^-t \o \o \o ^" 5 ^C CN hJ + H t 3 S ^X o" CN CO M-l f-H -M o> J PH CN J ^j O O *^ P. S Si 00 H o 'S a; g co -* CJ CD Q 1 *4 ao' ,= - N ?^ 11 O*ft *o- Efl CD Q 'o in CD rt Q tn CD in & h o 2 c ^ -S 3 ^ ! .2 'C 03 Vp -| , aS o ! O PQ S M P 296 bo rt CN CN CN C i_3 PO PO CN ^cT bb 3 .mum CN *-H >-H ^-H X 03 5? "o^ - c "ft s oT - o 57 CN bo oS T)< vo CN lO CN 1 CN '-i CN 10 CN oo" a * *3 OO OO IO " OO >O a CN --H -i ~y 'x X bb 03 1, > 1 T 1 -^H T-H ^ 1 ^X X - 03 1 > bo oS CN NO VO vO CN CM ^H -l ^H CN ^ NO CN 03 a t^. ^ _ ^ l^ u a 0) 3 H G > oS ^ bo 03 CN CN CN --i I-H CN o" 'ft a 00 oo 00 00 CO -H < | 1 1 1 T-H ? * 03 1 : : : : /: C 3 _i o naiinr* a si K os g 0^0 g -i n J o 0) 03 0) ft 'S o S 2 ,5 .bO S g Q rG +-> ^ o a ~o3 ^ 2 o3 oj o G I) Q o. O S K W PO bo LO t"* *-O CN VO PO c ,_, ,-H " 00 *: imum PO IO PO O PO CN -H CN PO CN ^ x O 3 3 33 o oS fc CN bo oS 1 O ^-^ '-i t^ <* o PO ^f CN '""' + + ft^< at 0) tt) ,T3 .3 ft o3 " B J j* a t ."7! '-+3 ^ O ^ o V3 +J +J Q "o3 Q 2 oS O G PQ S P^ W g o>"-^ , 5 B > *^ ' ^ 03 j__. t/3 hJ i) " c/3 ij r/"\ PH OJ "^ gj * KA 1as 2 fe CN DU 03 OO ^J< rj< OO OO i* -S 00 CN bo cd O O r^ t^ fo O "*~*rC G "^ ^_ < - h3 ^ ^H -H ^H M CN t^- IT! CN ** % ^ bi o C Q >> aximum CN CN *-i CN CN i * .0 0) M tj p.b rt S 3 rt >> Pll og^ -a 0) c > cd 3 o " 1 1 M |P| ^^ PH 00 CN of ro -H ro vo NO g-|| 2 ^ ^ S ^H" M ^H -rt P-" 5 P "*"* r CN H O . CN 00 NO + ON ^ cj H S'-N'O, 2 H >^ 00 CJ 0) Q 3 CN CN -H rt O 3 3 3 s^'ti 2 cd a 3 a 22^;^ Q X o3 Q S wl s 4) , S ^_^2 ^" ttj cd X cd efl ' " " i O S to ^ S ^ t/3 'C a , O ^ _O o ' CJ JS a ^ CU ^ cd aj;| J-tJ-c S CJ o a tU ' _>' rrt CQ rtj cd i "o <-l o to u +-> 03 Q 1 8 (O eg Q Pulses. . . . PH '5 o" 2 o .2 .bo s a r2 'S CO '<3 +J .d cd o3 o C o m S tf w 6The Apri as the combin; the curves of t 7 The Dece *o CJ "o to 0) Id Q S I4H O to 0) td Q to OJ ^ PH O ^ O o3 | 1 f a g O bo CD fi J3 *2j g S o3 cd o C O P5 S PH W 298 bo aS ^-H ^ i i CN ^ t CN o" CN *^ ^3 o u P | CO CO ro O ^^ cj 03 0) P bo cS CN J _ CM ^ _ ^, CN" CN - CN O ^ O S IO CN lO IA) 00 g ^H CN ^-H ^H S K; rt Q 3 3 = 3 bo cd O VO CN CN T-I CN CN CN 00~ ^1 CN u * 0) en | tS | IT) XO OO ^O '^ CN CN H CN CN -*-" 3 3 3 3 OS CJ O CO a Q '** o O s !}* 03 W fli CO cu W ^H x 1 15 o a) o g "o CO
  • _ S-g 0} .rH a I rt !* o S " c 2 OJ M !| S >- 0) s y >*> 5i 11 l-c J^ f 3g r=!rt O -g u B 299 bo a OO 00 OO *-l -H 00 h3 CN CN 2 cs ^ PH | ^ ^ ^ ^ ^ 1-c' 1 *4 v4 H * 03 d 3 3 3 3 i 00 CN bo rt * l CN *-i CN "-I CN CN ^ VO , ca cu ' fe bo o3 ^-l ^H ,-H CN CN CN o" CN CJ P S g O O O f- *^ co l ~~> 'x _j o3 333 I 1 s a G .2 O O o 6 W flj Cj ?i "o o <*-! o CO 1 3 VH (U 43 U3 (1) to (u TO in y o o 2 ^ y ^c c Jo .2 a, U3 P^ V-c Q O O J2 ^0 o fl 300 nant or more typical species. In the case of the total plankton some obscurity results at times from the inclusion of unusual proportions of -an adventitious population with flood waters. The selection of particular organisms as representative is also subject to some error, since seasonal changes in temperature and other more subtile causes often deflect or suppress their development. The totals of the ChlorophycecB, Bacillariacece, and chlorophyll-bearing Masti- gophora, and of the Rotifera and Entomostraca (PL I. -IV.) probably give as complete and accurate a delineation of the recurrent pulses as the statistical data afford, since they include relatively few adventitious organisms, cover the entire year, and swamp more or less completely individual and temporary divergences of particular species. The delineation of the pulses by statistical data is obviously more significant than the volumetric method, since it more clearly presents the results of the reproductive processes which lie at the foundation of the phenomenon of recurrent pulses ; and this method is also free from the unavoidable error arising from the presence of silt in the collections. The interval between collections introduces an error of consid- erable moment in any effort to determine with accuracy the duration of individual pulses, that is, the length of time between their minima or maxima. Daily collections would render this feasible, but with an interval of a week or more, not only the duration, but in some cases the probable separation of the pulses and location of their maxima, is to some undetermined degree obscured. The duration of the pulses of the five groups of plankton organ- isms shown graphically on Plates I. -IV., in the case of all chlorophyll- bearing organisms considered as a whole, is in 29 out of 36 instances between 21 and 35 days, less than 21 in 2 cases, and more than 35 in 5, reaching extreme limits of 14 and 49 days. They average 30.25 days between minima and 29.97 between maxima. The rotiferan data in the same months may be divided into 36 periods, in 33 of which pulses are traceable. The duration of pulses between minima lies between 2 1 and 3 5 d ay s in 2 3 of the 3 6 instances , falls below 21 in 5, and is above 35 in 8. The extreme limits are 14 and 49 days. In the case of the Entomostraca, where also the pulses are obscure in a few of the intervals, we find that 22 of the 36 are between 21 and 35 days between minima, 5 are below 21, and 9 are above 35. 301 The extreme limits are 12 and 49 days, and the average duration is 29.9 days. From the data here presented it is evident that the pulses are in the main from 3 to 5 weeks in duration, averaging approximately 29 + days a little less than one calendar month. The amplitude of the pulses is affected profoundly by seasonal and local influences, such as the factors of temperature and chemical constituents of the water, and the hydrographic conditions. These have been discussed in connection with the volumetric-data in Part I. and in the discussion of species in the first part of the present paper. Rising, or even uniform, temperatures, hydrographic stability, decaying vegetation or access of sewage or other fertilizing constitu- ents, all serve to increase the amplitude of the pulses. Declining temperatures, dilution or suspension of access of fertilizers, competi- tion of gross vegetation, access of flood waters and increase in current, all tend, in the main, to depress the amplitude of the pulses. The duration of the pulses is not, however, thereby essentially modified, though a tendency to override subsequent pulses and partially, rarely wholly, to submerge them is at times of major pulses often apparent in the data. The cause and significance of the phenomenon of recurrent pulses is not clearly and unmistakably evident, owing, on the one hand, to the irregularity of the data, and, on the other, to the great complex- ity of the problem, especially in the fluctuations and varying combinations of environmental factors. The plankton method itself is subject to great errors, but these are largely distributed, and careful examination, especially of the matter of dilution and computation, has failed to reveal any probable or even possible source in the method to which these recurrent pulses can be traced. It is not impossible that the rhythm here noted is merely a chance outcome of the statistical method and without biological significance; that it is wholly accidental, the resultant of the con- flicting and varying factors of the environment and not predomi- nantly or continuously initiated by any one factor. On the other hand, its nature, as we have described it, is such that we are led to look for some factor in the environment with which this rhythm of repetition in growth of the plankton organism might be correlated, or to some internal or inherent factor within the organisms constituting 302 the plankton, or to the interaction of environmental and internal factors. That there is a periodicity in the reproductive processes of organisms, of both plants and animals, is generally apparent. We see it in the flowering and fruiting seasons of the phanerogams, and in the breeding seasons of many invertebrates, of mollusks and insects, and of the vertebrates generally, of fishes, amphibians, reptiles, birds, and most mammals. Fluctuations in environmental conditions, notably in food and temperature, influence these re- productive processes. The phenomenon of rise and decline of the microscopic population in laboratory aquaria is likewise an illustra- tion of the periodicity of organisms, but usually within a briefer interval than that of the organisms above mentioned. The studies of Maupas ('88) and Calkins ('02) have shown that even in the seemingly uniform conditions of the laboratory, the reproduction of the ciliate Protozoa is essentially periodic. On a priori grounds it seems highly improbable that in the case of the organisms of the plankton, internal factors should determine the coincidence of the periods of growth and reproduction in several hundred species. While it is not impossible, or indeed improbable, that these species of the plankton if bred in pure cultures or uniform environment would still exhibit a periodic reproduction, it seems highly improbable that so diverse an assemblage of algae, diatoms, flagellates, protozoans, rotifers, and entomostracans as is found in the Illinois River, would exhibit in laboratory cultures under uniform conditions any such coincidence in the location and duration of their pulses as is found in the waters of the stream. Whatever the internal factors involved in the growth and reproduction of plankton organisms may be, it is patent that we must look for some environmental factor or factors lying at the foundation of the coincidence of seasons of growth and reproduction of plankton organisms, which results in the phenomenon of recurrent pulses in species, groups, and volumetric plankton. We may simplify the problem somewhat by recognizing at the outset the importance of nutrition in supplying the basis for the periodic growth of any organism. The rotifers and entomostracans, at least the limnetic types, depend in large measure, either directly or indirectly, upon the synthetic planktonts, such as the algae, dia- toms, and flagellates, for their food. Since the pulses of these animal 303 forms (cf. Plates III. and IV. with I. and II.) coincide with or follow shortly after those of the synthetic planktonts on which they feed, we may conclude that the cause of the periodic movement of these animal groups lies in the periodic fluctuations of their food supply. In the causes which control this periodic growth of the chlorophyll-bearing organisms will be found the solution of the general periodic phenomenon in plankton. This rhythm is primarily one of growth and reproduction, and its solution must be sought in the forms of matter anctenergy which affect these processes. The nutrition of the chlorophyll-bearing organisms is drawn from matter in the river water. The analyses contained in Part I., Table X., and graphically presented on Plates XLIII. to XLV. trace the seasonal fluctuations in the nitrates -one of the important constituents of plant food. Neither in the seasonal curves of this or other forms of nitrogen delineated in the plates is there any such rhythm of occurrence, though, as has been pointed out in the discussion of the chemical conditions, there are instances of apparent correlation of plankton and nitrate pulses. They occur at irregular intervals, and do not form a continuous series. That there might be a rhythm in the utilized nitrates (the analysis repre- sents only the unused residuum) is of course possible, or that it might occur in some other constituent of the food not determined in the analysis is not impossible, but we have no evidence of its existence. The chlorine in our river waters is a fair index of the amount of sewage or pollution by animal wastes. It is subject to considerable fluctuations, resulting in part from dilution by floods or concentra- tion in low waters, and there are other pulses not traceable to hydrographic conditions, which perhaps result from industrial wastes. These fluctuations in some instances coincide with those of the phytoplankton in question, but the instances are few and the correlation is incomplete. Upon investigation I find that sewage pumpage at Bridgeport, which discharged the sewage of Chicago River into the Illinois and Michigan Canal and thence into the Illinois River, was practically continuous, and could not produce the rhythm 'in question. The sewage of Peoria has a much more immediate effect upon the chemical conditions in the river at Havana than has that of Chicago. The sewers of this city, I am informed by Mr. H. E. Beasley, City Engineer, are flushed as 304 follows : " The method used is that of flushing with a hose, a crew of men being kept constantly at work, taking them about a period of three weeks to cover the entire system. The water is allowed to run through a fire-hose at each point for a period of about ten minutes." This system was in use during the years of our opera- tions, and it offers no occasion for the periodic pulses in growth of the organisms in question. Investigation of the discharges of dis- tillery and cattle-yard wastes into the stream has not revealed any periodic fertilization of the river waters from these sources. The available data thus fail to exhibit any periodic rhythm in food matters in solution and suspension in the river water with which these pulses of chlorophyll-bearing organisms might be correlated. Frequent reference has been made in previous pages to the appearance of pulses upon the decline of floods. Flood waters bring into the river, as shown by the chemical analyses, large quantities of silt and organic wastes in suspension and solution. They inundate great tracts of fertile territory rich in vegetation, and thus add to the available sources of food for the phytoplankton. Decline of the flood affords time for decay and solution of some of the food matters, and time also for breeding, and its run-off adds to the volume of the plankton in channel waters. A comparison of the hydrographs of the years in question (Part I., PL X.-XIII.) with these recurrent pulses (PL I.) will show that many if not most of the pulses appear on declining flood waters, and that many of the larger ones follow the major floods. Closer analysis, however, shows that there are sometimes two pulses of chlorophyll-bearing organisms on the decline of a single flood, and that they may also occur upon rising flood or even in its entire absence. Floods unquestionably affect the amplitude of the pulses, and to some extent modify their location. They seem inadequate, however, to explain their recurrence and their tendency toward a uniform interval. Minima between pulses also recur on declining floods. Energy as well as matter is necessary for the growth of the phytoplankton, and its source is primarily the radiant energy of the sun. A plot of the tri-daily air temperatures at Havana for 1894-1896 (Part I., p. 478, Fig. C) inclusive, exhibits many irregu- larities, a few of which partake of the nature of recurrent pulses at approximately monthly intervals, but they are too few and too irregu- lar to be the basis of the recurrent growth of the phytoplankton. 305 The importance of light for the photosynthesis of chlorophyll - bearing plants is unquestioned. The liberation of oxygen by the plant declines as the light fades, and is at its lowest ebb in darkness. The access of light to the phy toplankton is limited by several factors of the environment, principally by silt, which increases the turbidity, and by clouds, which interfere with the penetration of the sun 's rays. The fluctuations of the silt are chiefly the result of floods, and, as above stated, the floods do not exhibit a rhythmic pulse which can be correlated with that of the phytoplankton ; much less do the periods of rising water which are most silt-laden. The cloudiness of the sky varies greatly at different seasons of ' the year, being predominant at times in the autumn or winter months. It is sub- ject to pulse-like occurrences of variable duration, but an examina- tion of the records for central Illinois for the years under discussion does not disclose any periodic rhythm which can be correlated continuously with that revealed in the statistical records of the growth of the phytoplankton. Another factor of the environment which modifies the quantity of light which impinges upon the chlorophyll-bearing organisms of the plankton is the light from the moon. The amount of light, both absolute and relative, derived from this source is not great. According to the calculations of Zollner, the light from the sun is 618,000 times as bright as that from the full moon. In the pre- sent connection it is only important to know whether the moon- light contains an amount of solar energy sufficient to appreciably affect the photosynthesis of the phytoplankton. The amount of such energy utilized in photosynthesis is relatively a small propor- tion of the total, so that there is a possibility that moonlight may contribute to the process to an appreciable extent. This matter was investigated by Knauthe ( '98), who determined the fluctuations in the gaseous contents of the w*aters of carp ponds rich in Euglena. While this author does not report upon the plank- ton of the ponds investigated, it seems quite probable that carp ponds rich in Euglena would present conditions very similar to those found in the Illinois River, which has a remarkably well-developed Euglena water-bloom, and abounds also in carp. The following table presents the results of his work bearing upon the point in discussion. 55 a H fc O O O g|^ | 5 ^3 t || ^) V2 . ,_< ^J f * CD V- s^ o3 > ^ pj P3 m > l*tB O D ^ ^ C cu C CO 00 O bog iSt^ a; o ^ H) tj QJ O O3 Pi s rt r^ V3 (-H CT3 "^ o *^ tn 1 E H M^ti]"rH " I"" 3 ^ ^* | " ^ ^ r |xa t>i-i W !H N l^- r-- CN CN o3 ^"^ O- ON CN CN '-; t^. . . . . 03 cj O O O O r^ "ri O 03 O O ^~ ' ^"^ CN CN ^O 1O f^- t^* vO NO ^ ^ fO CN CO PO ^O l^" loiom loinio ^ CNCN t^l-^ OO OO CNCN * Tf CNCNCN 't l ^-^t l CNCN * * * ^H H ^H J OO OO -rt r-i CNCN OO OO OOO OOO OC OOO -^M t i ^-i 0) 9 S a 8 S ^j --2 - Z| o3 O O fa o ,d t/3 ^3 T^H *^ O O o o -^ "rt 0) w 2 - ' fl S-I o - ^ rZ-i ^ C3 x* ^ "S 'K ^ * S K* b/)' "~* M ?H S 3 -2 ^ S^^^ ".^T3 Pl,Q f- 3 4-^ '^ *^~ l 4-> '-' rl "^ rt -*- 1 O OTCfl<^ 03 o3 O s s ^ ^ ^ 3 2 "3 o3 IH U o UJ CO da rj ;j 0) CO 0) CO CO CO eS 03 fl El T) o3 o3 o3 o3 o3 o3 IH t-( In TH t-H IH D u O3 C PI c5 CTj 33:3 333 o3 03 ft WC/2C/2 C/3WC/2 C/3 t/2 C/D CO CO 00 ^ s : s : 6 ! r A d e 6 ! ^ 00 O3o3o3ftft ftp, ft ro rorOO OO1J O O O O ro fO ft o o o o CNl/IONlOON'-H - 1 ... CN-ON ^H T-H TjH ^ x >< x x x x"' x" x KJ kj rS rN 1 1 I 1 1 1 II 1 ^< 10 i/"; 1O ^ ^ CO fO fO 1 1 306 307 The amount of oxygen present in the water in the dark, or on dark nights, is reported as 0.20, 0.25, and 0.27 cm. 3 per 100 cm. 3 of water. In bright sunlight in the laboratory, and with the unusual abundance of Euglena due to the collection of the water sample from the region of the water-bloom, it rises to 2.05 cm. 3 In the case of the Spandauer samples it rises from 0.25 in the dark to 1.15 (an increase of 0.90 cm. 3 ) after "long" exposure to bright sunlight in the laboratory. The oxygen in this water at 11 :00 p. m., after exposure to moonlight, amounted to 0.45, or 0.20 cm. 3 more than was found in control water kept in the dark. In this instance the apparent increase due to moonlight is 2 / 9 of that due to sunlight. In the case of the moonlight the analysis was made at 11:00 p. m., after not more than three hours ' exposure. The moon was not at its greatest efficiency, since full moon occurred four days prior to the date of analysis. In the case of the sample exposed to the sunlight the analysis was made at 4:00 p. m., after "langer intens Sonnen- schein." It would seem probable that the effectiveness of moon- light in comparison with sunlight in photosynthesis by the phyto- plankton here indicated (2 to 9) is below the possible maximum and also above that of the average, since it was obtained when the moon was but four days past its maximum effectiveness. If we accept Knauthe 's data as sufficient to establish the effec- tiveness of moonlight in increasing photosynthesis, and thus the growth of the phytoplankton, we find in it a recurrent factor of the environment to whose influence we may seek to attribute the rhythm of growth of the chlorophyll-bearing organisms. On Plates I. and II. I have plotted the seasonal distribution of the totals of the Chlorophycece, of the Bacillariacecz, and of the Mastigophora from July, 1897, to April, 1899, and have indicated the times of full moon throughout this period by marks at the bot- tom of the diagram. The diagram shows clearly the occurrence of these recurrent pulses, their approximation in the three groups of chlorophyll-bearing organisms upon the same or adjacent dates, and the occurrence of their maxima in some cases at the time of full moon or within an interval of ten days thereafter. In the table which follows, I have given the data bearing on the pulses of the total of all chlorophyll-bearing organisms from July, 1895, to October, 1896, and from July, 1897, to March, 1899, inclu- sive, 36 months in all, stating the location of the pulse as determined (21) *8 \O ** co *"* '"' *** OOOON^NC 1 '" 1 -*^ r^ CN CN CN 04 ^ CN I s 03 IM 1" t t ,5 J 3 H^-, < co O *& P Q r d I* t- *** ^ ^> b 2 fli aj CO l2 ^ *" rt., C H 3 ^j; i ,pH^i Cca +J W3 (H .^ O c/3 Q) p> Qs t^* IO QQ ^ to l-HTfl-rHvOCO'-INOCO lONQCN-'-lt^OCOOO ' ^-H ^H CN >-c >-! CN --I S3 8 & J2 bC + + + + + + + + + + + + + + + ' . X >% oj *3 Oj 3 3 3 3 ^ O3 CB3333335 Q ^ 'O a | O) f~ ON OO "* O "^ LO t^ NO ^^ NO CO IO Tf* ^H CN CN -i CN '-H CN 8 03 1 1 d ,53 5 c Q O O 1 O ro -* O O I^-COIOCNIOCN'-'CN 'So i i 7 i i i 1 1 1 + + + 1 1 03 "o S 2 ^ 2 - ,0 - o ^-cs2-2cx>- 1 ^> bii "^ .j ^ d d "33 o O 0) OJ ^ < co o ^ Q Q g S 'a 1 -' ^ g fc^<^i , t j & 11 ^ CN ON *O NO ON CO CN CN CN CO H CO-^-COCNCNCOCNCN _C PQ C 03 S g "S ^ .S CO -( CN CO CO IO ON co t^ NO co NO NO ON O 11 -S.S CN CO CN CO CN CO *-H ^CNCOCOCNCNCN'* 1 s f> (A "3 a *o ^^S^SS^^^ ON .^ O 10 CN "- 1 i^ '""' CO _jj d .CO 3 < 1 o Q ,| _J b '^ c (uj^'bii'p. ^S^'-'^^^co ' ' o o o oo^-S^-S^-S ^ NO CN ^ N-4 ^ CN JQ ^^q^S^^^^ 1 1 1 1 1 1 1 s | | 1 | I I | 3 O o z 5 -H -H O O Ov Ov bi> w a V C/2 o Sept 6 . . 76 982 440 o o o o o " 13 49 515 000 7 500 o o o o " 20 63,144 000 o o o o o " 27. . 45 854 000 218 400 o o o o Oct. 4. . 48 193 000 251 000 1 800 000 o o o " 11 15,129 540 486 ' 000 o o o o " 18. . 17 367 000 25 000 o o o Q " 25 5 416 500 13 500 o o o Q Nov. 1 . . 25,325 500 2 000 o o o o " 8. . 14 564 000 o 3 600 000 25 000 o Q " 15 36 Oil 000 o o o o Q " 22 " 29 23,494,000 73,719 000 o 38 500 o o Dec. 6 . . 56 400 500 o o o o o " 13 148,740 000 o 1 800 000 o o o " 20 116,344 800 o o 247 200 6 000 o " 27 67 965 800 o o 69 600 o o Average 95,852 602 112 896 555 000 407 602 101 358 866 083 321 TABLE I continued. ORGANISMS PER CUBIC METER IN PLANKTON OF ILLINOIS RIVER IN 1898. (An asterisk at head of column indicates that all entries in it are based on filter-paper collections.) 1898 Dinobryon sertularia var. stipitatum Eudorina elegans Euglena acus II ^ Euglena oxyuris Euglena oxyuris* Jan 11.. " 21 100 " 25. . Feb. 3 " 8 " 15 " 22. . Mar 1 8. . " 15. . 3,600 40,000 40,000 " 22. . 800 " 29 Apr 5 . . 2,600 2,800 " 12 1,800 100 " 19 9,960 36,000 100 " 26 1,830,400 240,000 800 May 3 . . 4,883,200 240,000 3,200 " 10 24,608,000 48,800 " 17 28,800 32,800 90,000 180,000 " 24. . 1,000 " 31 .. 400 June 7 9,600 " 14. . 60,000 900 , 000 1,600 " 21 .. 30,400 2,400 " 28 4,000 1,800,000 July 5 . . 400 400 800 " 12 800 400 1,200 " 19 o 7,600 400 3,600,000 " 26. . 4,000 2,400 3,600,000 Aug. 2 ... 8,000 800 120,000 3,200 1,800,000 9 400 800 1,200 3,600,000 " 16 800 800 120,000 3,600,000 " 23. . 3,200 1,600 6,400 120,000 " 30. . . 2,400 800 3,200 4,500,000 Sept 6 40 1,600 900,000 10,400 5,400,000 " 13. . 500 1,500 3,600,000 " 20. ... 2,000 1,500 1,000 1,800,000 " 27 1,600 6,400 1,800,000 9,600 9,000,000 Oct. 4. . 1,500 3,600,000 1,000 2,700,000 " 11 1,000 1,800,000 500 1,800,000 " 18. ... 900,000 " 25. . Nov. ' 1 . . ' " ' 8 1,800,000 " 15 ' 22. .. 1,000 1,800,000 " 29 120,000 Dec. 6 . . o " 13 500 " 20. ... 22,000 " 27.. 603 911 14 362 375 214,807 963 960,769 322 TABLE I continued. ORGANISMS PER CUBIC METER IN PLANKTON OF ILLINOIS RIVER IN 1898. (An asterisk at head of column indicates that all entries in it are based on filter-paper collections.) 1898 O.eg J>2 "3>- ^ B Euglena viridis* Glenodinium cinctum Glenodinium cinctum* Gonium pectorale Lepocinclis ovum Lepocinclis ovum* Tan. 1 1 .... 80 000 o " 21. . o o 100 Q " 25. .. o o Q Feb 3 o o o Q 8. . o o Q " 15.. o Q " 22 o o Mar 1 . . o o Q " 8 " 15 " 22. . 200 400 400 200 200,000 240,000 4 260 000 o o Q " 29 240 000 o Apr 5 . . 240 000 o 200 o " 12. .. 120 000 200 Q " 19 400 1,200 240 000 o 800 Q " 26 May 3 3,200 360,000 120,000 22 400 o Q " 10. " 17. . 120,000 3,600,000 o 90 000 200 800 o Q " 24. . 630,000 o o o 200 1 800 000 " 31 60,000 o o 400 Q June 7 ... o 900 000 o o 180 000 " 14 1,600 2,700,000 o 60 000 800 420 000 " 21 " 28. . 3,200 7,200,000 900 000 o 7,200,000 o o 2,400 5 600 240,000 Q July 5 . . 120,000 o o 1 600 Q " 12.. 2 700 000 o o o 800 " 19. . 2 400 3 600 000 o 7 200 000 o 4 400 " 26. .. . 3,200 14 400 000 o 2 700 000 o 30 000 900 000 Aug 2 . . 1 600 7 200 000 20 000 12 600 000 o 50 400 9. . 4 800 7 200 000 400 25 200 000 o 6 400 " 16 5 400 000 o 5 400 000 o 800 720 000 " 23 4 800 4 500 000 o o o 14 400 " 30. . 8 000 2 700 000 800 * 900 000 800 43 200 Sept. 6 . . 800 3 600 000 900 000 o 11 200 240 000 " 13 " 20. . 1,000 3 000 1,800,000 500 120 000 o 1,000 5 000 " 27. . 6 400 1 800 000 o 3 200 8 000 Oct. 4 " 11 .. 6,300,000 120 000 o o o 2,500 2 000 1,800,000 Q " 18 o o o 500 120 000 " 25..' o o o o Q Nov. 1 . . o o o o 500 o 8 o o o o o Q " 15 o o o o o Q " 22. . o o o o o O Q " 29 o o o o o Q 120 000 Dec. 6. . o o o Q " 13. ... o 1 020 000 o 60 000 o Q Q " 20 o o o 900 000 o o o " 27. . o o o 960 000 Q Q Average 8 653 1 571 731 452 1 360 192 526 3 719 401 538 323 TABLE I continued. ORGANISMS PER CUBIC METER IN PLANKTON OF ILLINOIS RIVER IN 1898. (An asterisk at head of column indicates that all entries in it are based on filter-paper collections.) 1898 Mallomonas producta Pandorina morum Peridinium tabulatum Peridinium tabulatum* Phacus longicauda Phacus pleuronectes Platydorina caudata Pleodorina calif arnica Jan. 11 " 21. . 400 100 100 " 25 Feb 3 . . 100 8 400 " 15 " 22. Mar 1 . . 400 8 " IS. . 600 " 22. . " 29. . 200 600 Apr S . . 200 " 12. . 500 600 " 19 800 400 1,600 " 26. 48 , 000 3,200 May 3 . . 12,800 48 , 400 3,200 " 10 " 17. . 800 " 24. . " 31 400 June 7 . . 835,200 8,000 120,000 200 " 14 28 800 60,000 900,000 8,800 800 " 21 " 28. . 28,800 28,800 40,800 9,600 2,400 8,800 1,200,000 79,200,000 8,800 4,800 800 July 5 . . 400 2,000 5,400,000 4,800 " 12. 800 18,800 10,800,000 3,200 400 400 " 19. . 12,000 49 , 600 86,400,000 3,200 400 400 120 " 26. . 63,200 66,800 15,300,000 6,800 800 400 Aug. 2 9. . 800 59,200 1,200 12,000 7,200 120,000 120,000 11,200 4,800 2,000 " 16. .. 3,200 8,000 " 23 2,400 6,400 1,800,000 4,800 800 . 60 " 30. . 3,200 6,400 8,000 1,600 Sept. 6. . . . 2,400 12,800 1,600 " 13 3 000 3,000 1,000 " 20. . 1 1,500 7,000 500 " 27.. 1,600 100 4,800 35,200 4,800 Oct 4. . 7,000 " 11 " 18 S 500 1,500 1 ,000 " 25 500 Nov. 1 . . 500 8 " 15. . 1,000 1,000 " 22. . " 29. ... Dec. 6 . . " 13 180,000 " 20. . o " 27. .. Average. . 17 520 6 957 3 711 3 875 769 3,031 298 17 11 (22) 324 TABLE I continued. ORGANISMS PER CUBIC METER IN PLANKTON OF ILLINOIS RIVER IN 1898. (An asterisk at head of column indicates that all entries in it are based on filter-paper collections.) 1898 Syncrypta volvox ^ S to ?,5 <0 Synura uvella* Trachelomonas acuminata Trachelomonas acuminata* Trachelomonas hispida Trachelomonas volvocina* Jan. 11 " 21. . 100 5,600 3,600 3,800 " 25. . 7,740 387 Feb 3 . 1,600 4,600 480,000 8. . 800 800 60,000 " 15 10,800 3,600,000 28,800 " 22 Mar 1 . . 800 8,800 800 200,000 900,000 8 " 15. . 400 1,200 8,800 109,200 800 900,000 1,800,000 " 22. .. 221 ,600 60,000 10,800,000 " 29 Apr 5 . . 320,600 166,600 200 200 900,000 1,800,000 " 12 100 17,800 " 19 126,000 60,000 9,000 000 " 26 . 121 ,600 120,000 4,500,000 102 400 3 200 3 600 000 " 10. . 38,400 9,000 000 " 17. . 21 ,600 3,600,000 14,400,000 " 24 " 31 . 1,400 200 60,000 200 360,000 180,000 June 7 4,500,000 " 14 " 21 .. 1,600 800 120,000 7,200,000 7,200,000 147 600 000 " 28. . 800 6,300,000 38 700,000 Tuly 5 . 400 1 800 000 1 800 000 " 12! ! 1 ,200 800 900,000 10 800 000 " 19 800 3,600,000 86,400,000 " 26 2 000 3 600 000 9 200 42 300 000 Auff 2 . . 12,800 600,000 800 18 000 000 .? g 800 3,600,000 252,000,000 " 16.. 4,000 3,600,000 1 ,600 93,600,000 " 23. . 3 200 1 800 000 800 65 700 000 " 30. . 8 800 1 800 000 1 ,600 18 000 000 Sept. 6 " 13.. 4,000 5 , 400 , 000 800 1 000 16,200,000 6 300 000 " 20. . 4,000 1 500 1 800,000 " 27 Oct. 4. . 1,600 4,800 500 3,600,000 1 800 000 1,600 9,000,000 11 700 000 " 11 120 000 1 ,800 000 " 18 " 25 Nov. 1 . . 500 2,000 900,000 120,000 2,700,000 5,400,000 1,800,000 8 . 1 000 16 000 1 800 000 " 15.. 9 000 " 22 94,000 1 800 000 5 400,000 " 29 4,500 1 ,999,500 1 ,320,000 3,600,000 Dec. 6 . . 13,500 1 693 500 2 280 000 900,000 " 13 " 20. . 2,000 6 200 78,000 2 764 800 2,760,000 900 000 o 500 o 2,400,000 " 27... . 800 395 200 300 000 o 2 700 000 Average. . 625 179.138 150.000 873 t .094.615 1.251 17.672.692 325 TABLE I continued. ORGANISMS PER CUBIC METER IN PLANKTON OF ILLINOIS RIVER IN 1898. (An asterisk at head of column indicates that all entries in it are based on filter-paper collections.) 1898 Total Rhizopoda Arcella discoides A rcella vulgaris Centropyxis aculeata Centropyxis aculeata var. ecornis Cochliopodium bilimbosum Cyphoderia margaritacea Difflugia acummata Jan 11.. 440,500 100 " 21 32,800 100 200 100 " 25 66 338 387 387 387 1,161 20,898 774 1,935 Feb 3 . . 122,900 1,300 8 4 880 3,200 " 15 . 34 880 800 800 800 80 " 22. . 141 ,524 632 25,272 12,636 9,477 3,159 Mar 1 11 200 400 400 400 400 8. . 11 ,720 400 400 1,200 400 40 " 15. . 7,600 600 400 " 22 " 29 4,800 61 400 400 400 200 Apr 5 . . . 700 100 100 100 " 12 3 520 300 200 20 100 " 19 " 26 . 7,300 6,720 400 400 400 May 3 . . 26,000 400 " 10 " 17 " 24. . 49,800 23,800 9,320 2,400 600 1,600 800 200 400 80 " 31 June 7 . . 8,920 23 600 400 800 200 200 400 200 3,200 " 14. . 21,600 1,600 800 " 21 21 600 1 600 800 800 " 28. . 37 000 800 800 100 July 5 19 360 1 200 400 400 400 " 12 26 000 800 800 200 o 1 600 1 200 " 19. . 28 800 800 400 400 400 " 26. . 4 800 400 400 400 Aug 2 16 800 800 4 800 1 600 9. . 7 280 1 600 400 1,600 40 40 " 16. . 24 060 2 400 800 800 800 " 23 36 800 800 5 600 800 " 30 Sept 6 ... . 23,200 20 800 800 800 5,600 800 800 1,600 3,200 1 600 " 13 28,000 500 500 6,000 1 ,000 " 20 " 27. . 19,000 59 200 500 1 600 500 1 600 500 500 1,000 8,000 1,500 o 500 3 200 Oct. 4. . 912,580 40 40 500 500 " 11 9 000 1 000 " 18. . 10 000 500 2 000 1 000 " 25.. . 25,060 1,000 1,500 1,000 1,000 500 500 500 Nov 1. 32 060 500 1 000 1 000 2 000 500 500 8. . 37 060 1 000 1,000 1 ,000 1,000 1 000 " 15 42,000 1,000 5,000 4,000 " 22 190 400 2 000 4 000 6 000 o " 29. . 3 400 o 500 500 o Dec 6. 121 000 o 1 000 o " 13. . 600 600 " 20 1 040 40 1 ,000 " 27. . 220 o 55 364 465 1 098 570 604 1 284 198 315 326 TABLE I continued. ORGANISMS PER CUBIC METER IN PLANKTON OF ILLINOIS RIVER IN 1898. (An asterisk at head of column indicates that all entries in it are based on filter-paper collections.) 1898 Difflugia globulosa Difflugia lobostoma Difflugia pyrijormis Total Heliozoa Nuclearia delicatula a ll SG EH Amphileptus spp. Carchesium lachmanni Jan 11.. 100 100 593,420 26 500 " 21 400 200 197,100 37 000 " 25. . 9,675 7 353 190 017 13 545 45 666 Feb. 3 . . 500 100 1,246,300 1 600 54 700 8 " 15. . 800 9,200 80 2,000 629,680 1 016 000 800 4 400 197,600 164 800 " 22. . 6,318 632 518,954 50 544 Mar 1 . . 4,000 800 400 1 773 360 400 46 400 8. . 2,800 800 40 492 920 800 54 800 " 15. .. 2,600 1,400 200 324,200 200 89 600 " 22 1 600 800 2 000 267 800 400 22 000 " 29. . 200 400 241 420 400 10 200 Apr 5 100 500 241 440 o 3 100 " 12 " 19. . 1,000 1,600 800 1,200 100 100 1,342,500 1,340 800 300 2,400 13 200 " 26. ... 3,200 3,200 7,710 400 99*200 May 3 . . 22,400 3 200 17 404 800 83 200 " 10 30,400 3,200 18 260 800 6 400 " 17 800 800 21,654,400 1 600 " 24. . 3 640 200 200 990 800 o 200 " 31. . 3,840 400 2 282 400 o 600 June 7 " 14. . 9,600 5 600 8,000 1 600 200 800 o 7 , 800 , 000 6 480 000 o o " 21.. 5,600 800 3 200 3 200 12 010 400 o o " 28. ... 14,400 2,400 100 6 491 200 o 9 goo July 5 . . 8 800 2 000 160 495 640 o o " 12.. 10,000 2 400 800 400 400 3 900 920 o 400 " 19 12 800 2 000 2 000 2 000 721 640 o 400 " 26. . 2 400 400 14 400 14 400 487 'OOO o o Aug. 2 . . 5,600 800 17 600 17 600 4 474 400 o o " 9 " 16. . 2,800 8 000 400 800 3 200 78,400 13 600 78,400 13 600 69,000,200 253 600 o o " 23. . 12 800 2 400 20 800 20 800 728 000 o 1 600 " 30. . 6 400 800 800 7 200 7 200 122 400 o o Sept 6 . . 5 600 o 800 4 800 4 800 120 001 640 o 800 " 13. . 11 500 500 500 500 1 451 120 o 9 000 " 20. . . 8 500 500 18 000 18 000 1 923 'SOO o 2 500 " 27 25,600 1 600 1 600 65 000 65 600 851 400 o o Oct. 4. . 8 000 500 500 720 000 o o " 11. ... 2 500 1 000 843 540 o 3 500 " 18 " 25 Nov. 1 . . 8 " 15.. 2,000 15,000 15,000 5,000 17 000 1,000 1,000 2,000 2 000 60 60 2,000 500 500 o 500 500 o 1,744,000 1,334,000 985,560 965,000 488 100 500 500 2,000 1 000 5,000 35,000 31,000 22,000 28 000 " 22... 48 000 8 000 400 750 640 4 000 108 000 " 29 200 200 721 "00 o 47 500 Dec. 6 . . o o 720 580 40 7 000 " 13. ... o 1 573 800 100 16 400 " 20. . o o o o 487 120 o 16 600 " 27. . . 200 o o o 490 600 200 28 000 Average 7 194 1 158 368 4 871 4 760 15 812 346 630 26 546 327 TABLE I continued. ORGANISMS PER CUBIC METER IN PLANKTON OF ILLINOIS RIVER IN 1898. (An asterisk at head of column indicates that all entries in it are based on filter-paper collections.) 1898 Codonella cratera Halteria grandinella* Stentor cceruleus Tintinnidium fluviatile Total Suctoria Metacineta mystacina 2 11 ^ 1 Jan 11 300 80,000 300 6,580 " 21 " 25. . 300 58,437 40,000 28,800 11,997 100 49,240 126^03 Feb 3 5,900 1,000 1 1 , 496 " is! ! 8,000 5,200 800 800 14,160 31,040 " 22 15,795 720,000 48,649 Mar 1 . . 10,000 1,600 1,600 20,400 400 8,400 520 1,600 1,600 29,200 800 " 15 33,200 1,000 400 400 103,940 400 " 22 41 ,600 60,000 80 1,200 1,200 185,520 400 " 29 Apr 5 30,400 20,500 - 20 300 200 100 200 115,880 84,820 5,020 1,800 " 12 . 20,100 900,000 200 100 54,540 " 19 " 26 453,600 614,400 400 12,800 749,000 2,892,360 4,800 May 3 . . " 10. . 736,000 78,400 720,000 24,000 5,247,800 2,663,400 200 " 17 72 000 10,400 800 800 1,465,500 800 " 24 74,200 400 196,020 3,200 " 31 June 7 61,200 1 499 200 60 000 400 14,400 200 200 180,760 903,000 18,800 392,000 " 14 . 532 ,800 104,000 639,600 1,600 " 21 .. 195, ?00 74,400 800 2,601,200 3,200 " 28 45,600 3,600,000 33,600 1,118,400 July 5 " 12. . 13,600 35,600 2 700,000 4,800 5,600 7,200 400 7,200 400 153,000 184,500 800 " 19 24,000 2,800 400 400 946,080 " 26 2,000 120,000 3,600 370,200 Aug 2 . . 23,200 1 800,000 95,200 1,294,240 9 " 16 8,400 20,000 4,800 8,800 782,720 935,380 " 23. . 26,400 5,600 1,600 1,600 696,180 1,600 " 30 51,200 800 435,080 1,600 Sept 6 . . 13,600 40 422,840 " 13. . 49,000 120 2,000 197,960 " 20 34,500 20,000 500 475,860 1,000 " 27 92 800 200 22,400 1,792,700 14,400 Oct. "4. . 23,000 1,500 105,020 2,580 " 11 23 000 40 500 122,000 2,000 " 18. . 47,000 900,000 1,000 40 40 159,200 " 25. . 23,000 1,048,620 Nov. 1 . . 8. . 12,500 70 000 60 156,300 147,780 " 15. ... 35,000 100 180,600 " 22 " 29. . 2,000 2 000 128,400 66,000 Dec 6 40 64,280 " 13. . 300 1 080 000 159,740 " 20 200 120,000 191,320 " 27. 200 120 000 o 50,540 200 Average 101,024 255,769 882 22,590 332 301 592,416 328 TABLE I continued. ORGANISMS PER CUBIC METER IN PLANKTON OF ILLINOIS RIVER IN 1898. (An asterisk at head of column indicates that all entries in it are based on filter-paper collections.) 1898 Conochilus dossuarius Conochilus unicornis Total Bdelloida Philodina megalotrocha Rotifer neptunius 11 'S 2 CS .| "*. fr; Anuraa aculeata Jan. 11 .. 400 400 6,180 " 21 45 100 44 500 4 040 " 25. . 90,171 89,379 35 271 Feb 3 3 800 3 800 7 696 8 " 15. . 6,800 18,000 6,800 27,000 7,360 12 240 " 22 25 272 25 272 23 377 Mar 1 . . 1,600 400 800 18 320 8 400 4,040 40 4,000 23,960 400 " 15 400 22 160 80 400 19 200 80 980 40 " 22 " 29. .. 400 20 10,440 1,620 40 20 10,400 1,600 174,680 109,240 400 200 Apr. 5 . . " 12. .. 1,100 960 60 1,100 800 81,920 53,480 600 600 " 19 400 3 300 400 100 16 000 745 300 2 000 " 26. . 3,200 4,640 640 3,200 2 889 720 3 200 May 3 16 000 12 800 5 231 800 22 400 " 10 " 17 " 24. . . 200 800 3,200 14,400 20,800 1,040 80 1,600 6,400 520 11,200 10,400 400 2,647,200 1,438,300 191 780 35,600 22,400 4 000 " 31 o 18 600 880 80 200 600 161 080 1 400 June 7 .... " 14 392,000 1,600 800 600 800 400 200 507,000 637,400 1,600 800 " 21 3 200 1 100 300 800 2 593 600 " 28. . 1 900 800 300 1 112 500 July 5 800 2 480 80 1 600 800 146 920 o " 12 " 19. . 4,800 2 760 400 1 600 2,000 360 2,400 800 178,100 933 320 " 26. .. 120 60 268,480 Aug 2 1 400 560 40 1 260 840 o 9. . 1 200 12 000 775 920 o " 16. .. 4 120 120 4 000 907 260 o " 23 1,600 5,720 60 5,600 671 ,260 " 30 1 600 o 4 080 80 2 400 415 000 o Sept. 6 ... 9 640 2 400 40 4 800 413 200 o " 13 21,000 500 1,500 17,500 171,960 " 20. . 1 000 o 6 000 1 500 500 3 000 460 360 o " 27. . 14 400 o 13 300 8 000 300 4 800 1 744 200 o Oct 4 2 500 o 2 280 2 000 120 160 97 160 o " 11 .. 2 000 o 2 000 1 000 500 500 115 SOO o " 18. . o 540 40 188 160 o " 25 3,500 1 000 2,500 1,045 120 o Nov. 1 . . o 3 060 60 3 000 152 680 o 8. . . 1 180 120 60 1 000 146 600 " 15. o 100 100 180 500 o " 22. . o 400 400 126 000 o " 29.. o 66 000 o Dec. 6 . . o 20 20 64 260 o " 13.. o 600 600 159 140 o " 20. ... 191 320 " 27.. 200 o 20 o 20 o 50 120 20 Average 517 8 108 405 983 351 425 6 688 571 611 1 839 329 TAB-LE I continued. ORGANISMS PER CUBIC METER IN PLANKTON OF ILLINOIS RIVER IN 1898. (An asterisk at head of column indicates that all entries in it are based on filter-paper collections.) 1898 Anurcea cochlearis type and var. macracantha Anurcea cochlearis var. stipitata Anurcea cochlearis var. tecta Total Anurcea cochlearis Total eggs Anurcea cochlearis Atmrcea hypelasma Total eggs Anurcea hypelasma Jan 11.. 100 100 " 21 300 300 100 " 25 387 1,661 2,048 Feb 3 . . 500 100 600 300 8 80 80 " IS. . 80 80 " 22. . Mar 1 . 400 80 480 8. . 800 1,200 2,000 1,600 " IS. . . 2,200 600 2,800 1,400 " 22 3 200 800 4,000 2,800 " 29. 2,600 600 3,200 200 Apr. 5 . . 1,700 400 2,100 600 " 12 1 800 400 2,200 800 " 19. . 12,400 2,800 15,200 8,800 400 " 26. . 12,800 121,000 4,000 137,800 57,800 222 400 745,600 54,400 1,022,400 552,200 " 10. . 134,400 790,400 220,800 1,145,600 643,200 " 17. . 91 ,200 295,600 48 , 000 434,800 160,000 " 24 1 000 18 400 1,800 21,200 7,200 " 31 . 1 400 9,200 600 11,200 3,400 June 7 .... 32,000 32,000 3,200 " 14 28,000 1,600 29,600 7,800 2,400 " 21 .. 150,400 222,400 372,800 148,800 9,600 8,800 " 28. . 48,800 117,600 166,400 20,800 7,200 4,000 July 5. . 2,800 7,200 10 000 1,600 800 400 " 12. . 2,000 8,000 10,000 4,000 1,200 " 19. . 2,000 15,200 17,200 5,600 4,000 " 26 1 200 1 200 Aug 2 . . 4,800 2,400 9. . 2,000 4,000 " 16 16 000 8 800 " 23. . 9 600 3,200 " 30. . 800 800 Sept 6 " 13. . 500 500 1 ,000 " 20. . 3,500 8,500 12,000 6,000 4,000 1,000 " 27 19,200 35,200 54,400 16,000 43,200 54,400 Oct 4. . 4,000 2,000 4,000 500 2,000 500 " 11 ... 7,000 2,000 9,000 4,500 500 " 18 o 17 500 7 000 24 500 10 500 3 500 2,500 " 25. . 500 9,000 19,000 28,500 7,000 13,500 5,000 Nov. 1 . . 500 1,000 1,500 ,0 500 1 ,000 8 " 15. . " 22. ... " 29 500 1,000 8,500 10,000 500 Dec. 6 . . 1 , 000 1,020 2,200 20 " 13. . 500 1 700 5 100 7 300 1 800 " 20. .. 3 600 1 ,600 5,200 2,600 " ' 27 200 200 Average 9,421 44 540 15,432 69,165 32,358 2,390 1,917 330 TABLE I continued. ORGANISMS PER CUBIC METER IN PLANKTON OF ILLINOIS RIVER IN 1898. (An asterisk at head of column indicates that all entries in it are based on filter-paper collections.) 1898 Asplanchna brightwellii Asplanchna priodonta Brachionus angularis Brachionus angularis var. bidens Total Brachionus angularis Total eggs Brachionus angularis Brachionus bakeri var. brevispinus Jan 1 1 . " 21 .. 100 " 25 387 387 Feb 3 . . 8 " IS o " 22. . Mar 1 80 8. . " IS " 22 40 " 29 Apr 5 20 100 100 " 12 " 19. . " 26. .. May 3 . . 16 000 " 10. . 20,800 3,200 1,600 1,600 " 17 11 ,200 14,400 800 800 800 " 24 " 31 .. 400 200 2,120 2,000 1,400 200 200 1,400 June 7 . . " 14 " 21 . . 200 1 ,100 1,100 4,800 4,000 70,400 4,800 4,000 70,400 1,600 24,800 " 28 100 544,000 544,000 128,800 July 5 . 160 29,200 400 29,600 1,600 " 12. . 51,200 51,200 13,200 400 " 19 280 300,800 34,800 335,600 72,800 " 26 17 900 6 400 10,400 16,800 1,200 Aug. 2 ... 23,200 10,400 93,600 103,200 12,000 9 80 229,200 64,800 292,600 105,600 400 " 16 800 272,800 80,800 353,600 116,000 " 23. . 4,000 77,600 138,400 216,000 42,400 " 30 2,400 28,800 86,400 115,200 28,000 2,400 Sept 6. . 80,000 83,200 163,200 35,200 400 " 13. . 60 27,000 10,000 36,500 18,000 2,000 " 20 1,140 60 87,500 27,500 115,000 43,000 " 27 6,400 409 , 600 84,800 494,400 41,600 1,600 Oct. 4. . 500 19,000 9,000 28,000 2,000 " 11 1,000 8,000 1,000 9,000 2,000 " 18 8,000 500 8,500 2,500 " 25.. 60 11,500 11,500 5,500 Nov. 1 . . 1,000 , 1,000 8. ... " IS... 100 100 " 22 " 29 Dec. 6 . . 20 20 " 13 " 20. . 400 400 " 27 2 079 441 43 946 13,973 57,919 13,242 139 331 TABLE I continued. ORGANISMS PER CUBIC METER IN PLANKTON OF ILLINOIS RIVER IN 1898. (An asterisk at head of column indicates that all entries in it are based on filter-paper collections.) 1898 Brachionus bakeri var. clunior- bicularis Brachionus bakeri var. melhemi Brachionus bakeri var. obesus Brachionus bakeri var.rhenanus Brachionus bakeri var. tuberculus Total Brachionus bakeri II *|'l d g^ ocqj H Jan 11.. " 21 . " 25. . Feb 3 8 " IS. . " 22 Mar 1 . 8. . " IS " 22 40 " 29 Apr 5 . . " 12 " 19 . " 26. . May 3 " 10 . " 17. . " 24 " 31 40 40 " 14 " 21 " 28. . 800 100 900 Tulv 5 400 400 800 400 ' 12! 60 1,200 1,660 60 " 19. ... 920 1,200 40 2,160 2,520 " 26 Aug 2 . . 9. ... 400 40 840 800 " 16 800 1,600 2,400 5,600 " 23 " 30. . 800 800 800 800 5,600 2,400 Sept. 6 800 800 1,600 4,000 7,600 5,600 " 13 500 2,000 4,500 4,000 " 20. . 500 500 500 " 27.. 1,600 3,200 Oct 4 . 40 " 11 .. " 18. . 40 40 " 25 500 500 Nov 1 . . . 60 60 8 " IS. " 22. . " 29 Dec. 6. . " 13 " 20. . " 27. .. Average 90 49 41 118 155 592 420 332 TABLE I continued. ORGANISMS PER CUBIC METER IN PLANKTON OF ILLINOIS RIVER IN 1898. (An asterisk at head of column indicates that all entries in it are based on filter-paper collections.) 1898 Brachionus budapesti- nensis Brachionus militaris Brachionus mollis Brachionus pala Brachionus pala var. amphiceros Brachionus pala var. dorcas Brachionus pala var. dorcas forma spinosus Total Brachionus pala Jan. 1 1 ... " 21. . o o o 20 100 20 o 20 o o 60 100 " 25. . o o o o 387 387 Feb. 3 . . o o o o 200 o 200 8. . o o o o o o o o " IS. . . o o o o o o o " 22 o o o o o o o Mar 1 . . o o 80 o o o 80 8 o o o 80 o 80 " 15 " 22. . o o o 160 o o 360 1 720 o 520 1 720 " 29 o o o 200 o 140 o 340 Apr. 5 . . " 12. .. o o o 200 20 120 100 160 o 120 480 " 19 o o 2 800 1 200 800 o 4 goo " 26. . o o o 57 920 97 600 4 000 o 159 520 May 3 . . o o 32 000 419 200 o o 451 200 " 10 . o o o 19 200 57 600 o o 76 800 " 17. . o o o 5 600 69 600 800 1 700 77 700 " 24. . o 200 o 80 200 o o 280 " 31 June 7 . . o o o 200 o o o 200 " 14 o o o o o o o 1 000 " 21 " 28. 4 000 o o 800 o 200 o o o o July 5 . . 3 600 40 o 40 o o o 40 " 12 10 000 120 o o o o o " 19. . 85 600 80 o 800 5 600 o o 6 400 " 26. . 3 200 o o o 120 o o 120 Aug. 2 . . 9 600 40 o 6 400 o o 6 400 9.. 11 200 o 200 1 200 2 000 o o 3 200 " 16. . 20 000 o 800 800 37 600 o o 38 400 " 23 8 000 800 o o 35 200 o o 35 200 " 30 7 200 3 200 800 800 32 000 o o 32 800 Sept. 6. . 7 200 1 600 o o 19 200 o o 19 200 " 13 1 500 o o 500 4 000 o o 4 500 " 20 2 000 500 500 9 000 o o 9 500 " 27. . 44 800 1 600 4 800 4 800 78 400 o o 83 200 Oct. 4 . . 80 o 40 1 000 o o 1 040 " 11 500 o o o 500 " 18 " 25.. 1,000 o o 500 3 500 80 5 000 o o 580 8 500 Nov. 1 . . 8 " 15.. o o o 1 000 180 100 o o 180 1 100 " 22... o o o 400 400 o o 800 ' 29 Dec. 6. . o o o 1,000 320 500 1 160 20 o 1,500 1 500 " 13 " 20. ... " 27.. o o o 2,400 1,200 400 3,200 606 200 40 o 5,600 1,800 640 Average 4 211 147 137 2 693 17 071 170 33 19 969 333 TABLE I continued. ORGANISMS PER CUBIC METER IN PLANKTON OF ILLINOIS RIVER IN 1898. (An asterisk at head of column indicates that all entries in it are based on filter-paper collections.) 1898 Total eggs Brachionus pala Brachionus urceolaris Brachionus urceolaris var. bursarius Brachionus urceolaris var. rubens Total Brachionus urceolaris Total eggs Brachionus urceolaris Brachionus variabilis Brachionus free winter eggs Jan. 11.. " 21. . 100 100 40 40 100 " 25.. 1,161 1,548 Feb 3 100 8. . " 15. .. 800 2,400 " 22 632 3,791 Mar 1 . . 480 80 80 480 8. ... 160 840 " 15 1,000 160 160 400 " 22 2,920 2,000 2,000 400 440 " 29. 420 1,800 1,800 1,200 20 Apr. 5 . . 20 700 700 400 120 " 12 240 140 140 60 200 " 19 5,200 400 400 " 26. . 324,280 6,400 6,400 May 3 661,200 41,600 " 10. 118,400 9,600 " 17. . 101,700 800 " 24.. . 1,040 200 80 " 31 400 June 7 . . " 14. ... 400 400 " 21 100 800 800 " 28. . 100 100 100 100 July 5 . . 40 40 " 12 . 400 1,200 " 19. . 400 40 200 240 400 " 26. .. 800 400 400 1,200 Aug 2 1,200 o" 9. . 8,400 800 800 " 16. .. 5,600 800 800 800 " 23 " 30 14,400 12,000 2,400 2,400 800 4,800 Sept. 6. . . 22,400 5,600 5,600 800 " 13 " 20 " 27. . 3,000 5,500 32,000 500 500 500 . 1,600 Oct 4 500 500 " 11 500 " 18. . 500 . 540 " 25 4,500 500 Nov 1 . . 2,000 8. . 120 2,000 " 15 " 22. . 1,200 2,400 1,000 2,000 " 29. . 1,500 500 Dec 6 . . 860 100 100 " 13. .. 10,600 600 600 " 20 1,800 40 40 " 27. . 100 240 240 80 1,621 25 974 18 206 244 468 56 4 1,685 334 TABLE I continued. ORGANISMS PER CUBIC METER IN PLANKTON OF ILLINOIS RIVER IN 1898. (An asterisk at head of column indicates that all entries in it are based on filter-paper collections.) 1898 Mastigocerca carinata Metopidia solidus Monostyla bulla Monostyla lunaris Notholca striata var. acuminata Polyarthra platyptera Polyartha platyptera Male eggs Free Carried Jan 11 400 19,200 11,200 2,000 1,600 7,200 4,000 15,200 4,000 5,600 5,600 800 2,500 1,500 4,800 1,000 60 400 400 100 400 800 800 800 200 100 100 400 200 200 o 800 800 100 800 400 500 24 240 80 800 1,200 6,400 10,800 200 300 40 1,000 1,200 11,997 3,200 2,000 1,600 6,318 3,200 5,200 22,200 37,600 40,400 42,800 26,700 148,200 696,000 582,400 137,600 195,200 52,200 52,400 304,000 432,800 241,600 56,800 6,400 21,600 89,200 86,400 288,000 55,200 84,800 96,000 51,200 4,000 31,000 72,500 238,400 24,500 47,500 27,000 37,500 500 1,000 2,000 6,000 1,000 6,020 42,100 63,400 19,200 1,300 900 8,800 150,400 4,800 12,000 200 1,600 5,000 2,000 100 1,600 2,600 2,800 1,900 1,600 53,800 19,200 2,400 1,600 500 1,500 2,000 160 100 200 " 21 " 25. . Feb 3 8. " 15.. " 22 Mar 1 . . 8 " 15 " 22. . " 29. . Apr 5 . " 12. . 19 " 26 May 3 . . " 10 " 17 " 24. . " 31 June 7 . . " 14. . " 21 . " 28. . July 5 . . " 12 " 19. . " 26. . Auf? 2 " 9. . " 16. . " 23 " 30. . Sept. 6 . . " 13 " 20. . " 27. . Oct. 4. ... " 11. . " 18. . " 25 Nov. 1 . . 8 " 15 " 22. . " 29 Dec 6. . " 13. . " 20 " 27. . Average . 1,674 67 50 .57 388 86,674 3,598 ; 1,768 335 TABLE I continued. ORGANISMS PER CUBIC METER IN PLANKTON OF ILLINOIS RIVER IN 1898. (An asterisk at head of column indicates that all entries in it are based on filter^paper collections.) 1898 Polyarthra platyptera Winter eggs 2 2 It " If 8 *" Pterodina patina Rattulus tigris Schizocerca diversicornis Synchceta pectinata Synchceta stylata Total free winter eggs Synchceta Free Carried Jan 11.. 100 800 200 200 100 1,600 22,400 51,200 11,200 2,400 400 400 800 800 800 400 400 800 800 800 1,000 1,000 1,600 1,000 500 1,000 1,000 100 100 1,600 2,200 1 ,000 8,127 1,700 2,800 3,200 6,318 3,200 4,000 17,800 26,400 11,400 10,400 8,700 104,200 502,400 316,800 72,000 120,000 28,800 10,600 96,000 119,200 154,400 16,000 7,200 13,600 24,000 53,600 295,200 84,800 108,000 63,200 47,200 8,800 20,000 103,000 86,400 15,000 5,500 14,000 17,000 4,500 2,000 2,000 6,000 3,000 9,160 29,300 52,000 1 1 , 000 200 40 1,600 100 100 400 40 400 100 300 200 3,200 200 40 400 800 400 800 800 1,600 1,000 800 800 800 . 72 632 400 1,600 800 200 600 3,200 800 112,000 800 400 20,800 400 12,000 4,800 1,600 3,200 800 1,000 4,500 30,400 500 500 500 20 2,500 400 2, \20 300 4,257 1,000 1,200 800 6,400 4,800 15,200 58,000 47,000 2 1 , 000 11,500 368,000 954,400 1,139,000 233,600 206,400 60,480 61,600 48,000 19,200 795,200 22,400 22,800 9,600 64,800 8,000 170,400 52,000 18,400 24,800 1,600 14,000 27,000 265,600 5,000 27,000 77,000 824,500 110,500 97,000 110,000 38,000 39,000 42 , 500 55,720 59,200 17,640 100 1,200 160 6,400 9,600 6,400 800 800 3,200 400 800 800 60 . 500 500 100 60 " 21 " 25 . Feb 3 . . 8 " 15. . " 22. . Mar 1 8. . " 15. . " 22. . " 29 Apr 5 . . " 12. .. " 19 26""" May 3 . . . " 10 " 17. . " 24. . " 31 .. June 7 . . " 14. . " 21 ... " 28. . July 5 . . " 12. ... " 19 " 26. . Aug. 2 .... 9 " 16. . " 23. . " 30. . Sept 6 . . " 13 " 20... " 27 Oct. 4. . " 11 ... " 18. ... " 25.. Nov. 1 . . 8. . " 15. . " 22. .. " 29 . Dec. 6. . " 13 " 20. . " 27. .. Average . . 1,994 34 52,560 37 207 46 3,950 120,391 611- 336 TABLE I continued. ORGANISMS PER CUBIC METER IN PLANKTON OF ILLINOIS RIVER IN 1898. (An asterisk at head of column indicates that all entries in it are based on filter-paper collections.) 1898 Total eggs Synchceta Triarthra longiseta Triarthra longiseta eggs Pedalion mirum Total Entomostraca Total Ostracoda Total Cladocera Jan 11 500 700 100 " 21.. 200 1 380 440 " 25. .. 6,579 4 788 462 Feb 3 . . 300 216 24 8. . 400 320 " IS 5,600 1 200 80 " 22 Mar 1 . . 1,200 80 3,285 804 160 8 800 3 080 40 80 " IS. . 3 960 12 880 40 560 " 22. .. 4 000 40 40 19 440 320 800 " 29 3,200 100 22 180 160 360 Apr 5 . . 1,100 300 200 o 34 560 200 360 " 12 1,000 200 100 o 28 060 320 320 " 19 " 26. . 84,000 38 400 400 3 200 o 34,200 56 800 200 800 400 1 920 May 3 . . 278,400 9,600 o 204 800 2 800 " 10 91 600 38 400 o o 235 400 400 5 600 " 17. . 56 800 17 600 3 200 o 182 300 1 600 8 500 " 24 9,400 600 200 o 167 080 400 24 080 " 31 12 000 1 000 o 162 800 440 51 480 June 7 . . 11 200 200 o 438 800 1 600 136 000 " 14 13,600 o 211 400 400 29 200 " 21 257 600 800 100 83 100 200 2 300 " 28. . 51 200 800 500 45 600 400 10 100 July S " 12 " 19. . 47,200 16,800 34 800 400 1,600 4 000 400 1 , 600 1,200 9 200 4,920 1,620 14 040 440 60 o 640 360 1 240 " 26 4 800 28 000 1 600 99 600 23 000 o 9 960 Aug. 2 . . 178,400 18 400 1 ,600 30 400 22 160 40 10 520 " 9 20 000 4 400 1 200 4 000 4 120 o 1 080 " 16.. 10 460 3 200 22 400 4*500 800 1 320 " 23 35,200 4 000 17 600 17 340 180 1 820 ." 30. 7 200 6 400 o 14*400 27 080 o 4 040 Sept 6 .... 9 080 720 " 13....'.. 9,500 1 000 5 000 24 720 SOO 3 420 " 20. . 17 500 500 o 5 SOO 21 880 o 1 560 " 27.. 38 500 14 400 3 200 19 200 99*300 o 1 100 Oct. 4 . . 1 500 500 2 000 33 880 1 320 " 11 .. 2 000 o 2 000 34 060 o 2 000 " 18. . 10 500 1 500 500 500 25 640 2 120 " 25 78 000 500 o o 26*020 1 020 Nov. 1 . . 29 000 o o 60 8 600 o 120 8 41 060 IS 080 " IS 60,000 13 100 100 " 22 66,000 13 920 320 2 800 " 29.. 500 500 500 o 6 180 o 80 Dec. 6. ... o 9 740 260 " 13. . 800 o o o 21 740 o 240 " 20 2 600 o o 2 440 400 " 27 400 . 40 6 800 280 Average 31 620 3 147 255 4 524 47 042 191 6 241 337 TABLE I ^continued. ORGANISMS PER CUBIC METER IN PLANKTON OF ILLINOIS RIVER IN 1898. (An asterisk at head of column indicates that all entries in it are based on filter-paper collections.) 1898 ! Bosmina longirostris Ceriodaphnia scitula Chydorus spharicus Daphnia cucullata S 2 II tl a- Diaphanosoma brachyurum Moina micrura Jan 11 100 " 21 . 240 200 " 25 154 308 O, Feb 3 24 8. . " 15. . " 22 Mar 1 . 80 80 8. . 40 40 " 15 120 440 " 22 280 480 " 29 . 20 20 240 20 Apr 5 . . 20 100 40 200 " 12 20 60 20 200 20 " 19 200 100 o " 26 800 320 800 May 3 . . 2,800 " 10 3,600 400 600 600 " 17 200 3,500 400 3,300 300 " 24 " 31. . 480 40 5,920 33,920 2,960 8,720 7,880 5,040 440 1,000 160 720 40 200 62,800 55,800 600 3,400 11,600 " 14 6,000 10,600 200 2,400 9 200 " 21 . 100 1,500 400 100 200 " 28. . 200 700 100 Tulv 5 200 40 400 ' 12:: 180 60 120 " 19. . 160 40 1 040 " 26 180 120 o o o 8 580 1 080 Aug 2 . . 40 6 960 3 520 9. . 360 360 " 16 60 1 260 " 23 o o 1 020 900 " 30 40 2 520 1 440 Sept 6 40 o o o 240 440 " 13. ... 60 1,800 1 560 " 20 60 60 o 960 480 " 27. . 100 o 400 500 Oct 4 40 400 o 880 120 " 11 . 920 600 o 400 40 " 18. . 1 360 80 80 o 560 " 25.. 840 120 60 o Nov 1 60 o 60 o 8. . o o " 15.. 100 o o " 22 o o o o o " 29. o 80 o o Dec. 6. . 40 20 200 o " 13 ' 20. . 140 120 40 o 220 160 o o " 27. . o 280 o o o Average 36 2,441 1,539 422 181 417 479 261 338 TABLE I continued. ORGANISMS PER CUBIC METER IN PLANKTON OF ILLINOIS RIVER IN 1898 (An asterisk at head of column indicates that all entries in it are based on filter-paper collections.) 1898 Total Copepoda Canthocamptus spp. Cyclops albidus B ft. =0 O S ft ^ -2 ^t3 * Cyclops prasinus Cyclops virdis var. brevispinosus $ >2 " * IH "G-fe s $** Jan 11.. 600 160 o o " 21 940 40 160 o 40 " 25 4 326 77 308 o 308 Feb 3 ... 192 48 o o 8 320 160 o o " 15. . 1 120 80 o 80 o " 22. . 3 285 o o o Mar 1 644 o o o 8. . 2 960 40 120 o o " 15 12 280 40 400 o 80 120 " 22 " 29. . 18,320 21 660 200 100 20 80 60 o 20 80 o Apr. 5 . . 34 000 200 20 40 o " 12 27 420 1 120 o o 40 " 19. . 33 600 200 200 o 500 " 26 54 080 1 600 2 880 o o 4 160 May 3. 202 000 400 8 000 400 o 1 200 " 10. . 229 400 600 5 200 o o 600 2 200 " 17 172 200 800 200 600 o o 3 300 " 24 " 31. . 142,600 110 880 80 920 200 320 80 o 1,080 400 1,640 640 301 200 400 2 600 4 000 " 14 181,800 200 200 800 4 400 " 21 " 28. . 80,600 35 100 o o o o o 400 200 July S 3 840 o o o o o 120 " 12.. 1 200 o o o o 180 " 19. . 12 800 o o 40 o 40 240 " 26 13 040 o o 120 o 120 300 AUK 2 . . 11 600 o o o 40 AUK. *.- 3 040 o o 80 o 160 " 16. . 2 380 o o o 180 " 23 15,340 o 120 660 " 30. . 23 040 o o o 440 o 480 Sept 6 ... 8 360 o 40 80 " 13 20,800 120 240 " 20 20,320 120 60 360 " 27. . 98 200 100 700 o 300 o 200 700 Oct. 4. . 32,560 120 320 200 400 " 11 32,060 200 80 120 " 18. . 23 520 o 280 o o 40 40 " 25.. 25 000 o 60 60 120 240 Nov 1 . . 8 480 o 120 o o 60 8. . 15 080 o o 120 " 15 13 000 200 200 " 22 10 880 480 80 160 " 29. . 6 100 80 o 4 o o Dec. 6 ... 9 480 20 20 " 13. . 21 500 o 40 40 " 20. . . 2 040 40 o 160 40 " 27 6 520 120 40 Average 40,609 78 113 373 49 2 124 539 339 TABLE I continued. ORGANISMS PER CUBIC METER IN PLANKTON OF ILLINOIS RIVER IN 1908. (An asterisk at head of column indicates that all entries in it are based on filter-paper collections.) 1898 &| II S* Copepodan nauplii ll ! ?* Diaptomus siciloides a a a* Total Nematodes Total Oligochsetes Chironomus larva Jan 11.. 120 240 40 " 21 " 25 . 200 770 500 2,709 o 4,700 774 77 o Feb 3 72 72 24 100 " 8 80 80 o 80 o o " 15 . 160 800 400 o " 22. . 126 3,159 126 o Mar 1 4 640 o 4 o 8. . 800 2 000 o 400 40 " IS. . 220 11 ,200 40 40 o 600 o 120 " 22 1 ,120 16,800 40 40 40 40 " 29 760 20 700 o 40 20 80 Apr. 5 " 12 1,420 1 100 32,300 25 100 20 60 o 40 40 40 80 160 " 19 " 26. . 5,500 24,320 27,200 20 800 o o 100 320 100 300 300 May 3 19 600 182 400 o o o o 400 " 10 53 200 169 600 o o 200 400 " 17. . 27,900 166 400 o 900 4 800 200 700 " 24. . 12,160 126 400 o 840 40 80 280 " 31 5 680 103 800 o 160 40 80 440 June 7 . . 23,800 270 400 o 200 200 " 14 " 21 1 1 , 400 14 600 164,800 65 600 o o o 200 o o 400 " 28. . 4 500 30 400 o o o 300 100 400 July S " 12 1,320 780 2,400 180 o 60 o 40 180 480 300 " 19. . 1 ,680 10 800 o 60 o 80 480 " 26. . 840 11 600 o o o 120 Aug 2 . 360 11 200 o 80 o 40 80 9. . 400 2 400 o o " 16. . 600 1 600 o 60 60 60 " 23 3 360 11 200 o o 60 60 180 o " 30 . 5 520 13 600 o o 120 Sept 6 . . . 240 8,000 o o 40 160 80 " 13 4 320 16 000 60 o o 120 180 o " 20. . 19 000 o 60 120 60 60 " 27 Oct 4 2 600 96,000 29 000 o 200 o o 300 400 80 200 40 " 11. . 3 920 27 500 o 80 o 40 40 80 " 18. . 5 580 17 500 40 40 o 40 40 120 " 25 2 400 22 000 120 o o 120 60 o Nov. 1 ... 8. . 300 960 8,000 14,000 o 120 180 120 " 15 400 12 000 100 o 100 100 " 22. . 160 10 000 o 2 000 320 " 29. . 4 6 000 12 o 120 Dec 6. . 440 9 000 o o 500 " 13. ... 1 ,060 30 300 60 o " 20 1 800 20 o o o o " 27. . 920 5 400 o 20 40 4 780 36 707 11 10 39 318 76 124 (23) 340 TABLE I concluded. ORGANISMS PER CUBIC METER IN PLANKTON OF ILLINOIS RIVER IN 1898. (An asterisk at head of column indicates that all entries in it are based on filter-paper collections.) J898 li o^J H Plumatella statoblasts Total Glochidia Total Miscellaneous Total Phytoplank- ton Total Zooplankton ! ^ c cS rt oE h Jan 11 6 120 200 300 200 40 120 40 60 160 60 180 40 400 80 800 40 40 1,640 80 900 220 400 320 100 1,200 100 60 80 60 80 60 40 120 80 500 20 40 154 24 80 160 40 40 100 20 200 300 120 120 400 60 100 240 180 60 80 80 1,220 7,720 7,738 648 2,160 6,000 3,285 1,124 3,360 3,280 1,600 2,020 1,860 980 3,100 20,160 10,800 13,600 213,900 5,360 4,720 24,000 4,400 3,300 10,900 5,800 5,320 2,680 1,440 3,440 840 6,580 8,420 5,320 - 4,320 2,420 5,920 12,100 2,740 1,240 1,140 3,860 4,360 15,300 7,500 25,680 1,900 680 1,560 320 340 544,201,080 202,136,180 180,295,247 619,030,830 297,341,760 653,122,000 115,514,812 21,790,800 96,590,840 118,382,400 127,930,360 53,463,040 42,470,860 90,934,320 927,956,220 3,872,537,280 3,200,166,960 4,467,165,760 2,148,960,400 190,671,160 252,704,250 259,129,000 1,149,333,480 641,056,900 612,686,240 257,668,840 223,936,060 1,578,635,720 281,604,120 369,169,240 3,084,000,880 583,940,376 544,041,260 797,312,600 488,146,040 676,773,060 330,837,120 511,099,300 264,225,400 126,398,510 182,891,160 218,768,810 209,418,675 277,953,180 407,573,600 466,411,780 364,032,900 529,250,270 1,715,442,415 848,243,820 387,414,000 1,042,720 288,340 195,484 1,381,960 651,200 1,091,920 715,697 1,809,688 542,680 453,100 484,760 444,900 363,980 1,432,400 2,134,800 16,092,840 898,919,800 42,826,200 31,091,900 4,969,580 2,772,200 9,695,000 10,959,400 15,159,600 7,796,700 495,337,320 4,135,160 1,717,240 907,240 5,833,440 69,874,760 1,240,920 1,519,140 597,880 60,463,480 1,712,720 32,466,660 3,201,200 2,035.720 1,495,880 1,967,020 2,453,060 1,189,380 1,281,220 '732,300 1,109,040 799,980 916,780 1,757,440 682,440 548,700 545,243,800 202,424,520 180,490,731 620,412,790 279,992,960 654,213,920 116,230,509 23,600,488 97,133,520 118,835,500 128,451,120 53,907,940 42,834,834 102,366,720 930,091,020 3,848,630,120 4,099,086,760 4,509,991,960 2.180,052,300 195,640,740 255,476,450 268,824,000 1,160,292,880 656,216,500 620,482,940 753,000,160 228,071,220 1,580,352,960 282,511,360 375,002,680 3,153,875,640 585,181,296 545,560,400 797,910,480 548,609,520 678,485,780 363,303,780 514,300,500 266,261,120 127,894,390 184,858,180 221,221,870 210,608,055 279,234,400 408,305,900 467,520,820 364,832,880 530,167,050 1,717,199,855 848,926,260 387,926,700 " 21 " 25 Feb 3 . . 8 " 15 . " 22. . Mar 1 8. . " 15 " 22 " 29 . Apr 5 " 12 19. . " 26. ... May 3 . . " 10. . . " 17 " 24 " 31. . " 14 . " 21 .. " 28 July- 5. . " 12.. " 19 " 26 Aug 2 . . . . 9 " 16 " 23. . " 30. . Sept. 6 " 13. . " 20 " 27 Oct 4. . " 11 " 18 " 25. . Nov 1 8. . " 15. . " 22 " 29.. Dec. 6 " 13 " 20. . " 27 Average 37 135 52 9,393 723,283,871 34,226,468 756,548,801 BIBLIOGRAPHY. Amberg, C. '00. Beitrage zur Biologic des Katzensees. Inaug. Diss. 78 pp., 5 Taf. Zurich. Also in Vierteljahrsschr. d. naturf. Ges. Zurich, Jahrg. 45, pp. 59-136. Asper, G., und Heuscher, J. '85. Zur Naturgeschichte der Alpenseen. Jahresber. St. Gall. Naturwiss. Ges., 1885-86. 43 pp. Apstein, C. '96. Das Susswasserplankton. Methode und Resultate der quanti- tativen Untersuchung. Mit 113 Abbildungen. 200 pp., 5 Tab. Kiel und Leipzig. Bertram, [A.] '92 . Beitrage zur Kenntniss der Sarcosporidien nebst einem Anhange uber parasitische Schlauche in der Leibeshohle von Rotatorien. Zopl. Jahrb., Abth. f. Anat. u. Ont., Bd. V., pp. 581-604. Birge, E. A. '91. List of Crustacea Cladocera from Madison, Wisconsin. Trans. Wis. Acad., Vol. VIII., pp. 379-398, PL 13. '94. A Report on a Collection of Cladocera, mostly from Lake St. Clair, Michigan. Bull. Mich. Fish Comm., No. 4, pp. 45-49. 1 table. '95. Plankton Studies on Lake Mendota. I. The Vertical Dis- tribution of the Pelagic Crustacea during July, 1894. Trans. Wis. Acad. Sci., Arts, and Letters, Vol. X., pp. 421-484, PL VII.-X. '97. Plankton Studies on Lake Mendota. II. The Crustacea of the Plankton from July, 1894, to December, 1896. Trans. Wis. Acad. Sci., Arts, and Letters, Vol. XL, pp. 274-448, PL XV.-XLII. Borge, O. '00. Schwedisches Susswasserplankton. Botaniska Notiser, 1900, pp. 1-26, Taf. 1. 341 342 Brewer, A. D. '98. A Study of the Copepoda found in the Vicinity of Lincoln, Nebraska. Journ. Cin. Soc. Nat. Hist., Vol. XIX., pp. 119-138, PI. VII. Brunnthaler, Josef. '00. Plankton-Studien. I. Das Phytoplankton des Donaustromes bei Wien. Verhandl. d. k. k. zool.-botan. Ges. Wien, Jahrg. 1900, Bd. L., pp. 308-311. '01. Die Coloniebildenden Dinobryon-Arten. (Subgenus Eudino- bryon Lauterborn.) Verhandl. d. k. k. zool.-botan. Ges. Wien. Jahrg. 1901, Bd. LI., pp. 293-306. Mit 5 Fig. im Text. Biitschli, 0. '80- '89. Protozoa. Bronn's Klassen und Ordnungen des Thier- reich, Bd. I., Abth. I.-III. 2035 pp., 79 Taf. Leipzig und Heidelberg. Burckhardt, G. '00. Faunistische und systematische Studien iiber das Zooplankton der grosseren Seen der Schweiz und ihrer Grenzgebiete. Rev. Suisse de Zoologie, T. VII., pp. 353-713, PI. 19-22. 'OOa. Quantitative Studien iiber das Zooplankton des Vierwald- stattersees. Mitteil. d. naturf. Ges. Luzern, 1900, Heft 3, pp. 129-439; Separatabdruck, 311 pp. Burrill, T. J. '02. Report of the University of Illinois. Report of Streams Ex- amination, Chemic and Bacteriologic, of the water between Lake Michigan at Chicago and the Mississippi River at St. Louis, * * * published by authority of the Trustees of the Sanitary District of Chicago, pp. 97-119, PI. XXI.-XXX. (in text), Tab. 1-80. '04. River Pollution and Purification: A Study of the Effect of Chicago Sewage upon the Water Supply of St. Louis. Trans. Am. Micr. Soc., Vol. XXV., pp. 105-120,' Tab. I., PL XV.-XVII. Calkins, G. N. '02. Studies on the Life-History of Protozoa. I. The Life-Cycle of Paramcecium caudatum. Arch. f. Entwicklungsmechanik d. Org., Bd. XV., pp. 139-186. 4 fig. in text. 343 Caullery, M., et Mesnil, F. '04. Sur les affinites des Actinomyxidies. Compt. Rend. Soc. Biol. Paris, T. 56, pp. 410-412. Cohn, Ludwig. '03. Untersuchungen iiber des Plankton des Lowentin und einiger anderer Seen Masurens. Zeitschr. f. Fischerei, Bd. X., pp. 201 331, Taf. 1-12. Daday, E. v. '85. Neue Thierarten aus der Siisswasserfauna von Budapest. Termeszetr. Fiizetek, Vol. IX., pp. 208-215, Taf. XL Forbes, S. A. '78. The Food of Illinois Fishes. Bull. 111. State Lab. Nat. Hist., Vol. I., No. 2, pp. 71-89. '80. On the Food of Young Fishes. Bull. 111. State Lab. Nat. Hist., Vol. L, No. 3, pp. 66-79; 2d ed., 1903, pp. 71-85. '82. On some Entomostraca of Lake Michigan and Adjacent Waters. Am. Nat., Vol. XVI., pp. 537-542, 640-650, PI. VIII. , IX. '83 . The first Food of the Common Whiteftsh (Coregonus clupeiformis Mitch.). Bull. 111. State Lab. Nat. Hist., Vol. L, No. 6, pp. 95-109. '90. On some Lake Superior Entomostraca. Rep. U. S. Comm. Fish and Fisheries, 1887, pp. 701-718, PI. I.- IV. '93. A Preliminary Report on the Aquatic Invertebrate Fauna of the Yellowstone National Park, Wyoming, and the Flathead Region of Montana. Bull. U. S. Fish Comm., Vol. XL, pp. 207-258, PI. XXXVII.-XLII. Forbes, E. B. '97. A Contribution to a Knowledge of North American Fresh- water Cyclopidae. Bull. 111. State Lab. Nat. Hist., Vol. V., pp. 27-82, PI. VIII.-XX. Fordyce, C. '00. The Cladocera of Nebraska. Trans. Am. Micr. Soc., Vol. XXII. , pp. 119-174. 4 pi. Also, in 1901, in Studies from the Zool. Lab. Univ. Nebr., No. 42, PI. XXII.-XXV. Fri6, A., und Vavra, V. '01. Untersuchungen iiber die Fauna der Gewasser Bohmens. V. Untersuchung des Elbeflusses und seiner Altwasser. Arch, der Naturwiss. Landesdurchforsch. v. Bohmen, Bd. XL, No. 3, pp. 1-156. Mit 119 Abb. im Text. 344 Friedlander, B. '01. Herrn Alfred Goldsborough Mayer's Entdeckung eines " Atlantischen Palolo" und deren Bedeutung fur die Frage nach unbekannten Kosmischen Einfliissen auf biologische Vorgange. Zugleich eine Beleuchtung der darwinistischen Betrachtungsweise. Biol. Centralbl., Bd. XXL, pp. 312-317, 352-366. Fuhrmann, O. '00. Beitrag zur Biologic des Neuenburger Sees. Biol. Centralbl., Bd. XX., pp. 85-96, 120-128. 1 Fig. Garbini, A. '98. Due nuovi Rizopodi limnetici (Difflugia cyclotellina Het- erophrys Pavesii). Zool. Anz., Bd. XXL, pp. 667-670. 2 Fig. Haeckel, E. '90. Plankton Studien. Jenaische Zeitschr.,Bd. XXV., pp. 232-336. Hart, C. A. '95. On the Entomology of the Illinois River and Adjacent Waters. Bull. 111. State Lab. Nat. Hist., Vol. IV., pp. 149-273, PI. I.-XV. Hempel, A. '96. Descriptions of New Species of Rotifera and Protozoa from the Illinois River and Adjacent Waters. Bull. 111. State Lab. Nat. Hist., Vol. IV., pp. 310-317, PI. XXII.-XXVI. '99. A List of the Protozoa and Rotifera found in the Illinois River and Adjacent Lakes at Havana, 111. Bull. 111. State Lab. Nat. Hist., Vol. V., pp. 301-388. 5 fig. Hensen, V. '87. Ueber die Bestimmung des Plankton's oder des im Meere treibenden Materials an Pflanzen und Thieren. Fiinft. Ber. d. Komm. zu wiss. Untersuch. d. deutsch. Meere in Kiel, pp. 1-107 + I.-XVIII. 6 Tab. Herrick, C. L. '84. A Final Report on the Crustacea of Minnesota included in the Orders Cladocera and Copepoda. Twelfth Ann. Rep. Geol. and Nat. Hist. Surv. Minn., pp. 1-191, PI A-V. 345 Herrick, C. L., and Turner, C. H. '95. Synopsis pf the Entomostraca of Minnesota. Geol. and Nat. Hist. Surv. Minn., Zool. Series, II. 337 pp. and 81 pi. Heuscher, J. '85. (See Asper, G., und Heuscher, J.) Huitfeldt-Kaas, H. '98. Plankton in norwegischen Binnenseen. Biol. Centralbl., Bd. XVIII., pp. 625-636. 2 Fig. Jennings, H. S. '94. The Rotatoria of the Great Lakes and of some of the Inland Lakes of Michigan. Bull. Mich. Fish Comm., No. 3, pp. 1-34, PI. 1 and 2. '96. Report on the Rotatoria. Bull. Mich. Fish Comm., No. 6, pp. 85-93, Fig. 1 and 2. '00. Rotatoria of the United States, with especial Reference to those of the Great Lakes. Bull. U. S. Fish Comm., Vol. XIX., pp. 67-104, PI. 14-22. 'OOa. A Report of Work on the Protozoa of Lake Erie with especial Reference to the Laws of their Movements. Bull. U. S. Fish Comm., Vol. XIX., pp. 105-114. Jordan, E. O. '00. Some Observations on the Bacterial Self-Purification of Streams. Journ. Exper. Med., Vol. V., pp. 271-314, PI. XX. Kellicott, D. S. '88. Partial List of Rotifera of Shiawassee River at Corunna, Michigan. Proc. Am. Soc. Microscopists, Vol. X., pp. 84-96. 3 fig. '97. The Rotifera of Sandusky Bay (Second Paper). Proc. Am. Micr. Soc., Vol. XIX., pp. 43-54. Kelly, H. M. '99. A Statistical Study of the Parasites of the Unionidae. Bull. 111. State Lab. Nat. Hist., Vol. V., pp. 399-418. Kertesz, K. '94. Budapest es Kornyekenek Rotatoria-faunaja [The Rotifer fauna of Budapest and Neighborhood]. Inaug. Diss. 55 pp., 1 pi. Budapest, 1894. 346 Knauthe, K. '98. Der Kreislauf der Gase in unseren Gewassern. Biol. Centralbl., Bd. XVIII. , pp. 785-805. Kofoid, C. A. '95. A Report on the Protozoa observed in Lake Michigan and the Inland Lakes in the Neighborhood of Charlevoix during the Summer of 1894. Bull. Mich. Fish Comm., No. 6, pp. 76-84. '97. Plankton Studies. I. Methods and Apparatus in Use in Plank- ton Investigations at the Biological Experiment Station of the University of Illinois. Bull. 111. State Lab. Nat. Hist., Vol. V., pp. 1-25, PL I.-VII. '03. Plankton Studies. IV. The Plankton of the Illinois River, 1894-1899, with Introductory Notes on the Hydrography of the Illinois River and its Basin. Part I. Quantitative Investigations and General Results. Bull. III. State Lab. Nat. Hist., Vol. VI., pp. vi-xviii, 95-629. 5 text fig. and 50 pi. Kramer, Aug. '97. Ueber den Bau der Korallenriffe und die Planktonvertheilung an den Samoanischer Kiisten. Mit Anhang: Ueber den Palolo- wurm, von Dr. A. Collin. 174 pp., 1 Karte. Kiel und Leipzig. Lauterborn, R. '93. Ueber Periodicitat in Auftreten und in der Fortpflanzung einiger pelagischer Organismen des Rheins und seiner Altwasser. Verhandl. d. Naturhist.-Med. Vereins Heidelberg, N. F., Bd. V., pp. 103-124. '93a. Beitrage zur Rotatorienfauna des Rheins und seiner Alt- wasser. Zool. Jahrb., Abth. f. Syst., Geogr., u. Biol. d. Thiere, Bd. VII., pp. 254-273, Taf. 11. '94. Ueber die Winterfauna einiger Gewasser der Oberrheinebene. Mit Beschreibungen neuer Protozoen. Biol. Centralbl., Bd. XIV., pp. 390-398. '98. Vorlaufige Mittheilung iiber den Variationskreis von Anurcea cochlearis Gosse. Zool. Anz., Bd. XXL, pp. 597-604. '98a. Ueber die zyklische Fortpflanzung limnetischer Rotatorien. Biol. Centralbl., Bd. XVIII., pp. 173-183. 347 Leger, L. '04. Considerations sur le genre Triactinomyxon et les Actinomyx- idies. Compt. Rend. Soc. Biol. Paris, T. 56, pp. 846-848. Leidy, Joseph. '79. Fresh-Water Rhizopods of North America. Rep. U. S. Geol. Surv. Terr., Vol. XII. 324 pp., 48 pi. '87. Asplanchna ebbesbornii. Proc. Acad. Nat. Sci, Phila., 1887, p. 157. Lemmermann, E. '00. Beitrage zur Kenntniss der Planktonalgen. XL Die Gattung Dinobryon Ehrenb. Ber. d. deutsch. Bot. Ges., Bd. XVIII., pp. 500-524, Taf. XVIII., XIX. Levander, K. M. '94. Kleine Beitrage zur Kenntniss des Thierlebens tmter dicker Eisdecke in einiger Gewassern Finlands. Meddel. Soc. pro Fauna et Flora Fennica, Bd. XX., pp. 66-71. '00. Zur Kenntniss der Fauna und Flora finnischer Binnenseen. Acta Soc. pro Fauna et Flora Fennica, Vol. XIX., No. 2, pp. 1-55. 1 Fig. Marsh, C. D wight. '93. On the Cyclopidae and Calanidae of Central Wisconsin. Trans. Wis. Acad. Sci., Arts, and Letters, Vol. IX., pp. 189-224, PI. III.-VI. '95. On the Cyclopidae and Calanidae of Lake St. Clair, Lake Michi- gan, and certain of the Inland Lakes of Michigan. Bull. Mich. Fish Comm., No. 5. 24 pp., 5 pi. '97. On the Limnetic Crustacea of Green Lake. Trans. Wis. Acad. Sci., Arts, and Letters, Vol. XL, pp. 179-224, PL V.-XIV. Marsson, M. '00. Zur Kenntnis der Planktonverhaltnisse einiger Gewasser der Umgebung von Berlin. Forschungsber. aus der Biol. Station zu ' Plon, Teil 8, pp. 86-119. Maskell, W. M. '87. On the Fresh-water Infusoria of the Wellington District. Trans. New Zealand Inst., Vol. XIX., pp. 49-61, PI. III.-V. 348 Maupas, E. '88. Recherches experimentales sur la multiplication des Infusoires cilies. Arch, de Zool. Exper. (2), T. VI., pp. 165-277. 4 pi. Mayer, A. G. '00. An Atlantic "Palolo," Staurocephalus gregarious. Bull. Mus. Comp. Zool., Vol; XXXVI. , pp. 1-14, PI. I.-III. Meissner, V. '02. Notiz iiber niedere Crustaceen des Wolga-Flusses bei Saratow. Zool. Anz., Bd. XXVI., pp. 51-55. '03. Contribution to the Fauna of the lower Crustacea of the River Volga. Year-book of the Volga Biol. Station, 1903. 43 pp. ,2 tab., PI. II.-IV. (Russian.) Minkiewitsch, R. '98. Some Observations on the Protozoa, made at the Biological Station at Bologoe. Rep. Imp. Soc. Nat. St. Petersburg, Vol. XXIX., pp. 239-246, 267-269. Mrazek, A. '00. [Review of Stole, '99.] Zool. Centralbl., Bd. VII., pp. 594-595. Hit 2 Fig. im Text. Needham, J. G., and Hart, C. A. '01. The Dragon-flies (Odonata) of Illinois, with Descriptions of the Immature Stages. Part I. Petaluridae, vEschnidae, and Gomphi- dae. Bull. 111. State Lab. Nat. Hist., Vol. VI., pp. 1-94, PI. I. Ostwald, Wolfgang. '03. Theoretische Planktonstudien. Zool. Jahrb., Abth. f. Syst., Geogr., u. Biol. d. Thiere, Bd. XVIII., pp. 1-62. Mit 4 Curven und 2 Abb. im Text. '03a. Ueber eine neue theoretische Betrachtungsweise in der Plank- tologie. Forschungsber. a. d. Biol. Station zu Plon, Teil X., pp. 1-49. Penard, E. '02. Faune Rhizopodique du Bassin du Leman. Avec nomb reuses figures dans le texte. 714pp. Geneve. Plate, Ludwig. '85. Beitrage zur Naturgeschichte der Rotatorien. Jenaische Zeitschr., Bd. XIX., pp. 1-120, Taf. I.-III. 349 Prowazek, S. '00. Synedra hyalina, eine apochlorotische Bacillarie. Oesterr. botan. Zeitschr., Bd. 50, pp. 69-73. 2 Fig. Przesmycki, A. M. '01. Ueber parasitische Protozoen aus dem Inneren der Rotatorien. Anz. Akad. Wiss. Krakow, 1901, pp. 358-408. 3 Taf. Ross, L. S. '97. A New Species of Daphnia, and Brief Notes on other Cladocera of Iowa. Proc. Iowa Acad. Sci., Vol. IV., pp. 162-166. 1 fig. Rossinski, D. M. '92. Materialien zur Kenntniss der Evertebratenfauna des Moskwa- flusses. Bull. Soc. Hist. Nat., Anthrop., Ethnogr., T. LXVII., Suppl. a Nr. 6, pp. 1-38. Rousselet, C. F. '97. Brachionus Bakeri and its Varieties. Journ. Quekett Micr. Club., Ser. II., Vol. VI., pp. 328-332, PI. XVI. '02. Third List of New Rotifers since 1889. Journ. R. Micr. Soc., 1902, pp. 148-154. '02a. The Genus Synchaeta: A Monographic Study with Descrip- tions of five New Species. Journ. R. Micr. Soc., 1902, pp. 269-290, 393-411, PI. III. -VIII. Roux, J. '99. Observations sur quelques Infusoires Cilies des Environs de Geneve, avec la description de nouvelles especes. Rev. Suisse de Zool., T. 6, pp. 555-636, PI. XIII, XIV. '01. Faune Infusorienne des Eaux stagnantes des Environs de Geneve. Mem. Inst. Nat. Geneve, T. XIX. 148 pp., 8 pi. Sand, R. '01. Etude monographique sur le groupe des Infusoires Tenta- culiferes. 441 pp., 24 pi. Bruxelles, 1901. Published also in Ann. de la Soc. Beige' de Micr., T. XXIV., pp. 57-189; T. XXV., pp. 7-205; T. XXVI. , pp. 11-119. Say, T. '18. An Account of the Crustacea of the United States. Journ. Acad. Nat. Sci. Phila., Vol. I., pp. 423-458. 350 Schacht, F. W. '97. The North American Species of Diaptomus. Bull. 111. State Lab. Nat. Hist., Vol. V., pp. 97-207, PI. XXI.-XXXV. '98. The North American Centropagidae belonging to the Genera Osphranticum, Limnocalanus, and Epischura. Bull. 111. State Lab. Nat. Hist., Vol. V., pp. 225-269. SchewiakonY Wl. '93. tiber die geographische Verbreitung der Siisswasser-Protozoen. Mem. 1'Acad. Imper. des Sci. de St.-Petersbourg, Ser. VII., T. XLL, No. 8. 201 pp., 4 Taf. Schorler, B. '00. Das Plankton der Elbe bei Dresden. Zeitschr. f. Gewasser- kunde, Bd. III., pp. 1-27. Schroder, B. '97. Attheya, Rhizosolenia und andere Planktonorganismen in Teiche des botanischen Gartens zu Breslau. Ber. d. deutsch. botan. Ges., Bd. XV., pp. 367-374, Taf. XVII. '97a. Ueber das Plankton der Oder. Ber. d. deutsch. botan. Ges., Bd. XV., pp. 482-492, Taf. XXV. Scott, Thomas. '99. The Invertebrate Fauna of the Inland Waters of Scotland. Report on Special Investigation. Ann. Rep. Fish. Board Scot- land, Vol. XVII., pp. 132-204, PI. VII. Scourneld, D. J. '98. The Entomostraca of Epping Forest, with some General Re- marks on the Group. Essex Naturalist, Vol. X., pp. 193-210, 259-274, -3 13-334. Seligo, A. '00. Untersuchungen in den Stuhmer Seen. Nebst einem Anhange: Das Pflanzenplankton preussischer Seen, von B. Schroder. 88 pp., 9 Tab., und 10 Taf. Danzig. Sharpe, R. W. '97. Contribution to a Knowledge of the North American Fresh- water Ostracoda included in the Families Cytheridae and Cyprid- idas. Bull. 111. State Lab. Nat. Hist., Vol. IV., pp. 414-482, PI. xxxix.-XLvm. 351 Skorikow, A. S. '96. Rotifera of the Neighborhood of Charkow. Contrib. Soc. Nat. Imp. Univ. Charkow, Vol. XXX., pp. 209-374, PI. VI.-IX. (Russian.) '97. Contribution to an Investigation of the Distribution of the Invertebrates of the River Udy. Contrib. Soc. Nat. Imp. Univ. Charkow, Vol. XXXI. , pp. 39-48. (Russian.) '02. Die Erforschung des Potamoplanktons in Russland. Biol. Centralbl., Bd. XXII., pp. 551-570. Smith, F. '94. List of the Protozoa and Mollusca observed in Lake St. Clair in the Summer of 1893. Bull. Mich. Fish Comm., No. 4, pp. 42-44. '00. Notes on Species of North American Oligochaeta. III. List of Species found in Illinois, and Descriptions of Illinois Tubificidae. Bull. 111. State Lab. Nat. Hist., Vol. V., pp. 441-458, PI. XXXIX., XL. Smith, S. I. '74. Sketch of the Invertebrate Fauna of Lake Superior. Rep. U. S. Fish Comm., 1872-73, Part II., pp. 690-707. Sowinski, W. K. '88. Beschreibung der Fauna der Susswassercrustaceen aus der Umgebung Kiews und des nordlichen Teiles des Gouvernement Kiew. Schriften der Kiew 'schen-Naturf . Ges., 1888, Bd. IX., pp. 225-298. Hit Karte. Stenroos, K. E. '97. Zur Kenntnis der Crustaceen-Fauna von Russisch-Karelien. Cladocera, Calanidag. Acta Soc. pro Fauna et Flora Fennica, Bd. XV., No. 2. 72 pp. Mit einer Tafel und einer Karte. '98. Das Thierleben im Nurmijarvi-See. Acta Soc. pro Fauna et Flora Fennica, Bd. XVII., pp. 1-259, Taf. I-III. 1 Karte. Steuer, Adolf. '01. Die Entomostrakenfauna der "alten Donau" bei Wien. Eine ethologische Studie. Mit einem Anhang: Zur Frage uber Ursprung und Verbreitung der Entomostrakenfauna des Siisswas- sers. Zool. Jahrb., Abth. f. Syst., Geogr., u. Biol. d. Thiere, Bd. XV., pp. 1-156, Taf. I -XII. Mit 20 Abb. im Text. 352 Stingelin, Th. '97. Ueber jahreszeitliche, individuelle und locale Variation bei Crustaceen, nebst einigen Bemerkungen iiber die Fortpflanzung bei Daphniden und Lynceiden. Forschungsber. a. d. Biol. Station zu Plon, Th. V., pp. 150-165. Hit 5 Fig. im Text. Stokes, A. C. '86- '88. A Preliminary Contribution toward a History of the Fresh- water Infusoria of the United States. Journ. Trenton Nat. Hist. Soc., Vol. I., pp. 71-319, PI. I.-XIII. '94. Notices of presumably undescribed Infusoria. Proc. Am. Phil. Soc., Vol. XXXIII., p: 338-345, PL XXI. Stole, A. '99. Actinomyxidia, nova Mesozoii, ' pfibuzna Myxosporidiim. Rozp. Ceske Ak. Cis. Frant. Jos. II., VIII., 22. 12 pp., 3 pi. Thorpe, V. G. '93. The Rotifera of China. Journ. R. Micr. Soc., 1893, pp. 145- 152. 2 pi. Verworn, M. '97. Die polare Erregung der lebendigen Substanz durch den con- stanten Strom. Archiv f. Physiologic (Pfluger), Bd. 65, pp. 63-81, Taf. IV. Volk, R. '03. Hamburgische Elb-Untersuchung. I. Allgemeines iiber die biologischen Verhaltnisse der Elbe bei Hamburg *und iiber die Einwirkung der Sielwasser auf die Organismen des Stromes. Mitth. a. d. Naturhist. Mus. in Hamburg, Bd. XIX., pp. 65-164, Taf. I. -VI I. Ward, H. B. r 96. A Biological Examination of Lake Michigan in the Traverse Bay Region. Bull. Mich. Fish Comm., No. 6. 100 pp., 5 pi. Weber, E. F. '98. Faune Rotatorienne du Bassin du Le"man (2 me partie). Rev. Suisse de Zool., T. V., pp. 355-785, PI. 16-25. Weismann, A. '79. Beitrage zur Naturgeschichte der Daphnoiden. VI. Samen und Begattung der Daphnoiden. VII. Die Enstehung der cyclischen Fortpflanzung bei den Daphnoiden. Zeitschr. f. wiss. Zool., Bd. XXXIII., pp. 55-270, Taf. VIII.-XIII. 353 Wesenb erg-Lund, C. '98. ^Uber danische Rotiferen und iiber die Fortpflanzungsverhalt- nisse der Rotiferen. Zool. Anz., Bd. XXL, pp. 200-211. '00. Von dem Abhangigkeitsverhaltnis zwischen dem Bau der Planktonorganismen und dem spezifischen Gewicht des Siisswas- sers. Biol. Centralbl., Bd. XX., Nr. 18 und 19, pp. 606-619, 644-656. Whipple, G. C. '94. Some Observations on the Growth of Diatoms in Surface Wa- ters. Tech. Quart., Vol. VII., pp. 214-231. Whipple, G. C., and Jackson, D. D. '99. Asterionella, its Biology, its Chemistry, and its Effect on Water Supplies. Journ. N. E. Waterworks Ass'n, Vol. XIV., pp. 1-25. 4 pi. Woodworth, W. McM. '97. On some Turbellaria from Illinois. Bull. Mus. Comp. Zool., Vol. XXXL, pp. 1-16. 1 pi. Zacharias, O. '94. Faunistische Mittheilungen. Forschungsber. a. d. Biol. Station zu Plon, Th. II., pp. 57-90, Taf. I., II. Mit 2 Fig. im Text. '97. Neue Beitrage zur Kenntniss des Susswasserplanktons. For- schungsber. a. d. Biol. Station zu Plon, Th. V., pp. 1-9, Taf. I. '97a. Die Fauna der Versuchsteiche. Zeit. f. Fischerei, Bd. V., pp. 37-51. '98. Das Heleoplankton. Zool. Anz., Bd. XXL, pp. 24-32. '98a. Das Potamoplankton. Zool. Anz., Bd. XXL, pp. 41-48. '98b. . Untersuchungen iiber das Plankton der Teichgewasser. For- schungsber. a. d. Biol. Station zu Plon, Teil VI., pp. 89-139, Taf. IV. '00. Trichodina pediculus Ehrb. als Mitglied des Planktons der Bin- nenseen. Biol. Centralbl., Bd. XX., p. 463. Zernow, S. A. '01. Remarks on the Zooplankton of the Rivers Schoschma and Wjatka (Region Malmyz, Government Wjatka). Bull. Imp. Soc. Friends Sci. Moscow, Zool. Section, Vol. 3, pp. 28-35, 36. 1 pi. 354 Zimmer, C., und Schroder, B. '99. Das Plankton des Oderstromes. Forschungsber. a. d. Biol. Station zu Plon, Teil VII., pp. 1-24. Zumstein, H. '99. Zur Morphologic und Physiologic der Euglena gracilis Klebs. Inaug. Diss. 50 pp., 1 Taf. Leipzig. Also, in 1900, in Jahrb. f. wiss. Botanik, Bd. XXXIV., pp. 149-198, Taf. VI. Zykoff, W. '00. Das Potamoplankton der Wolga bei Saratow. Zool. Anz., Bd. XXIII., pp. 625-627. '03. Report on the Fauna of the Volga and the Aquatic Fauna of the Government of Saratoff. Bull. Soc. Naturalists of Moscow, 1903, pp. 1-148, PI. I., II. (Russian.) EXPLANATION OF PLATES. PLATE I. Seasonal distribution of synthetic groups of planktonts, Chlorophycece, Bacillariacea, and Mastigophora, from July 1, 1895, to October 171896. Note changes of scale indicated at bottom of diagram. Numbers in column at left apply only to 1895. In this plate and in II. and IV., apices exceeding the limit of the diagram are dropped down between dotted lines to show location. Circles at bottom indicate location of day of full moon. PLATE II. The same as above, from July 1, 1897, to April 1, 1899. Note change in scale from previous plate. PLATE III. Seasonal distribution of total Rotifera and Crustacea from July 1, 1895, to October 1, 1896. The Crustacea included, belong almost exclusively to the Ento- mostraca. Apices exceeding the limits of the diagram are dropped down between dotted lines to show location. Totals include both adult and immature stages of the Entomostraca when detached from parent, and both free and attached eggs of the Rotifera. PLATE IV. The same as above, from July 1, 1897, to April 1, 1899. PLATE V. Seasonal distribution of Polyarthra platyptera. Total number of individuals, not including eggs, represented by ordinants, parts of which exceeding 200,000 are represented by diagonal lines instead of solid vertical lines. Thus parts of a seasonal plot which overlap those above it on the plate are represented by the diagonally-lined ordinants. (24) 355 356 357 358 359 360 ERRATA AND ADDENDA. Page 58, line 7, for ovalis read ovata. Page 85, line 8, for longicaudus read longicauda, and just above Phacus pleuro- nectes read the following paragraph: Phacus longicauda var. torta, n. var. This variety, for which I propose the name torta because of the twisted body, is figured by Stein ( '78, Taf7207 Fig. 3). It occurred sparingly in midsummer from July to September, rarely in October, in .1896 and 1897. Page 91, line 18, after T. caudata Ehrb. read T. lagenella Stein. Pages]153, line 3 from bottom, 168, line 16, and 178, line 14, for '98 read '98a. Pages 156, line 11, 159, line 16, and 161, line 5 from bottom, for '93 read '98a. 361 14 DAY USE RETURN TO DESK FROM WHICH BORROWED | BIOLOGY LIBRARY TEL. NO. 642-2532 This book is due on the last date stamped below, or on the date to which renewed. Renewed books are subject to immediate recall. _SEE LD 21A-12m-5,'68 (J401slO)476 General Library _ University of California Berkeley >** ' ' *'*' V "7"*---^ . '. V river, 1894-1899 Lib. JUN 13 194S M246526 B1OLOY THE UNIVERSITY OF CALIFORNIA LIBRARY