BIOLOGY 
 LIBRARY 
 
GENEEAL ZOOLOGY 
 
 BY 
 
 A. S. PEARSE 
 
 f 
 
 NEW YORK 
 HENRY HOLT AND COMPANY 
 
 1917 
 
"R3.T 
 
 BIOLOGY 
 
 LIBRARY 
 
 G 
 
 COPYBIQHT, 1917 
 BT 
 
 HENRY HOLT AND COMPANY 
 
 T H K MAPLE PRESS YORK. PA 
 
WHO, THOUGH WHOLLY UNSCIENTIFIC, 
 
 HAS LOYALLY DEVOTED FIFTEEN 
 
 YEARS TO THE ADVANCEMENT 
 
 OF SCIENCE 
 
 376237 
 
PREFACE 
 
 This book has been written to be read by students of 
 eighteen to twenty years of age. With this in mind the 
 .chapters have been made short, and an attempt has been 
 made to enliven the text by interpolating a considerable 
 number of illustrations that lean more toward natural 
 history than anatomy. 
 
 The purpose of the book is to give a general survey of 
 the important points relating to the chief groups of animals. 
 The first four chapters deal largely with generalities and 
 may well follow chapters which come later if such an ar- 
 rangement is more expedient for any particular course. 
 An important place is given to insects because they afford 
 the most available material for collecting, classification, 
 and dissection students can acquire a variety of first-hand 
 knowledge from them as from no other group of animals. 
 
 The writer is convinced that students of biology get the 
 best training in scientific methods of thought from practical 
 work, and that it should constitute the chief part of any 
 course; also that a text-book should not repeat what is 
 observed in laboratory or field, but supplement and general- 
 ize upon such information. The book ought, therefore, 
 to contain more of natural history and general biological 
 theory than anatomy. Some chapters in the present work 
 (e.g., X) are perhaps , without much unity or apparent 
 purpose, and are intended primarily to serve for reference 
 in connection with laboratory or field studies. 
 
 Anyone who teaches is stimulated by association with 
 great teachers men of originality who are scientific, yet 
 human, and who are ever willing to help or encourage 
 students. Though this book intends to be original, the 
 writer is fully aware that he has consciously and uncon- 
 sciously used the ideas of others. Foremost among those 
 
vi PREFACE 
 
 who have thus unwittingly contributed are: G. H. Parker, 
 W. C. Curtis, and M. F. Guyer. 
 
 In the preparation of the book the writer has been under 
 obligation to a number of persons, and it is a pleasure to 
 acknowledge the debt. Professor M. F. Guyer read the 
 manuscript for chapters I to V, XXVII, XXVIII, XXX, 
 and helped in other ways; Professor George Wagner, read 
 chapters XI to XX; Professor W. S. Marshall, V to XI; 
 Professor W. J. Meek, XXVII; Dr. John N. Lowe, I to 
 IV; Miss G. M. White, XXI and XXII; Messrs. A. R. 
 Cahn and T. C. Nelson, XXV and XX respectively. Dr. 
 A. G. Ruthven and Professor E. C. Case, of the University 
 of Michigan, read chapters XXIII, XXIV, XXVI and 
 XXIX. Most of all I am indebted to Miss Hattie J. 
 Wakeman, who drew all the original figures but two. Fig. 
 46 was drawn by Lydia Wakeman, and Mr. A. R. Cahn 
 furnished the prints for Fig. 103. 
 
 UNIVERSITY OP WISCONSIN, 
 March 1, 1917. 
 
CONTENTS 
 
 CHAPTER PAGE 
 
 I. INTRODUCTION 3 
 
 II. BASIS FOR THE CLASSIFICATION OF ANIMALS . . 15 
 
 III. LIFE AND LIVING THINGS 25 
 
 IV. CELLS 37 
 
 V. PHYLUM ARTHROPODA, CLASS CRUSTACEA, LAW OF 
 
 BIOGENESIS . 45 
 
 VI. PHYLUM ARTHROPODA, CLASSES ONYCOPHORA, 
 
 MYRIAPODA, INSECTA 63 
 
 VII. ORDER ORTHOPTERA, THE RED-LEGGED LOCUST . 76 
 
 VIII. THE RED-LEGGED LOCUST 84 
 
 IX. PHYLUM ARTHROPODA, CLASS INSECTA .... 95 
 
 X. PHYLUM ARTHROPODA, CLASS INSECTA ..... 106 
 
 XI. PHYLUM ARTHROPODA, CLASS ARACHNID A . . .123 
 
 XII. PHYLUM PROTOZOA 134 
 
 XIII. THE ORIGIN AND CHARACTERISTICS OF THE 
 METAZOA 147 
 
 XIV. PHYLUM PORIFERA SPONGES 157 
 
 XV. PHYLUM COELENTERATA; PHYLUM CTENOPHORA . 163 
 
 XVI. PHYLUM PLATYHELMIA 174 
 
 XVII. PHYLUM NEMATOIDEA; PHYLA: ROTIFERA, 
 
 BRACHIOPODA, BRYOZOA 183 
 
 XVIII. PHYLUM ECHINODERMATA . . . , 192 
 
 XIX. PHYLUM ANNELIDA 203 
 
 XX. PHYLUM MOLLUSCA . . . 213 
 
 XXI. PHYLUM CHORDATA 224 
 
 XXII. PHYLUM CHORDATA, CLASS PISCES 235 
 
 XXIII. PHYLUM CHORDATA, CLASS AMPHIBIA 246 
 
 XXIV. PHYLUM CHORDATA, CLASS REPTILIA 257 
 
 XXV. PHYLUM CHORDATA, CLASS AVES 269 
 
 XXVI. PHYLUM CHORDATA, CLASS MAMMALIA .... 286 
 
 XXVII. MAN. SPECIAL FEATURES 297 
 
 XXVIII. MAN. GENERAL FEATURES 311 
 
 XXIX. ANIMALS OF THE PAST 327 
 
 XXX. EVOLUTION AND HEREDITY 342 
 
 INDEX 361 
 
 vii 
 
GENERAL ZOOLOGY 
 
CHAPTER I 
 THE ABUNDANCE OF ANIMAL LIFE 
 
 ' Most persons do not realize what an enormous number of 
 animals exist on the earth, and what a variety of habitats 
 they occupy. The forest not only harbors monkeys, 
 squirrels, tree frogs, and other familiar animals which are 
 suited to such a situation, but also supports a host of 
 minute insects, worms, snails, and other things which 
 escape ordinary notice. Many of these are active only at 
 night, on humid days, or on various infrequent occasions. 
 The fields and prairies have a characteristic fauna of 
 prairie-dogs, grasshoppers, antelope, etc. Lakes, ponds, 
 and streams swarm with aquatic animals. Particularly 
 in winter, bodies of water serve as refuges for many animals 
 which may be found elsewhere during warmer seasons. 
 Some animals, such as the mole, the mole-cricket, and the 
 earthworm, pass their whole lives burrowing in the soil, and 
 show structural adaptations which fit them particularly for 
 such a habitat. 
 
 Unsuspected residents may occur in all sorts of situations 
 and in countless numbers. Darwin was once interested 
 in the distribution of animals and plants on the feet of birds. 
 In this connection he tried to ascertain how many living 
 things there were on the muddy shore of a little puddle. 
 From three spoonfuls of mud he raised 537 separate plants. 
 More recently one of the investigators for the United States 
 Department of Agriculture, in studying the food of birds, 
 took a census of all the animal and plant objects in a space 
 two feet square and as deep as a bird might scratch, with 
 the following result: 
 
 Animal Plant 
 
 objects objects 
 
 Forest 112 194 
 
 Meadow.. 1254 3113 
 
' o^ 
 4 GENERAL ZOOLOGY 
 
 Probably most readers have never heard of nematode 
 worms, yet Cobb in a paper on these animals says: 
 
 "Not the least interesting thing about nematodes is the astounding 
 variety of their habitats. They occur in arid deserts, at the bottoms 
 of lakes and rivers, in the waters of hot springs and in polar seas where 
 the temperature is constantly below the freezing point of fresh water. 
 They were thawed out alive from Antarctic ice in the far south by mem- 
 bers of the Shackleton expedition. ... A thimbleful of mud from the 
 bottom of river or ocean may contain hundreds of specimens. The 
 nematodes from a 10-acre field, if arranged in single file, would form a 
 
 FIG. 1. A nematode worm which lives on the roots of plants. Greatly mag- 
 nified. (From Cobb, Yearbook; U. S. Department of Agriculture, 1914.) 
 
 procession long enough to reach around the world. A lump of soil no 
 larger than the end of one's thumb may contain hundreds, even thou- 
 sands of nematodes, and yet present few points that would distinguish it 
 from a lump of soil destitute of these organisms. ... In short, if all 
 other matter in the universe except the nematodes were swept away, 
 our world would still be dimly recognizable, and if, as disembodied 
 spirits, we could then investigate it, we should find its mountains, hills, 
 vales, rivers, lakes, and oceans represented by a film of nematodes. The 
 location of towns would be decipherable, since for every massing of 
 human beings there would be corresponding massing of certain nema- 
 todes. Trees would stand in ghostly rows representing our streets and 
 highways." 
 
 Instances of this kind might be multiplied. There are 
 animals in the soil which even exceed the nematodes in 
 numbers. 
 
THE ABUNDANCE OF ANIMAL LIFE 5 
 
 DEFINITIONS 
 
 Biology is the branch of natural science which includes 
 all studies pertaining to living things. There are two great 
 subdivisions: Botany, which deals with plants; and Zool- 
 ogy, which is concerned with animals. Zoology, then, is 
 the body of facts and theories derived from the scientific 
 study of animals. Our science is founded on facts, and 
 over this superstructure there have been erected a number 
 of theories which are intended to blend zoology into a 
 harmonious whole. We must be scientific in our study. 
 This is the hardest task a zoologist has from day to day. 
 A scientist's aim is to discover the truth about phenomena. 
 A real scientist formulates his conclusions and general laws 
 from a study of the available facts, and he does this without 
 prejudice, or superstition, or thought of self interest. Let 
 us be scientific! This means accurate observation and 
 sound, thoughtful conclusions. 
 
 On account of the amount of material which has been 
 accumulated from zoological studies, men's interests have 
 been divided and a number of subsciences established. 
 Chief among these are the following: 
 
 Systematic Zoology deals with the description of species 
 and their classification according to accepted systems. 
 About 522,400 species have been described, and the mere 
 cataloguing of these requires considerable work by special- 
 ists. There are many branches of systematic zoology, 
 such as: entomology, relating to the classification of insects ; 
 ornithology, birds; conchology, molluscs, etc. 
 
 Distributional Zoology has two aspects. Under Zoogeog- 
 raphy are included facts relating to the present arrange- 
 ment of animals on the earth. Five great realms which 
 possess characteristic animals are recognized (Holoarctic, 
 African, Indo-Malayan, Neotropical, Australian), and 
 these have each been subdivided into smaller parts. 
 Paleozoology deals with the distribution of animals during 
 
6 GENERAL ZOOLOGY 
 
 the past, as shown by fossil remains. Zoogeography, then, 
 has reference to space; Paleozoology, to time. 
 
 Morphology is the science of structure. Anatomy deals 
 with dissection; embryology, with the changes which take 
 place as individuals develop to maturity; histology, with 
 minute anatomy or the study of tissues; pathology, with the 
 structure of diseased tissues; neurology, with the make up 
 of nerves; paleontology, with the characteristics of fossil 
 animals. 
 
 Physiology treats of the functions of the parts of animals 
 i.e., how the structures which make up the mechanism of 
 any animal work. As under morphology there are a num- 
 ber of subdivisions: psychology is concerned with the work- 
 ing of the mind; psychiatry, with the operations of diseased 
 or abnormal minds. 
 
 Ecology considers the relations of animals to their en- 
 vironment. Here would properly come such topics as the 
 relations of parasites to their hosts, colonial habits, the 
 denizens of particular habitats, etc. 
 
 Evolutionary Zoology relates to the origin and descent of 
 species. Heredity is concerned with how characteristics 
 are transmitted from parent to offspring. Eugenics deals 
 particularly with heredity in man, with the aim of improv- 
 ing the human race. 
 
 SHORT HISTORY OF ZOOLOGY 
 
 If we are going to be zoologists, even in a small way, it is 
 fitting that we should know something about those who 
 have laid the foundations of our science. The following 
 ten names have, therefore, been selected to serve as " mile- 
 stones of zoological progress." 
 
 ARISTOTLE, the " Father of Natural History," was a 
 Greek scholar who lived from 384 to 322 B.C. He wrote in 
 all about three hundred works on philosophy, metaphys- 
 ics, psychology, and rhetoric, but his most noteworthy 
 writings were in the field of natural history. His greatest 
 
HISTORY 
 
 contribution to zoology was his method of study. He 
 gathered first-hand knowledge about animals and presented 
 his facts in a scientific way. Locy says, "the influence of 
 Aristotle was in the right direction. He made a direct 
 appeal to nature for his facts, and founded his Natural 
 History only on observation of the structure, physiology, 
 and development of animals." Aristotle's best books, the 
 " History of Animals," the " Parts of Animals," and the 
 " Generation of Animals," were authoritative for twenty 
 centuries. Aristotle led an active public life, being a 
 student of Plato and a teacher of Alexander the Great. 
 
 FIG. 2. Aristotle. (From Locy, 
 Biology and Its Makers.) 
 
 FIG. 3. Pliny. (From Locy, 
 Biology and Its Makers.) 
 
 PLINY, 23-79 A.D., was a Roman general and writer. 
 He wrote thirty-seven voluminous and uncritical volumes 
 on natural history. His works were filled with tales of 
 dragons, gorgons, and other fabled monsters, so that it 
 was impossible to separate fact from fiction. His influence 
 was highly detrimental to the progress of zoological thought, 
 but his works, nevertheless, were authoritative during the 
 middle ages. 
 
 GALEN, 130-200 A.D., was a Greek physician. He was 
 the great anatomist of antiquity and his clear and forceful 
 descriptions were the sole anatomical guides in the medical 
 schools for twelve centuries after his time. He was a 
 sound thinker and accurate observer. Galen probably 
 never actually dissected the human body, but wrote his 
 
8 
 
 GENERAL ZOOLOGY 
 
 excellent works from studies and comparisons of other 
 animals. 
 
 During the Middle Ages there was no progress in zoo- 
 logical thought. All questions were referred to ancient 
 authorities. Matters of learning were almost wholly in 
 the hands of the clergy, who deemed everything unworthy 
 which did not pertain directly to religious life. The ad- 
 
 FIG. 4. Galen. (From Locy, 
 Biology and Its Makers.) 
 
 FIG. 5. Vesalius. (From Locy, 
 Biology and Its Makers.) 
 
 monition to "shun the world " was taken all too seriously 
 by the early Christians. There was stagnation in all 
 fields of learning. 
 
 VESALIUS (1514-1564, Belgian), more than any man, 
 threw off the yoke of tradition and the respect for authority 
 which had characterized the scholars of the Middle Ages 
 and opened the way for the free discovery of new knowledge 
 which we enjoy today. He was a man of great courage 
 and honesty. Despite the superstitious traditions of his 
 time, he dissected the human body. His eminent book, 
 the " Structure of the Human Body, " and his direct methods 
 
HISTORY 
 
 9 
 
 of teaching revolutionized anatomical methods. His 
 greatest work, however, was the opening of men's minds 
 to the opportunities for scientific progress to those who 
 would make observations for themselves and draw their 
 own conclusions. 
 
 HARVEY (1578-1667, English), following the anatomical 
 discoveries of Vesalius, laid the foundations of our modern 
 methods of experimental investigation in biological science. 
 His great work was the demonstration of the circulation 
 of the blood, an epoch-making discovery. Before his time 
 it was supposed that there was a sort of an ebb and flow 
 
 FIG. 6. Harvey. (From Locy, Biology 
 and Its Makers.) 
 
 FIG 7. Linnaeus. (From Locy, 
 Biology and Its Makers.) 
 
 in the blood-vessels, that the arteries contained blood mixed 
 with animal spirits from the lungs, and that the veins held 
 the crude blood. Harvey showed conclusively that the 
 blood traversed a regular route through the body. This 
 may seem like a simple everyday fact at the present time, 
 but in the fifteenth century Harvey's assertions, though 
 they were finally accepted, excited great controversy and 
 astonished the whole scientific world. 
 
 LINNAEUS (1707-1778) was the founder of modern 
 systematic zoology. Before his time species were rather 
 
10 
 
 GENERAL ZOOLOGY 
 
 indefinitely known from verbose Latin descriptions. This 
 great Swedish naturalist inaugurated our present binomial 
 nomenclature, giving to each species a scientific name con- 
 sisting of two words, the genus and species. He attempted 
 to describe and catalogue all species of plants and animals. 
 In the tenth edition of his great work, the Systema Naturae, 
 4236 species of animals were described. The general classi- 
 fication instituted by Linnaeus was not very satisfactory 
 
 and has now been considerably 
 amplified. In his later life Lin- 
 naeus was a professor in the 
 University of Upsala and the 
 foremost figure in the zoological 
 world. His influence made 
 classification and naming new 
 species the most studied fields 
 of biology for some time after 
 his death. 
 
 CUVIER (1769-1832) was an 
 eminent educator, being direc- 
 tor of the higher institutions of 
 learning under Napoleon Bona- 
 parte. Early in life he con- 
 ceived the idea of making a very comprehensive study of 
 comparative anatomy and was so successful that this sub- 
 ject became the leading field of zoology until the time of 
 Darwin. He also made very thorough studies of the anat- 
 omy of all the chief groups of animals which had existed 
 in bygone ages. Cuvier would not accept the idea of an 
 evolution of animals, but believed the earth had been re- 
 populated again and again after a series of "catastro- 
 phisms" which wiped out everything alive. 
 
 DARWIN (1809-1882) was an Englishman who received 
 most of his zoological training during a five-year cruise as 
 naturalist on the ship Beagle, which made a voyage around 
 the world. After his return to England, he began with 
 great care to accumulate data for his theory of evolution. 
 
 FIQ. 8. Cuvier. (From Locy, 
 Biology and Its Makers.} 
 
HISTORY 
 
 11 
 
 This theory is well set forth in the " Origin of Species/' 
 published in 1859. His great work for Zoology, and the 
 world, was to convince scientific men that there had been 
 a change in the past that one type of animal had evolved 
 from a somewhat different preexisting type. His theory 
 involved four ideas: (1) More animals are produced than 
 can find a place to live and hence there is a " struggle for 
 existence;" (2) qualities tend to be transmitted unchanged 
 from parent to offspring; but (3) there is always slight varia- 
 tion which may make one animal a little better than others 
 of its own species; and (4) there may thus be a " survival 
 of the fittest" in the struggle, which constitutes the 
 
 FIG. 9. Darwin. (From Locy, 
 Biology and Its Makers.) 
 
 FIG. 10. Agassiz. (From Locy, 
 Biology and Its Makers.) 
 
 ' ' natural selection ' ' 
 improved after 
 
 through which Darwin supposed animals 
 a number of generations. This theory 
 gave a wonderful impetus to all branches of scientific work 
 because it opened a vast field for investigation by clearing 
 men's minds of the idea that natural phenomena had always 
 been and always would be as they are, and were, therefore, 
 not open to experimental study. 
 
 AGASSIZ (1807-1873), though a Swiss by birth, is of 
 particular interest to all American zoologists. He was a 
 comparative anatomist of the school of Cuvier who came 
 
12 
 
 GENERAL ZOOLOGY 
 
 to America when a young man and introduced the scientific 
 methods of Europe into this country. Though he was the 
 first to work out the close relation between the fossil record 
 and the embryonic stages in the development of individual 
 animals, he would never accept his discoveries as evidences 
 of evolution, and died an opponent of Darwin. 
 
 HUXLEY (1825-1895), though primarily a student of 
 fossil animals, did a great work along general zoological 
 lines for all English-speaking people. More than any other 
 
 zoologist he was able to talk 
 and write in an understand- 
 able way to popular audiences. 
 His contribution to zoological 
 progress, then, was to popu- 
 larize zoology and make 
 general biological laws mat- 
 ters of common everyday 
 knowledge. 
 
 From this brief historical 
 survey it is plain that zoology 
 has not come about all at 
 once. There has been a long 
 struggle to accumulate facts 
 
 and to make theories to fit them. No one nation, nor 
 class of people has done the good work, but a host of 
 fair-minded energetic workers have contributed to progress. 
 One of the fine things about science is its method of work. 
 There is no secrecy nor selfishness only desire to discover 
 truth at any cost. For science men have died of fever 
 during explorations; carried on life-long struggles with 
 poverty, superstition, or other obstacles; or been shunned 
 by their fellows for advocating new ideas. But through 
 the courage and inspiration of such workers zoology has 
 gone forward; we are not yet at the end of our discov- 
 eries; new things are flashing into the light of knowledge 
 almost every day. 
 
 FIG. 11. Huxley. (From Locy, 
 Biology and Its Makers.) 
 
CLASSIFICATION 13 
 
 THE CLASSIFICATION OF ANIMALS 
 
 At the present time there are about 523,000 species of 
 animals known, with perhaps an equal number yet to be 
 described. It goes without saying that such a number of 
 distinct kinds requires some means of classification, if only 
 for convenience in cataloguing. There are in general two 
 systems of classification natural and artificial. The first 
 is the result of an attempt to show genetic, or blood, rela- 
 tionships; the second term is applied to any system of 
 grouping which will enable one to bring animals together 
 which show certain points of similarity, whether such 
 characteristics are of fundamental importance or not. la 
 times past the barnacles were classed with molluscs be- 
 cause they possessed a calcareous shell, but when their 
 embryology was studied it was discovered that all the early 
 stages were like those of crustaceans, and they have since 
 been classified with other members of that group such as 
 crabs, lobsters, and water-fleas. The former classification 
 for the barnacles was artificial, the latter, it is believed, is 
 more natural. Systematic zoologists are continually striv- 
 ing for a perfect natural classification. 
 
 The animal kingdom is divided into smaller and smaller 
 units just as a military regiment is divided into battalions, 
 companies, platoons, squads, and individuals. The largest 
 groups are called phyla, these are subdivided into classes, 
 etc. In going from larger to smaller, the groups would be 
 arranged as follows : Phylum, Class, Order, Family, Genus, 
 Species. 
 
 Each species of animal is known by its scientific name, 
 which consists of the name of its genus and its own particu- 
 lar species. Taking man as an example, we may classify 
 him. He belongs to the species sapiens and the genus 
 Homo. This genus also includes some other species of 
 men, none of which are now living. The scientific name of 
 man would be, Homo sapiens Linnaeus; the last word 
 showing that Linnaeus first described this particular species. 
 
14 GENERAL ZOOLOGY 
 
 The genus Homo is grouped with a number of others con- 
 taining ape-like animals in the family Hominidse, and this 
 family in turn is one of those which make up the order 
 Primates, including monkeys of all kinds. Primates is one 
 of the orders of the class Mammalia, which includes all 
 animals which have hair and suckle their young such as 
 bats, seals, whales, hoofed animals, cats, dogs, etc. The 
 class Mammalia is one of half a dozen in the phylum 
 Chordata, one of the primary divisions of the animal 
 kingdom. 
 
CHAPTER II 
 BASIS FOR THE CLASSIFICATION OF ANIMALS 
 
 If one is going to classify 600,000 different kinds of 
 animals, it will be necessary to have some general plan for 
 making large divisions which will include a considerable 
 number of species having common characteristics. To 
 
 FIG. 12. A bit of cork showing cells as represented by Hook, the discoverer of 
 cells. (From Locy, Biology and Its Makers.) 
 
 make a natural classification the seven following criteria 
 have been found of most value: 
 
 1. Body Composed of One Cell or of Many. All living 
 things are made up of little living units, the cells (Fig. 12), 
 just as a brick house is made up of bricks. A cell, like a 
 brick, may thus exist as part of a structure or as an inde- 
 pendent unit. One great division of animals, Protozoa, is 
 made by grouping all those together whose bodies are com- 
 
 15 
 
16 
 
 GENERAL ZOOLOGY 
 
 posed of one single cell; another, Metazoa, by grouping 
 together those whose bodies are multicellular. 
 
 2. Diplo- or Triplo-blastic (Fig. 13). Animals made up 
 of many cells show differences in arrangement. In the 
 sponges the cells do not form layers but in other meta- 
 zoans they form definite sheets. We may have, therefore, 
 
 FIG. 13. Sections across the bodies of Hydra and Planaria showing diplo- 
 blastic and triploblastic structure. In the first there are two layers of cells be- 
 tween the digestive cavity and the outside; in the second, there are three. 
 
 diploblastic animals with two layers of cells the ectoderm 
 and entoderm; or triploblastic animals with three ectoderm, 
 mesoderm, and entoderm. These primitive sheets of cells 
 which appear in the development of most Metazoa are 
 called the germ layers. 
 
 3. Body Metameric or Non-metameric (Fig. 14). The 
 bodies of many animals show a condition of metamerism, 
 
 FIG. 14. A slug and a leech, showing the absence and presence of metamer- 
 ism. The slug has no subdivisions in the body, but in the leech the body is made 
 up of a series of similar parts. 
 
 or segmentation. Similar parts are arranged one after 
 the other and form a sort of a chain. Each of the seg- 
 ments has a more or less complete set of organs, and these 
 are repeated in the successive segments of the body. Man 
 
CLASSIFICATION OF ANIMALS 17 
 
 shows evidences of metamerism in the arrangement of his 
 ribs, vertebrae, spinal nerves, muscles, etc. The common 
 earthworm is a better example for its body is composed of 
 a chain of very similar metameres. 
 
 4. Symmetry Radial or Bilateral (Fig. 15). Some animals 
 have the chief organs arranged around a central axis. The 
 form is usually disc-like or spherical, perhaps also with pro- 
 jecting arms. In such radially symmetrical animals there 
 are a number of antimeres, or body parts which might be 
 interchanged without destroying the original symmetry, 
 just as one could exchange two quarters of a pie without 
 
 FIG. 15. A jelly-fish and a turtle, showing radial and bilateral symmetry 
 
 respectively. 
 
 spoiling its general shape. Jelly-fishes, corals, and star- 
 fishes are examples. Those which grow fixed to some ob- 
 ject have distal (free) and proximal (attached) ends; those 
 which move freely have no "head" or "tail" end, but 
 possess oral (mouth) and aboral surfaces. In bilaterally 
 symmetrical animals, on the other hand, the body has a 
 front or anterior end, which often is distinguished as a head. 
 The opposite end is the posterior or hind portion; there are 
 right and left sides; a dorsal or back, and a ventral, or belly 
 side. Such animals are symmetrical on two sides of a plane 
 which passes down the middle of the dorsal and ventral 
 sides. The portions on either side of this plane are called 
 
18 
 
 GENERAL ZOOLOGY 
 
 lateral. Man may serve as an example of a bilaterally 
 symmetrical animal. 
 
 5. Body Sac-like or a Tube Within a Tube, i.e., Accelomate 
 or Ccelomate (Fig. 16). Some animals, like the fresh-water 
 
 FIG. 16. A Hydra and the anterior end of an earthworm. Both have been 
 split lengthwise so that the inside of the body is exposed. The Hydra has only 
 one cavity, the enteron, or digestive cavity; the earthworm has another cavity, 
 the ccelom, between the digestive tube and the body wall. 
 
 FIG. 17. Showing endo- and exoskeletons. The bones in a man's leg are 
 surrounded by muscles; the skeleton of a grasshopper's leg consists of tubes 
 with muscles inside. 
 
 hydra, have only one cavity within the body and the out- 
 side wall is solid. There is only one opening into the cavity, 
 the mouth. Other animals, like the earthworm, have a 
 
CLASSIFICATION OF ANIMALS 19 
 
 tubular digestive system, with a mouth at one end and an 
 anus at the other. Around this tube there is a fluid-filled 
 space, the ccelom or body-cavity, -which intervenes between 
 it and the solid body wall. The earthworm is again a good 
 illustration of this point. 
 
 6. Appendages. Many animals possess appendages to 
 the body proper in the form of paddle-like swimming 
 organs, fins, jointed legs, etc. 
 
 7. The Type of Skeleton (Fig. 17). Animals may have 
 hard structures on the outside (exoskeleton) or inside 
 (endoskeletori) of the body. In the vertebrates and their 
 relatives the skeleton is typically of the latter type and 
 its central axis always originates as a rod-like organ, the 
 notochordj or chorda, which extends longitudinally through 
 the body. In the vertebrates proper the chorda is re- 
 placed during early development by bony vertebrae which 
 form the spinal column, or " backbone." 
 
 Using such criteria, it is possible to divide the animal 
 kingdom into fifteen great groups, or phyla, which may be 
 identified by means of the following "key": 
 
 KEY TO PHYLA OF ANIMALS* 
 
 1 (2) Body composed of one cell or a colony of similar cells; mostly 
 
 microscopic Phylum I, PROTOZOA. 
 
 2 (1) Body composed of many cells arranged in tissues 3 
 
 3 (24) Body non-metameric 4 
 
 4 (11) Radially symmetrical. Have proximal (attached) and distal 
 
 (free) ends, or have oral (mouth) and aboral surfaces, but have 
 no head or tail. Parts of body radiate around a chief axis. . .5 
 
 5 (6) Body filled with many minute pores; no definite mouth. 
 
 Sponges Phylum II, PORIFERA. 
 
 6 (5) Body not filled with pores ; a mouth present 7 
 
 7 (8) Body with eight rows of comb-like plates arranged radially. 
 
 Phylum IV, CTENOPHORA. 
 
 8 (7) Body without eight rows of radially arranged paddle-plates. . 9 
 
 9 (10) Body sac-like, often with tentacles. Polyps, medusae, jelly- 
 
 fishes, corals, sea-pens. .. .Phylum III, CXELENTERATA. 
 
 * In using the key there are always two alternatives. The first number 
 is to be compared elsewhere with the one in parenthesis after it. 
 
20 GENERAL ZOOLOGY 
 
 10 (9) Skin usually spiny; body a tube within a tube; marine. Star- 
 
 fishes, sea-urchins, brittle stars, sea-cucumbers, stone-lilies: 
 
 Phylum IX, ECHINODERMATA. 
 
 11 (4) Bilaterally symmetrical. Have anterior (front), posterior 
 
 (hind), right and left, dorsal (back), and ventral (belly) 
 regions 12 
 
 12 (15) Body soft, flat, worm-like 13 
 
 13 (14) One opening into digestive cavity; or no digestive cavity. 
 
 Phylum V, PLATYHELMIA. 
 
 14 (13) With mouth and anus Phylum VI, NEMERTINA. 
 
 15 (12) Body not flat and leaf-like 16 
 
 16 (17) Body cylindrical, worm-like; without anterior ciliated* lobes. 
 
 Phylum VII, NEMATOIDEA. 
 
 17 (16) Body not as described under 16 18 
 
 18 (21) Small, often microscopic, aquatic animals; solitary or growing 
 
 in branching plant-like colonies 19 
 
 19 (20) Microscopic worm-like animals with two ciliated* lobes at the 
 
 anterior end and a chewing stomach within the body. 
 
 Phylum VIII, ROTIFERA. 
 
 20 (19) Attached colonial animals which have a superficial radial 
 
 symmetry Phylum X, BRYOZOA. 
 
 21 (18) Larger animals; often possessing a hard shell which may cover 
 
 the body 22 
 
 22 (23) Stalked attached marine animals with a shell composed of two 
 
 (dorsal and ventral) valves. Lamp shells. 
 
 Phylum IX, BRACHIOPODA. 
 
 23 (22) Soft-bodied animals, usually with a shell and a ventral muscular 
 
 "foot" Phylum XIV, MOLLUSCA. 
 
 24 (3) Body metameric and bilaterally symmetrical 25 
 
 25 (28) No jointed appendages, body worm-like (snakes belong 
 
 under 30) 26 
 
 26 (27) Microscopic animals, with ciliated lobes at the anterior end of 
 
 the body and a chewing stomach within. 
 
 Phylum VIII, ROTIFERA. 
 
 27 (26) No ciliated lobes at anterior end; body with minute bristles 
 
 along the sides or with an adhesive sucker at one or both ends. 
 
 Phylum XII, ANNELIDA. 
 
 28 (25) Jointed appendages (except in limbless amphibians, snakes, 
 
 and limbless lizards) 29 
 
 29 (30) External skeleton (turtles, which have both exo- and endo- 
 
 skeletons, belong under 30) ; no chorda. 
 
 Phylum XIII, ARTHROPODA. 
 
 30 (29) Endoskeleton; chorda or backbone present. 
 
 Phylum XV, CHORDATA. 
 * Cilia are microscopic hair-like processes which usually wave actively. 
 
CLASSIFICATION OF ANIMALS 21 
 
 CHIEF PHYLA OF THE ANIMAL KINGDOM 
 
 Subkingdom PROTOZOA 
 
 1. Protozoa (protos, first; zoon, animal). Animals made 
 up of a single cell. The cells may grow together in colonies 
 but never show any division of labor as tissues. Examples: 
 Amoeba, Euglena, Paramoecium, malarial parasite, Volvox. 
 8500 species. 
 
 Subkingdom METAZOA 
 
 2. Porifera (porus, pore; fero, carry). The body radial 
 or without definite symmetry, filled with numerous small 
 inhalent pores, and with one or more large apertures where 
 water is expelled. Cellular tissues and germ, or repro- 
 ductive, cells are well developed, but not laid down in 
 fundamental germ layers. A skeleton made up of lime, 
 silica, or horny material is often present throughout the 
 body. Peculiar cells bearing collars line some of the canals 
 cf the interior. Tissues are well developed but there are 
 no organs. This phylum includes the sponges, which are 
 attached and usually grow up from the bottom somewhat 
 like water plants. 2500 species. 
 
 3. Coelenterata (koilos, hollow; enteron, intestine). The 
 body radially symmetrical, usually with four or six anti- 
 meres; diploblastic (with two primary cell-layers); with a 
 single gastrd-vascular cavity and no anus. The body-wall 
 is provided with peculiar stinging structures the nemato- 
 cysts. Examples: Hydra, jellyfishes, zoophytes, polyps, 
 and corals. 4400 species. 
 
 4. Ctenophora (ktenos, a comb; phoreo, I bear). Animals 
 possessing radial combined with bilateral symmetry; with 
 eight radially arranged rows of paddle plates; triploblastic 
 (with three primary cell layers) ; anus usually present. Sea 
 Walnuts or Comb-jellies. 100 species. 
 
 5. Platyhelmia (platys, flat; helminthos, a worm). Flat, 
 bilaterally symmetrical, triploblastic animals; with a single 
 gastro- vascular cavity having no anus, or without a diges- 
 
22 GENERAL ZOOLOGY 
 
 tive system; a very small coelom present. A well-developed 
 system of excretory tubes present. Flatworms : planarians, 
 flukes, tapeworms. 5000 species. 
 
 6. Nemertina. Flattened worms which resemble the 
 Platyhelmia in general appearance, but are more special- 
 ized. An anus is present at the posterior end of the body. 
 The nemerteans. 280 species. 
 
 7. Nematoidea. (Nematos, a thread). Cylindrical or 
 thread-like, bilaterally symmetrical, non-met americ, triplo- 
 blastic worms. A mouth and anus are present and the 
 body is of the " tube-within-a-tube type." The body cav- 
 ity is, however, unlined and hence not a true ccelom. 
 Round- worms: Trichina, hook-worm, vinegar-" eels," etc. 
 1500 species. 
 
 8. Rotifera (rota, a wheel; /ero, I carry). Microscopic, 
 bilaterally symmetrical, metameric, triploblastic animals; 
 with a coelom; two ciliated lobes, which often look like 
 rotating wheels, at the anterior end. The digestive system 
 is tubular, with a mouth, anus, and several distinct regions. 
 There are chewing teeth in the stomach. A pair of water 
 canals are present which serve for excretion. The wheel- 
 animalcules. 500 species. 
 
 9. Brachiopoda (brachion, the arm; pous, a foot). 
 Marine, bilaterally symmetrical, triploblastic animals; 
 which possess a bivalve shell (valves dorsal and ventral) 
 and are usually attached by a stalk. A peculiar horseshoe- 
 shaped structure in front of the mouth gives the name to 
 this phylum. The body is very small in proportion to the 
 size of the shell. The lamp shells, which are mostly extinct, 
 are examples. 500 living species. 
 
 10. Bryozoa (6rwm, moss; zoon, animals) . Small, usually 
 colonial animals, which are mostly marine. The colony 
 usually has a branching plant-like form. At the tips of the 
 branches are the individual animals which have a super- 
 ficial radial symmetry. A number of tentacles surround 
 the mouth and a U-shaped digestive tube leads to the 
 anus. 1200 species. 
 
CLASSIFICATION OF ANIMALS 23 
 
 11. Echinodermata (echinos, hedgehog; derma, skin). 
 Radially symmetrical animals, usually with five antimeres. 
 A large coelom is present, and there is a peculiar system 
 of tubes called the water-vascular system. An anus may 
 be lacking and is often non-functional when present. 
 There is usually a calcareous, spiny skeleton. Starfishes, 
 brittle-stars, sea-cucumbers, sea-urchins, stone-lilies. 4000 
 species. 
 
 12. Annelida (annulus, ring). Metameric, bilaterally 
 symmetrical, triploblastic , soft-bodied worms. Body clearly 
 a tube within a tube i.e., coelom very well developed ; paired 
 excretory organs (nephridia) in each segment of the body; 
 no j ointed appendages. Earthworm, marine worms, leeches. 
 4000 species. 
 
 13. Arthropoda (arthron, joint; pous, foot). Metameric, 
 bilaterally symmetrical animals; with a chitinous exo- 
 skeleton arid jointed appendages; coelom poorly developed. 
 Crustaceans, insects, spiders, centipedes, scorpions, ticks. 
 400,000 species. 
 
 14. Mollusca (molis, soft). Non-met americ, bilaterally 
 symmetrical, triploblastic animals; in which there is a 
 well-developed digestive system with mouth and anus, a 
 small coelom, and usually a calcareous exoskeleton in the 
 form of a shell. Clams, mussels, snails, slugs, devil-fishes, 
 octopi. 60,000 species. 
 
 15. Chordata (chorda, cord or string). Triploblastic, 
 metameric, bilaterally symmetrical animals; with well- 
 developed coelom and an endoskeleton the chorda or other 
 supporting structures. Paired appendages are usually 
 present, and these are often jointed. Fishes, newts, frogs, 
 reptiles, birds, mammals. 36,000 species. 
 
 The relationships of the phyla are shown in Fig. 18. 
 
24 
 
 GENERAL ZOOLOGY 
 
 Mammalia 
 
 Protozoa 
 FIG. 18. Showing the probable relationships of the phyla of animals. 
 
CHAPTER III 
 
 LIFE AND LIVING THINGS 
 
 "Just as the search for the philosopher's stone that was to transmute 
 the baser metals into gold, led through alchemy to the foundations of 
 modern chemistry, and to a richer reward than the long-sought stone, 
 and as the vain pursuit of the elusive elixir vitce, that was to renew youth 
 and vigour and give unending life at the prime, merged into the begin- 
 nings of scientific medicine; so the inquiry into spontaneous generation, 
 or the origin of life, opened up the whole of our modern knowledge of the 
 causation of disease through the discoveries of Pasteur, and onward 
 beyond that laid the broad foundations for the wonderful developments 
 of modern surgery which arose from the noble lif ework of Lister. Millions 
 of lives have been saved, and untold misery and suffering averted, 
 by practical discoveries which arose from apparently purely philosoph- 
 ical enquiries dealing with theories which might have been dismissed as 
 chimerical." Moore. 
 
 What is life? No one knows! Definitions have been 
 formulated which will enable us to separate living from 
 non-living things, but when it comes to knowing why a 
 living thing is alive we have little that is satisfying from a 
 scientific point of view. Things are alive because they 
 have life. It may seem idle to pursue a discussion which 
 in the end will lead us back to where we started, but we 
 may, nevertheless, look with profit into the facts and 
 theories which relate to living things. Because we cannot 
 answer a question is no reason why we should not try. 
 Maybe, sometime, by trying, the answer will be found. 
 
 ORIGIN OF LIFE 
 
 In the past many curious notions have been held in regard 
 to the origin of individual animals. It was at one time 
 commonly believed that small aquatic animals generated 
 from mud, and that decaying meat was transformed into 
 
 25 
 
26 GENERAL ZOOLOGY 
 
 maggots. Van Helmont, a deservedly eminent scientist of 
 the sixteenth century, was a firm believer in such spon- 
 taneous generation, and soberly stated that mice could be 
 produced by placing some dirty linen in a receptacle, to- 
 gether with a few grains of wheat or a piece of cheese. 
 Such crude ideas show the low level of the scientific thought 
 of the time. The belief that organisms commonly origi- 
 nated from inorganic materials or decaying substances was 
 prevalent. 
 
 The first critical experiments which helped to do away 
 with the belief in spontaneous generation were performed 
 by an Italian poet and physician, Franchesca Redi. He 
 demonstrated clearly that fly maggots were not engendered 
 spontaneously in spoiled meat, but developed from eggs 
 deposited by flies. He set three jars on his window sill- 
 one was uncovered; one covered with gauze so that the air 
 might enter freely but no flies could come in; the other was 
 corked tightly. Though the meat in all the bottles spoiled, 
 maggots appeared only in the uncovered jar. The flies 
 were seen to lay their eggs, and the hatching of maggots 
 was observed. Redi's epoch-making discovery seems very 
 simple to us, but it aroused a storm of discussion and dis- 
 pute in his time. 
 
 It would perhaps be expected that the invention of the 
 microscope would at once have furnished the means to dis- 
 prove spontaneous generation, but it had the opposite 
 effect. This instrument opened up a new world of minute 
 " animalculae," and with these the advocates of spon- 
 taneous generation made their last stand. It was found 
 that if dry hay was put in water, cooked to destroy any 
 living germs which might be present, and allowed to stand 
 for a day or two, that the " infusion" was filled with myriads 
 of " animalcules." This experiment was believed to prove 
 that the organisms were generated from the disintegrating 
 hay. Later, however, it was urged that the animalcules 
 might enter the infusion from the air. Experiments were 
 tried in which infusions were sealed up in tubes, both before 
 
LIFE AND LIVING THINGS 27 
 
 and after boiling. Spallanzani, in the middle of the 
 eighteenth century, wrote: "I used hermetically sealed 
 vessels. I kept them for an hour in boiling water, and 
 after opening and examining their contents after a reason- 
 able interval, I found not the slightest trace of animalcule, 
 though I examined with the microscope the infusions from 
 nineteen different vessels." The question was still dis- 
 cussed, however, and even as late as 1858 a French scientist 
 read a paper before the National Academy in which he 
 announced that animalcule had been raised in boiled in- 
 fusions exposed only to " artificial air," or oxygen. He 
 maintained that the organisms could not have arrived as 
 air-borne particles. 
 
 This paper excited the interest of Pasteur and led to the 
 work which finally laid the idea of spontaneous generation 
 to rest. "He showe'd that sterilized cultures always be- 
 came infected when exposed to air; that properly filtered or 
 sterilized air never caused infection; that Alpine air almost 
 free from germs scarcely ever produced a growth of organ- 
 isms; that city air nearly always produced contamination; 
 and that, in absence of added germs from without, culture 
 media remained sterile for years. The sources of error in 
 the work of his opponents were elucidated, and their con- 
 trary results explained on such grounds." Tyndall alfe'# 
 furnished evidence of a physical nature which corroborated, 
 the discoveries of Pasteur. He was trying to get a beam of 
 light which was perfectly free from dust particles and made 
 an apparatus in which the light was passed through win- 
 dows in an air-tight box. When the box was absolutely 
 dust-free there was no fermentation within it, thus demon- 
 strating that the origin of bacteria and similar organisms 
 was due to air-borne particles. 
 
 At the present time, the only generalization we can make 
 concerning the origin of living things is, "all life comes from 
 life." Every living organism originates from a preexisting 
 individual of its own sort. This conclusion is unsatisfying, 
 
28 GENERAL ZOOLOGY 
 
 from a scientific point of view, and, of course, does not ex- 
 plain the origin of life. 
 
 PHYSICAL BASIS OF LIFE 
 
 If we cannot explain the origin of life we may at least look 
 into the nature of living matter. The name, protoplasm, 
 has been given to the complex mixture of chemical com- 
 pounds which makes up the living substance. It is a trans- 
 parent semi-fluid material, somewhat like the white of an 
 egg. At one time there was considerable dispute in regard 
 to the structure of protoplasm. One school maintained 
 that there were granules which were all-important; others, 
 that little fibers were the living elements; a third group 
 that little bubbles were the necessary feature. Modern 
 investigation has shown, however, that none of these things 
 are of prime importance. The same bit of protoplasm may 
 at different times be granular, or reticular, or alveolar. It 
 is more or less clear and jelly-like; it shows spontaneous 
 internal movements; it is alive these are the only constant 
 features to be seen even through the best microscopes. 
 
 Protoplasm is somewhat variable in its composition be- 
 cause it is unstable chemically, and is continually chang- 
 ing. Nevertheless, it shows a general similarity of struc- 
 ture and elemental composition in all plants and animals. 
 It is made up of twelve of the eighty-odd known chemical 
 elements. These are : 
 
 Carbon, oxygen, hydrogen, nitrogen, sulphur 99 percent. 
 
 Chlorine, phosphorus, potassium, sodium, magnesium, 
 calcium, iron 1 per cent. 
 
 Protoplasm shows some chemical peculiarities which are 
 noteworthy. (1) The element carbon has remarkable 
 ability to form extremely complex systems of combinations 
 with other elements and its compounds are most important 
 (Fig. 19). (2) Protoplasm is very labile, or unstable. 
 Living substance may readily incorporate a little more 
 water (or other compound) or lose a little as the necessity 
 
LIFE AND LIVING THINGS 29 
 
 arises. This ability to change continually yet be essentially 
 the same is perhaps the most striking quality of protoplasm. 
 (3) The most important and characteristic chemical com- 
 pounds in protoplasm are colloidal i.e.j they consist of 
 enormous molecules, which mix readily with water but 
 never completely dissolve, as many crystalline substances 
 do. The most important colloidal substances are the 
 proteins. These contain carbon, oxygen, hydrogen, and 
 nitrogen and possess various unique properties. They are 
 made up of enormous molecules which may be readily 
 
 H S W, CH a 
 
 /VHCCCCCOOH HC - CCOOH. 
 
 C H H H H C t H s H 
 
 NH* 
 
 Jso/euc/ne 
 Arginine 
 
 
 CH 3 /V/i A/H 2 H H H 
 
 HCCKCCOOH. H-kCCC CCOOH 
 
 CH 3 H H H H H H 
 
 Leucine Lys/ne 
 
 FIG. 19. Some of the chemical compounds formed when protoplasm breaks 
 down. In these diagrams the significance of the letters is as follows: c, carbon; 
 h, hydrogen; n, nitrogen; o, oxygen. 
 
 modified by adding or eliminating whole groups of atoms. 
 If we compare an atom to a soldier, we may liken a protein 
 molecule to an army corps in which a regiment of infantry, 
 or a hospital unit, or some other part is continually being 
 transferred from one position to another. One like the 
 other has a certain recognizable unity yet the internal 
 arrangements are never continuously the same. 
 
 The chemical compounds which make up protoplasm are 
 not all organic. Inorganic compounds are present in con- 
 siderable quantity. Water is an universal constituent of 
 protoplasm and there are always mineral salts, such as the 
 
30 GENERAL ZOOLOGY 
 
 phosphates, chlorides, and carbonates of calcium, potas- 
 sium, magnesium, and iron. Some jelly fishes are more than 
 99 per cent, water. 
 
 Organic compounds were formerly believed to come only 
 from living things but now the chemist may make thou- 
 sands of them in his laboratory, and many are of vast 
 commercial importance. The chief classes of organic com- 
 pounds are proteins, carbohydrates, and fats. Proteins 
 differ from the others in that they contain nitrogen in 
 addition to carbon, oxygen, and hydrogen. They consti- 
 tute the greater, and most essential, part of the protoplasm. 
 Lean meat and white of egg are examples of substances 
 which are largely made up of proteins. Carbohydrates, of 
 which sugars and starches are examples, have their oxygen 
 and hydrogen in the same proportions (H 2 O) in which they 
 occur in water. Fats, or hydrocarbons, contain the same 
 elements as carbohydrates but in different proportions, so 
 that there is a smaller percentage of oxygen. 
 
 It is possible for the chemist to analyze protoplasm to 
 some extent and to learn the exact formula? for some of the 
 chemical compounds present, but its protein molecules are 
 so large that their exact composition and arrnagement is 
 still a matter of uncertainty in most cases. Here again, 
 then, we are met with an unsatisfactory termination in our 
 search for the fundaments of life. When we try to analyze 
 living substance, it dies, and we have left a number of com- 
 plex compounds (Fig. 19) which, though perhaps simpler 
 than those present during life, are still of such complicated 
 structure that they are imperfectly known. 
 
 MECHANISM AND VITALISM 
 
 Notwithstanding our comparative ignorance concerning 
 the phenomena of life men have not been backward in 
 putting forth theories to explain the operation of living 
 things. There has been long and bitter controversy be- 
 tween the mechanists, who maintain that living matter is 
 
LIFE AND LIVING THINGS 31 
 
 not essentially different from non-living, and the vitalists, 
 who hold that living things possess a vital principle which 
 makes them different from non-living. 
 
 The mechanist believes it is possible that no living ma- 
 chine is beyond experimental analysis. He expects as his 
 knowledge grows broader to know more and more about the 
 mechanism of life, and even hopes, sometime, to be able to 
 make a living organism himself. From a scientific point 
 of view it is certainly not satisfying to believe that all 
 things were created as they are; that living things are alive 
 because they are alive, and that it is of no avail to try to 
 discover how they came to live and are living. It would be 
 equally unscientific to say that living things were not 
 created. There is no proof either way. Yet the mechanist 
 has much on his side. 
 
 The first big victory for the mechanists was the dis- 
 covery that organic compounds can be made synthetically. 
 When Woeler in 1828 made urea in his laboratory, he 
 opened up a new field which has been very fruitful. It had 
 been believed that such substances could only be elabor- 
 ated within the bodies of living organisms. For example, 
 Liebig, who was a man of good standing scientifically, 
 manufactured a beef extract which he claimed was nourish- 
 ing because it possessed something "vital." One of his 
 opponents tested the extract, to see if such was the case. 
 He took a litter of kittens from their mother. Half of 
 them were fed nothing and the others received beef extract. 
 The starved lot lived longer than the others, showing that 
 the extract had little food value, but was stimulating. 
 When it became known that not only urea but many other 
 organic compounds could be made synthetically, people 
 began to doubt the " vital principle." 
 
 There is no question that every organism is a machine. 
 It is composed of matter which may be separated into well- 
 known chemical elements and compounds. It consists 
 of levers, pulleys, and other things which work according 
 to the laws of mechanics. Many complicated activities 
 
3 
 
 32 GENERAL ZOOLOGY 
 
 which were once believed to show the presence of some 
 vital quality are now understood and may be controlled 
 at the will of the experimenter. A turtle's heart may be 
 taken from its body and placed in saline solution where it 
 will continue to beat rhythmically for two or three days. 
 Furthermore, its rate of beat may be regulated. If a cer- 
 tain mineral salt is added to the solution in which it lies, 
 it beats faster; if another is added, it slows down and even 
 ceases to pulsate, but will begin again if the solution is 
 restored to its original condition. 
 
 Another stroke for mechanism came with the knowledge 
 t of the control of " vital" activities through enzymes and 
 ^4jormones. For example, there are in every man a pair 
 ? of glands, the thyroids, on either side of the neck which 
 \ are very necessary to "vital" activity. They give off a 
 substance, thyroidin, which enters the blood stream and is 
 constantly necessary for the proper nourishment of the 
 body. Goitre is the name given to a diseased, swollen 
 condition which may interfere with the proper function- 
 ing of the thyroids. It was in trying to find a cure for 
 this trouble that some interesting facts were discovered. 
 It was found that, if diseased glands were removed, the 
 body would waste away and the patient finally died. But 
 if, after taking out the glands, thyroidin from a healthy 
 sheep was injected hypodermically, there was no dis- 
 turbance. Furthermore the same result could be obtained 
 by grafting a fresh thyroid beneath the skin. Thyroidin 
 is, then, necessary in the body of man, but it makes little 
 difference how it is supplied. The machine needs a certain 
 amount of the substance to run properly but the thyroid 
 gland is not necessary any more than an oil cup is neces- 
 sary for an engine if oil is supplied in some other way. 
 Many other substances control or facilitate various activi- 
 ties which were once believed to be dominated by " vital 
 spirits." 
 
 Arguments might be multiplied which tend to show that 
 living animals are machines, but let us look at the other 
 
LIFE AND LIVING THINGS 33 
 
 side. The vitalist has only one argument no mechanist s 
 has ever made a living thing or brought a dead thing to * 
 life. He believes that even though we could understand 
 living mechanism and its workings completely, we would 
 still not be able to produce life without some preexisting 
 life to start it off. Vitalism involves more than this, 
 there is belief in a fundamental difference between the 
 living and non-living. There must be something ' 'vital' 7 
 present without which an organism will cease to be alive. 
 Some vitalists call this vital principle the soul. There are 
 those who believe that all living things have souls, and 
 others who feel that such a quality is confined to a few 
 of the higher animals or to man alone. 
 
 Until we learn more, there can be no final decision be- 
 tween vitalism and mechanism. The radical mechanist 
 can continue to say that we have no proof of anything vital, 
 or divine, and can assert, if he chooses, that "vitality" 
 is simply a name for a lot of things we do not yet under- 
 stand. The vitalist can still hold that the mechanist has 
 never proved his point living things are different from 
 non-living and, through scientific reasoning, we cannot 
 show why. A scientific attitude compels us to accept 
 nothing until it is proved. Very little is yet proved. If 
 we knew more we probably would not argue so much. 
 Professor Wilson in a recent address said, "And so, if 
 you ask whether I look to a day when we shall know the 
 whole truth in regard to organic mechanism and organic 
 evolution, I answer: No! but let us go forward." 
 
 CHARACTERISTICS OF LIVING THINGS 
 
 It is difficult to formulate absolute differences which will 
 always separate living from non-living objects. It is 
 possible, however, with some confidence to set forth the 
 following six qualities which will serve to characterize living 
 organisms. 
 
 1. Determine Size and Structure. All animals and plants 
 vary to a considerable extent, in fact, it is often said that 
 
34 
 
 GENERAL ZOOLOGY 
 
 no two are alike, yet each species when adult has a certain 
 average size and form which enables one to recognize it. 
 A cat is never as large as a cow; a birch tree is never found 
 with the bark or leaves of an oak. Living things cling to 
 such specificities generation after generation. 
 
 2. Precise Elementary Composition. Living matter, 
 though highly variable, is made up of only twelve of the 
 eighty odd chemical elements available in nature. These 
 
 Excretions 
 
 Oxtjqen 
 
 Oxidation 
 O\erqy 
 
 Rxeccs food material 
 
 FIG. 20. A scheme showing metabolism, or the chemical changes continu- 
 ally going on in protoplasm. No living thing can stand still. It must change 
 continually. 
 
 occur in definite proportions and form only certain kinds 
 of compounds. 
 
 3. Definite Organization. Living organisms are composed 
 of a variety of unlike parts which are generally incapable 
 of independent existence. The smallest unit of organic 
 structure is a cell and in large animals there may be billions. 
 The parts of an animal will not exist separately you can- 
 not remove a kidney or a stomach and have it remain 
 
LIFE AND LIVING THINGS 35 
 
 active because the functioning of such an organ depends 
 upon its surroundings. 
 
 4. Metabolism is the name for the complex of chemical 
 changes continually taking place in living things. Noth- 
 ing which is alive can stand still, it must change. Some 
 changes are constructive (anabolism), some are destructive 
 (catabolism). In general there is an orderly sequence in 
 the transformation of substance. Material is appro- 
 priated, digested (changed chemically and dissolved), and 
 assimilated (or converted into living substance). If any 
 of the material is not fit for food, it is rejected as fceces. 
 In order to furnish energy to transform materials and 
 build substance, oxidation takes place, and this makes 
 
 FIG. 21. Scheme showing the life cycle through which every living thing must 
 
 pass. 
 
 respiration necessary to supply the oxygen. After living 
 substance has been broken down a residue is left which 
 passes out as an excretion (Fig. 20). 
 
 In addition to this continual daily rhythm there is 
 another series of periodic changes which is characteristic 
 of living things. This is called the Life Cycle, during which 
 an organism passes through the stages of youth, maturity, 
 and old age. A young animal has great vitality it grows 
 rapidly and has an abundance of energy. A mature animal 
 is at its best the machine is complete and perfect. Usu- 
 ally at this stage reproduction takes place and a new 
 cycle is started by setting free a young animal. In old 
 age the machine wears out and death finally puts an end 
 
36 GENERAL ZOOLOGY 
 
 to the cycle. Every animal must run in the race of life; 
 and the goal for all is the same death.* 
 
 5. Reproduction. All living things have the power to 
 perpetuate their race. Some animals split themselves in 
 two, or break up into fragments, and each of the pieces 
 grows into a complete new individual. Such multiplica- 
 tion increases the number of organisms easily and quickly. 
 At times, however, most animals have some means of proper 
 reproduction during which there is fusion of products 
 from two individuals to form a third and this new indi- 
 vidual starts out with all the vigor of youth, though its 
 parents may have been at the other end of the cycle. 
 
 6. Adaptability or Reactiveness. All organisms are able 
 to adjust themselves to their surroundings and this ability 
 to meet changing conditions is perhaps the most character- 
 istic attribute of living things. Garden spiders always 
 spin webs of the same type, yet one may fill a square, and 
 another a triangular space. Individuality is thus retained 
 but adapted to varying conditions. The success of any man 
 depends upon his adaptability how well he can meet the 
 difficulties and changing conditions of life. 
 
 * Except in some Protozoa, which may escape death from old age by 
 becoming young again (see Chapter XII). 
 
CHAPTER IV 
 CELLS 
 
 Protoplasm, or living substance, occurs only in the form 
 of cells. These " units of life" are in a certain sense like 
 the bricks from which men make houses. Bricks are of 
 various colors and shapes, they may be built into houses 
 or they may exist as separate chunks of burnt clay, yet 
 anyone recognizes a brick when he sees it. Brick houses 
 may have doors, various arrangements of windows, porches, 
 and other embellishments without changing the essential 
 character of the bricks which furnish the bulk of their 
 mass. 
 
 FIG. 22. Cells scraped from the inside of a man's cheek. 
 
 Living substance always occurs in the form of cells. 
 Many of these " units of life" are independent (Protozoa); 
 they are able to reproduce themselves, carry on metabolism, 
 and maintain all the other activities characteristic of living 
 things, as described in Chapter III. Other cells live as 
 small units in the midst of great living machines and are 
 incapable of independent existence. A recent estimate 
 places the number of the nerve cells in the cortex of 
 the human brain at 9,280,000,000, and yet these would 
 together occupy only about one cubic inch of space. The 
 
 37 
 
38 
 
 GENERAL ZOOLOGY 
 
 greater part of the brain substance is made up of nerve 
 fibers, other types of cells, and- their products. The whole 
 body contains a vast number of such units. Though cells 
 are highly variable in size and shape, they have certain 
 structures which are familiar to everyone who has looked 
 through a microscope at any thing living. 
 
 If you scrape a little of the membrane from the inside of 
 your cheek and place it under a microscope, the fragments 
 will appear like Fig. 22. The individual cells are flattened 
 and irregularly hexagonal in form. Each one has, at the 
 center, a body of somewhat denser material, the nucleus. 
 
 FIG. 23. Examples of cells. A, cartilage; c, cells; s, the substance between 
 the cells. B, a muscle cell from a round- worm; m, muscular or contractile 
 portion; n, nucleus. C, nerve cell; a, axis cylinder process; d, dendrites, or root- 
 like projections; n, nucleus. 
 
 The rest of the cell substance, i.e., the outside part which 
 surrounds the nucleus, is the cytoplasm. Other parts of 
 the body will show cells somewhat like those from the 
 cheek. All have a nucleus and cytoplasm, but there may 
 be great variation in both. Some cells have walls of 
 secreted material around them which forms " cell-walls," 
 others have the cytoplasm drawn out into long projections, 
 or vary in other ways. Fig. 23 shows a few examples of 
 cells. 
 
 Fig. 24 gives a general scheme of a typical cell with all 
 
CELLS 
 
 39 
 
 the structures commonly present. The cytoplasm is usu- 
 ally made up of fibers, granules, and fluid plasma. Other 
 bodies such as crystals, oil globules, and water vacuoles, 
 may be present. Such " extra" bodies are called plastids, 
 metaplasm, etc. The nucleus is covered by a membrane 
 and has, besides some fluid plasma, a network of linin 
 fibers to which are attached irregular masses of chromatin. 
 Just outside the nucleus is the centrosphere which contains 
 two centrosomes. 
 
 CELL 
 
 MEMBRANE: 
 
 NUCLEAR _ i 
 MEMBRANE. 
 
 PLASTIDS'- 
 
 CYTOPLASMIC 
 ' NETWORK 
 
 FIG. 24. A typical cell showing all the parts. 
 
 There is a division of labor among the structures men- 
 tioned. The plasma membrane on the outside of a cell 
 is semi-permeable, allowing certain substances to pass into 
 or out of the cytoplasm but not admitting others. The 
 cytoplasm does the work which is the particular task of 
 each cell. In a muscle cell it is specialized for contraction; 
 in a nerve cell it carries the nervous impulses. The cyto- 
 plasm may also store food or even waste products. The 
 general function of the nucleus appears to be the control 
 of synthetic metabolism. A cell without a nucleus may 
 work until its reserve energy is exhausted, but it can never 
 build up any new material. The chromatin in the nucleus 
 
40 GENERAL ZOOLOGY 
 
 appears to be the chief bearer of hereditary qualities from 
 one cell to another. How the transfer of hereditary char- 
 acters is believed to take place will be explained later 
 (Chapter XXX). The centrosomes are apparently inac- 
 tive except when the cell divides. They then commonly 
 form dynamic centers which are concerned with the 
 separation of the chromatin material. 
 
 Cells rarely exceed the size which is usual for their par- 
 ticular kind. The largest in animals, such as some nerve 
 cells, are two or three feet long. Most cells, however, are 
 much smaller and usually cannot be seen with the naked 
 eye. The smallest bacteria are less then one twenty-five 
 thousandth of an inch in diameter. When a cell has grown 
 so that it threatens to exceed the maximum set for its kind 
 by natural causes, it divides. Cell-division is important 
 in several ways. It allows cells to grow without becoming 
 too large, and it increases the number of cells. In many 
 celled animals, growth is closely bound up with cell- 
 division and cell-differentiation. Cell-division is, there- 
 fore, a very important biological phenomenon. There are 
 two types: mitotic and amitotic. 
 
 MITOSIS OR INDIRECT CELL-DIVISION 
 
 Mitotic cell-division is a rather complicated process 
 which results in the accurate division of the chromatin of 
 the nucleus and a somewhat less precise separation of the 
 other parts of the cell. The changes which take place are 
 grouped under five stages (shown in Fig. 25) which may be 
 described as follows: 
 
 1. Resting Cell. A resting cell ready for division does 
 not differ from any other cell, except that the nucleus is 
 large in proportion to the cytoplasm. The centrosomes 
 lie at one side of the nucleus and the chromatin is scattered 
 irregularly through the linin network. 
 
 2. Prophase. The centrosomes move apart and take 
 positions on opposite sides of the nucleus. Long strands 
 
CELLS 
 
 41 
 
 stretch out from them in all directions and in the zone 
 between the centrosomes these form a framework, the 
 spindle. The chromatin has meantime formed a long 
 thread, the spireme, and this later breaks up into a number 
 of compact rods of chromatin material, the chromosomes. 
 The nuclear membrane now breaks down and the chromo- 
 somes arrange themselves around the center of the spindle 
 halfway between the centrosomes. 
 
 3. Metaphase. Each chromosome splits into two equal 
 parts. 
 
 >ndle 
 
 Spin 
 
 chromosomes 
 
 Prophose 
 
 F 
 Mefophase A/iapha^e Te/ophase 
 
 FIG. 25. Mitosis, or indirect cell division. The ehromatiii is very accurately 
 divided because it is the bearer of hereditary qualities. 
 
 4. Anaphase. The new chromosomes formed by split- 
 ting in the last stage migrate toward the nearest centro- 
 some. Thus half go toward each pole of the cell. 
 
 5. Telophase. The chromosomes break up into scattered 
 bits of chromatin and a nuclear membrane is formed about 
 each of the two groups. The centrosomes divide and the 
 new pairs come to rest beside their respective nuclei. A 
 separation takes place in the cytoplasm between the two 
 nuclei, thus completing mitosis and forming two cells. 
 
 It seems strange that the division of a minute bit of liv- 
 ing matter should involve such a complicated mechanism 
 
42 GENERAL ZOOLOGY 
 
 as has been described. The purpose of mitosis is appar- 
 ently to secure the accurate division of the chromosomes 
 so that each daughter cell may receive exactly the same 
 amount of chromatin. The chromosomes are believed to 
 be the chief bearers of hereditary qualities and have been 
 known as the " vehicles of inheritance." This perhaps 
 explains why such care is taken to insure accurate division. 
 There is a characteristic number of chromosomes for 
 each plant or animal and when one of its cells divides the 
 same number is always reformed from the scattered chro- 
 matin of the nucleus. Thus the number in several animals 
 is as follows: 
 
 Ascaris, a nematode 2 or 4 Rana, a frog; and man 24 
 
 Aphis, a plant louse 8 Lumbricus, an earthworm 3% 
 
 Gryllotalpa, a cricket 12 Crepidula, a snail 60 
 
 Rat 16 Artemia, a brine-shrimp 168 
 
 It is known that species in the same genus show constant 
 differences in the number of chromosomes. Each shows its 
 characteristic number generation after generation. Some- 
 times there is an odd chromosome. This peculiarity is 
 particularly characteristic of the sex cells of certain animals. 
 In such cases, the minute male sex cell, or spermatozoon, 
 may carry the odd chromosome and is believed to produce 
 a female when it "fertilizes" an egg; one which has none 
 will give rise to a male. The odd chromosome, therefore, 
 perhaps serves to determine sex. 
 
 AMITOSIS OR INDIRECT CELL-DIVISION 
 
 When the cell divides without mitosis the process is very 
 simple. The nucleus elongates, becomes constricted in the 
 middle like a dumbbell, and finally is pinched in two (Fig. 
 26). The two daughter nuclei may differ greatly in size 
 and. in the amount of chromatin they contain. Amitosis 
 often takes place in old, weak, or degenerate cells and may 
 be looked upon, at least in some cases, as a sign of a general 
 wearing out of the cell mechanism. Some students of cell 
 physiology dispute this interpretation, however. 
 
CELLS 43 
 
 Amitotic cell-division is often a means of increasing the 
 nuclear surface in a cell. In many active cells the nucleus 
 becomes very much branched, and it is only a step beyond 
 this to have it break up into pieces. It will be remembered 
 that the nucleus controls synthetic metabolism in the cyto- 
 plasm around it. The cell can build up faster with the in- 
 creased surface brought about by division, just as man can 
 freeze ice cream faster with small pieces of ice than he can 
 with large chunks. 
 
 Cell-division does not take long in some cases from ten 
 minutes to half an hour. Many cells in your body have 
 divided mitotically since you started to read this chapter. 
 As to differences between the two types mitosis is pri- 
 
 Fio. 26. Amitosis, or direct cell division. The nucleus elongates and pinches 
 in two. A membrane is formed between the two new nuclei, thus making two 
 cells. Amitosis is a quick method of increasing nuclear surface. 
 
 marily for the exact division of the chromatin; amitosis is 
 chiefly for increasing the amount of nuclear surface in pro- 
 portion to the cytoplasm. 
 
 THE CELL THEORY 
 
 The modern "cell theory" asserts that cells are the units 
 which build every living organism. No plant or animal 
 exists which does not have the protoplasm divided upon 
 into cells. Though this view is generally accepted by the 
 scientific world, it is less than a hundred years since it was 
 pronounced. 
 
 In 1665 an Englishman named Hooke, when making a 
 careful examination of a piece of bark, discovered that it 
 was made up of little compartments arranged in regular 
 
44 GENERAL ZOOLOGY 
 
 rows (Fig. 12). Likening these to the rooms in monasteries 
 Hooke gave them the name, cells. 
 
 Later workers found cells in various plants and animals, 
 but they were merely looked upon as. interesting features 
 without particular importance. It was in 1833 that 
 another Englishman, Brown, discovered the nucleus, which 
 he described in certain plant cells. Not until 1838-1839 
 was the Schleiden-Schwann Cell Theory brought before the 
 scientific world by the two Germans whose name it bears. 
 In 1838 Schleiden asserted that all plants were made up 
 of cells; and in the next year, Schwann published a great 
 work which convinced everyone that not only plants but 
 animals as well were made up wholly of cells. 
 
 No generalization, unless it be Darwin's " Origin of 
 Species, " has so stimulated biological investigation as the 
 cell theory. A new field was opened up which led to the 
 discovery of the method of the fertilization of eggs and other 
 fundamental matters which had not been previously 
 understood. 
 
 It is unfortunate that the name "cell" was given to pro- 
 toplasmic units. One thinks of a cell as a box or hollow 
 thing. A biological cell, however, is cytoplasm and nucleus. 
 The cell-wall, which originally gave the name, is not a con- 
 stant feature, and in animal cells is more often absent than 
 present. 
 
CHAPTER V 
 
 PHYLUM ARTHROPODA, CLASS CRUSTACEA, 
 LAW OF BIOGENESIS 
 
 The animals belonging to the phylum Arthropoda are 
 readily distinguished from all others by the presence of 
 an exoskeleton and by the uniform occurrence of paired, 
 jointed appendages arranged metamerically. This is an 
 important phylum numerically for it contains more species, 
 than* all the rest of the animal kingdom 400,000 or more. 
 It includes the crayfishes, crabs, centipedes, insects, spiders, 
 ticks, scorpions, etc. 
 
 In order to understand the general structure and rela- 
 tionships of the arthropods, examine Fig. 27. This shows 
 the structure of an hypothetical ancestor which may be 
 supposed to have given rise to all the existing groups of 
 arthropods. This imaginary animal was worm-like, meta- 
 meric, and had a tubular digestive system with a mouth 
 at one end and an anus at the other. There was a hard 
 exoskeleton, covering every segment of the body, and 
 metameric appendages. The appendages were rather 
 broad and flat; each consisted of a jointed basal piece and 
 two distal portions hence, was a biramous appendage. 
 
 Anterior and posterior ends were well differentiated. 
 Several segments at the anterior end had grown together 
 to form a head which bore a pair of antennae, a pair of 
 eyes, and a pair of biting jaws or mandibles. A pair or 
 two of appendages, the maxillae, which were added to the 
 head when the metameres just behind fused with it, had 
 come to serve with the mandibles as " mouth parts" for 
 manipulating the food. At the posterior end of the body 
 the appendages had perhaps become specialized for purposes 
 of reproduction. 
 
 45 
 
46 
 
 GENERAL ZOOLOGY 
 
 Inside the body there was a tubular digestive system 
 
 extending from mouth to anus. 
 This began as a gullet or 
 esophagus, was enlarged a little 
 behind the head to form a 
 stomach, and continued 
 through the rest of the body 
 as a straight intestine. Above 
 the alimentary canal there was 
 a long tubular heart which 
 received blood through paired 
 openings in each metamere and 
 pumped it forward. The ner- 
 vous system consisted of a 
 " brain" (the supraesophageal 
 ganglion), dorsal to the esopha- 
 gus, and a chain of metameric 
 masses of nerve cells, or gang- 
 lia, on the ventral side of the 
 alimentary canal. There were 
 nerves from the ventral chain 
 of ganglia to each of the ap- 
 pendages and to the internal 
 organs; branches from the 
 brain supplied the optic (or 
 eye) ganglion, the antennae, 
 and some of the mouth parts. 
 There was a pair of kidneys 
 in each metamere which opened 
 to the outside through pores 
 in the exoskeleton. Near the 
 posterior end of the body were 
 a pair of tubes (the gonads, or 
 sex glands), which shed egg- 
 or sperm-cells through the 
 anus. 
 
 From such an ancestor the five existing classes of arthro- 
 
ARTHROPODA 47 
 
 pods probably arose and have become differentiated by 
 specializing along particular lines. This does not mean 
 that modern arthropods have necessarily become better 
 than their remote ancestors. Specialization, in zoology, 
 means that an animal is far from primitive ancestral con- 
 ditions. Some specializations have been along progres- 
 sive lines, others have made arthropods simpler in struc- 
 ture. Modifications have not taken place hit or miss in 
 any direction, but have followed rather definite lines. The 
 metameres of the body have been grouped into three 
 regions the head, thorax, and abdomen. In some cases 
 the head and thorax have again fused so that there are 
 only two regions the cephalo thorax and abdomen. The 
 appendages have been greatly modified in many instances 
 one branch has been lost, so that an uniramous condition 
 has come about; often the segments are lost, combined, or 
 greatly changed in shape; sometimes the appendages are 
 wholly absent from the abdomen. In most arthropods 
 the ancestral number of mouth parts has been found to be 
 insufficient and the walking legs just behind have been 
 pressed into service. In one group five pairs of thoracic 
 legs have been turned forward to help the mouth and only 
 three are left for walking. The nervous system has shown 
 a tendency to fusion in some arthropods the ventral 
 ganglia are no longer distinct but have fused into one great 
 mass of nervous tissue. The sense organs (eyes, ears, etc.) 
 have become very complicated in some cases. The kidneys 
 seldom retained the primitive metameric arrangement but 
 have usually been restricted to a single segment of the 
 body, or wholly lost and replaced by a new type of excretory 
 organ which opens into the intestine. The arthropods \ 
 have, then, become specialized by losing parts of the body, ) 
 by adding new organs, by condensation and combination, / 
 and in other ways but all have retained the same general / 
 fundamental plan of structure, which shows that they are/ 
 related. ^ 
 
 As the arthropod line developed, it early split into two 
 
48 
 
 GENERAL ZOOLOGY 
 
 groups. One remained aquatic, finally developing gills for 
 respiration in the water, and gave rise to the class Crustacea. 
 All the other arthropods became terrestrial and were hence 
 
 ^^^^ 
 Onycophora 
 
 dncestral arthropod 
 
 FIG. 28. A scheme to show how the arthropods have developed from their 
 ancient ancestor. The branches are not intended to represent actual relation- 
 ships, but to indicate the lines of specialization which have been followed. 
 
 obliged to acquire the ability to breathe air. Most of the 
 air-breathing arthropods have developed a branching 
 system of minute tubes, the tracheae, which carry air to 
 
ARTHROPODA 49 
 
 all parts of the body. We can, therefore, make two 
 divisions of the Phylum Arthropoda: 
 
 Division I. Branchiata breathing in water, through the general 
 
 surface of the body or through gills. 
 Division II. Tracheata breathing air, through tracheae or in some 
 
 other way. 
 
 The first division contains only the class Crustacea, and 
 the Tracheata therefore include the other four classes. 
 Two of the latter have retained the ancestral worm-like 
 form, with metameric appendages, but are easily dif- 
 ferentiated from each other because one class (Myriapoda) 
 has lost the primitive metameric kidneys completely and 
 the other has retained them (Onycophora). The other 
 two classes have departed from the primitive worm-like 
 form and are highly specialized. They have lost the 
 appendages on the posterior end of the body; lost the 
 metameric kidneys and developed a new type of excretory 
 organ (the Malpighian tubules); and become specialized 
 in many other ways. One of these last two classes (Insecta) 
 has kept only six walking legs and has developed wings, 
 the other (Arachnida) has retained eight walking legs and 
 never possesses wings. We may at present, therefore, 
 classify the Phylum Arthropoda on the following basis: 
 
 Division I. Branchiata. Aquatic, breathe through skin or gills. 
 
 Class 1. Crustacea. Aquatic arthropods, with primitive but 
 
 nori-metameric kidneys. 
 
 Division II. Tracheata. Terrestrial (or aquatic), breathe air, usually 
 through tracheae. 
 
 Class 2. Onycophora. Worm-ljke, with paired metameric kid- 
 neys throughout the body. 
 
 Class 3. Myriapoda. Worm-like; excretion through Malpig- 
 hian tubules which .open into the intestine just behind the 
 stomach. 
 
 Class 4. Insecta. Three pairs of walking legs; one or two pairs 
 of wings usually present; no metameric kidneys, Malpighian 
 tubules present. 
 
 Class 5. Arachnida. Four pairs of walking legs; no wings; 
 Malpighian tubules present. 
 
50 
 
 GENERAL ZOOLOGY 
 
 The relationships of the classes of arthropods are shown 
 in Fig. 28. Let us now consider each of them in more detail. 
 The remainder of this chapter will be devoted to the 
 Crustacea. 
 
 CLASS 1. CRUSTACEA 
 
 This class may be defined as including arthropods which 
 live for the most part in water, and usually breathe through 
 gills. We can perhaps best get an idea of one of these 
 animals by considering the activities of an example which is 
 familiar to everyone the common crayfish, or crawfish, 
 
 FIG. 29. Crayfishes. 
 
 found in fresh water the world over. The daily life of this, 
 or any other, animal is shaped primarily for three ends 
 self-maintenance, self-protection, and race preservation. 
 To use the words of Alcock "the three great exigencies: 
 to find something to eat, to avoid being oneself eaten, and 
 to disseminate one's species, give rise to a perpetual struggle 
 in which the fittest is successful. " 
 
 THE CRAYFISH, Cambarus virilis Hagen 
 
 Self -maintenance. Crayfishes (Fig. 29) eat nearly any- 
 thing that is available. They prefer animal food, and lurk 
 
CRUSTACEA 51 
 
 in sheltered nooks waiting for some unwary fish or insect 
 to swim by. They catch such animals in their big pin- 
 chers, or chelipeds, and tear them to pieces. If living 
 animal food fails, crayfishes eat plants, or any sort of dead 
 organic matter and are hence good scavengers. Fine 
 particles of food are retained by the little bristles along the 
 margins of the chelipeds and the mouth parts, so that noth- 
 ing of value is lost. 
 
 A crayfish is well equipped with sense organs to find its 
 food. There are two pairs of antennae at the anterior end 
 of the body and the shorter pair, the antennules, is not only 
 provided with sensory bristles for receiving touch stimuli, 
 but possesses organs of smell as well. A crayfish is able 
 to taste with the nerve endings on the mouth parts. The 
 compound eyes are set on stalks so that they can be moved 
 about in different directions. Each is made up of hundreds 
 of minute but complete eyes. Together these little eyes 
 permit a crayfish to see a complete but somewhat broken 
 picture of its surroundings a mosaic image. If a crayfish 
 is blind in one of its simple eyes, there is a space in its field 
 of vision where nothing is visible. 
 
 Once the food is secured, it must be converted into living 
 substance or modified so as to serve as fuel to run the living 
 machine. It is pulled apart by the chelipeds, torn into 
 shreds by the mouth parts, then passes down into the stom- 
 ach, where it is further chewed by three strong projecting 
 teeth and strained between little bristles. It is mixed with 
 the digestive secretion from the " liver" and finally absorbed 
 through the walls of the intestine. What cannot be digested 
 and absorbed passes out of the anus as faeces. The digested 
 food which passes through the wall of the alimentary canal 
 enters great blood spaces, the sinuses, and ultimately reaches 
 the small muscular heart, in the middle of the back, which 
 pumps blood to all parts of the body. The heart works only 
 when it contracts. It is passive during expansion, being 
 stretched out by little elastic strands which lead from it to 
 the walls of the chamber in which it lies. The blood of the 
 
52 GENERAL ZOOLOGY 
 
 crayfish is colorless, but carries food, oxygen, and waste 
 products. 
 
 A crayfish labors under one great disadvantage the 
 exoskeleton is so rigid that it cannot grow from day to day, 
 as do animals with pliable skins. This shell is an admir- 
 able protection but prohibits growth. This difficulty is 
 surmounted by a periodical casting of the entire exo- 
 skeleton including the lining of the esophagus and stom- 
 ach. After such a moult the crayfish is a "soft shell" for a 
 day. During this period the body absorbs a great deal of 
 water, and there is very rapid increase in size. The shell 
 then hardens and no more growth can take place until the 
 next moult. Just before the shell is shed a little stony 
 secretion, the gastrolith, is formed on either side of the 
 stomach. This is a reserve supply of lime which is rapidly 
 transferred by the blood to the new shell after moulting. 
 
 The crayfish has a special respiratory system for securing 
 oxygen, which consists of a number of plume-like gills along 
 the sides of the thorax. These delicate breathing organs 
 are protected by the hard shell, or carapace, which covers 
 the cephalothorax. One of the mouth parts has a little 
 paddle which projects into the respiratory chamber and by 
 its movements keeps a current of water passing forward over 
 the gills and out under the head. The gills are full of 
 moving blood which receives oxygen from the passing 
 water. The carapace fits over the thorax much as a man's 
 coat covers his body, and the gill chamber is kept from 
 getting dirty by little bristles along its edge which strain 
 the incoming water. 
 
 The excretions resulting from general metabolism in the 
 crayfishes' body are eliminated through the " green gland" 
 a form of kidney consisting of a long tube which is ex- 
 panded into a bladder near the discharg ; ng end. Excre- 
 tions are absorbed from the blood by the green gland and 
 passed out through a pore which opens through the basal 
 segment of the second antenna. There is only one pair of 
 green glands and these are wholly within the head. 
 
CRUSTACEA 53 
 
 From what has been said, it is apparent that a crayfish 
 is well able to find substances fit for food and to reject that 
 which is unfit. Elaborate and specialized systems of 
 organs for digestion, assimilation, circulation, respiration, 
 and excretion are present. 
 
 Self -protection. The solitary crayfish has many ene- 
 mies fish which seek to devour it, parasites which suck 
 its blood or sap its vitality by infesting the interior of the 
 body, minute moulds or bacteria which cause diseased 
 conditions. How does it escape these? 
 
 Larger enemies are usually avoided by hiding in crevices 
 between stones or water plants. If a crayfish is dislodged 
 from its retreat, it swims quickly backward by flipping the 
 powerful tail fin and often finds a new hiding place (Fig. 29). 
 But if captured, it does not surrender without a struggle. 
 The great claws inflict painful wounds and the smaller legs 
 scratch the cornered animal fights literally " tooth and toe- 
 nail," until it is bested or escapes. Even if a leg or two is 
 lost the damage is not serious for a new one is formed 
 beneath the exoskeleton, and after the next moult is nearly 
 as large as before. On account of the large spaces within 
 the body, a crayfish easily bleeds to death if the body wall 
 is punctured. In the appendages, however, excessive 
 bleeding is prevented by a special arrangement. A crushed 
 leg is thrown off at a " casting joint" near its base, where 
 there is a very small aperture from the blood sinuses and 
 the opening quickly closes. 
 
 The color of a crayfish is usually more or less like the 
 surroundings, and can be changed somewhat by the con- 
 traction or expansion of little pigment cells in the skin. 
 Such " protective resemblance" makes it less conspicuous. 
 
 The insidious enemies which attack the crayfish as para- 
 sites are warded A off chiefly by sanitary measures. Scrupu- 
 lous cleanliness is observed. There are special combs and 
 bristles on the appendages for cleaning the exoskeleton 
 and it is continually 'scraped and polished. If there is a 
 break in the outside covering, the way is opened for various 
 
54 GENERAL ZOOLOGY 
 
 moulds and bacteria to gain an entrance. These are fought 
 off by little motile cells, the amoebocytes, until the wound 
 can heal. 
 
 But a crayfish must struggle against other things besides 
 aggressive enemies and stealthy parasites. It. is obliged 
 to meet the changing conditions of environment so as not 
 to be destroyed by storms, or cold, or other unfavorable 
 variations. Crayfishes are resistant to low temperatures 
 and live a slow, yet active, life beneath the ice where there 
 is a winter season. They do not fare so well if the water 
 becomes too warm, and are therefore less abundant in the 
 tropics than elsewhere. They can live for a considerable 
 time out of water, particularly if they can rest in some 
 damp place, as among fallen leaves. 
 
 Crayfishes quickly succumb to foul water and sometimes 
 migrate over the land to escape from a stagnant pool. If 
 the pond in which they have been living dries up, they 
 burrow into the soil and remain quiet until water is again 
 available. Some crayfishes have become expert diggers 
 and make burrows far from water; in fact, some enter the 
 water only during the breeding season. Such species often 
 do damage by making holes in fields or by perforating dykes. 
 They are easily killed by dropping unslaked lime down their 
 burrows and closing them tightly. 
 
 Race Preservation. Crayfishes are of separate sexes. 
 Mature males are readily distinguished from females by 
 their larger chelipeds and narrower abdomens. The repro-*J 
 ductive gland, or testis, of the male lies in the posterior 
 portion of the thorax and is connected by two tortuous 
 ducts, the vasa deferentia, with small pores in the basal 
 joints of the last pair of walking legs. The first two pairs 
 of abdominal appendages in the male are peculiarly modi- 
 fied so that they may be used in transferring sperm to the 
 female. 
 
 During the breeding season the males move about 
 actively in search of females. When one is encountered, 
 a little packet (the spermatophore) full of sperm cells is 
 
CRUSTACEA 55 
 
 transferred to her. In some species the females have a 
 small pouch in the exoskeleton (the annulus) into which 
 the spermatophore is thrust by the male. 
 
 A female has an ovary in the thorax which connects by 
 two short ducts with openings in the basal segments of the 
 third pair of walking legs. After receiving a spermato- 
 phore she prepares with great care to lay eggs. The 
 abdomen and its appendages are cleaned for two or three 
 days by scraping them with the last pair of walking legs. 
 The female then lies on her side, bends the abdomen a little, 
 and secretes a gelatinous apron over the underside of the 
 body (Fig. 29). The eggs are then ejected from the opening 
 in the third legs; the spermatophore dissolves and the 
 sperm cells "fertilize" the eggs inside the gelatinous apron. 
 
 The jelly of the apron soon shrivels and breaks up; the 
 eggs becoming fixed to the abdominal appendages by little 
 stalks. The mother crayfish takes good care of the eggs; 
 they are kept clean and frequently aerated by elevating 
 the abdomen and waving the appendages which bear them 
 (Fig. 29). There is yolk material stored within each egg- 
 shell to nourish the developing embryo until it hatches. 
 When the little crayfish emerges, it is attached to the old 
 egg-shell by a slender filament which issues from the anus. 
 It remains thus fastened to the mother even after its first 
 moult (Fig. 30), but when the skin is shed the second time, 
 the little creature is free and soon able to shift for itself. 
 A female crayfish carries from three to six hundred eggs 
 at a time and after becoming mature usually breeds 
 annually. A crayfish usually lives to be six or seven years 
 old. 
 
 A mature crayfish reacts to its surroundings so as to be 
 successful as long as possible, and thus continue its race. 
 How much mental ability does it display in responding 
 to the stimuli which it receives ? Very little. Its behavior 
 is largely made up of reflexes and instinctive activities. 
 If a large object appears in its field of vision, the crayfish 
 crouches down into its crevice; if a small animal moves, 
 
56 GENERAL ZOOLOGY 
 
 it rushes forth, seizes the intruder and tries to eat it. When 
 hunger is satisfied the crayfish sits in its lair without any 
 movement, except for respiration, for days at a time. Its 
 activities are for the most part strictly utilitarian. Yet a 
 crayfish has some ability to profit by experience. If a large 
 shadow is cast over one repeatedly, it first avoids the shadow 
 at each repetition and puts itself in an attitude for defense, 
 but finally learns to ignore this new thing which has come 
 into its life. Professor Yerkes taught a crayfish to go 
 
 Fio. 30. A young crayfish attached to a portion of one of its mother's 
 abdominal appendages. Until after its second moult the young crayfish is 
 firmly fastened by an "anal filament." (Adapted from Andrews.) 
 
 through a simple labyrinth where it had equal chance to 
 enter a blind alley or to go through an opening into the 
 water. At first it took the wrong course half of the time, 
 but after two hundred and fifty trials made no more 
 mistakes. 
 
 If we compare a crayfish with the ancestral arthropod 
 described at the beginning of this chapter, we see that it 
 is specialized in several respects. The abdomen is the 
 most primitive part of the body, being clearly metameric 
 with a pair of biramous appendages on each segment. The 
 thorax is overgrown by the great carapace and shows evi- 
 
CRUSTACEA 57 
 
 dence of segmentation only on the ventral side. The 
 thoracic appendages are specialized; most of them have lost 
 the outer branch and become uniramous. Yet with careful 
 examination in the laboratory we can demonstrate that a 
 fundamentally similar plan of structure is shown by all 
 of the appendages on the body of the crayfish, except the 
 eye. If time permitted it would be possible to show many 
 other ways in which the crayfish is specialized, but we must 
 pass on to consider some of its relatives. 
 
 CLASSIFICATION OF CRUSTACEA 
 
 The Crustacea have in their specialization followed various 
 lines and we can divide the class into two great groups, or 
 subclasses, and a large number of orders. It will probably 
 not be profitable for the student to try to learn the follow- 
 ing list, but it is given in full in order that one may be able 
 to see for himself what a degree of diversity has been at- 
 tained in ihigL most primitive class of arthropods, contain- 
 ing only about 16,000 living species. Before we pass to 
 the table of classification it will be well to point out the 
 chief lines which specialization has followed : 
 
 1. The heart has been greatly shortened so that in some crustaceans 
 it occupies only one or two metameres. 
 
 2. The kidneys are always restricted to one pair. 
 
 3. Gills may be developed along the sides of the thorax, or on the 
 thoracic or abdominal appendages. 
 
 4. The primitive biramous appendage is frequently modified by the 
 dropping out of the external ramus or by the loss of segments. 
 
 5. A carapace may be developed. This may cover only a part or 
 the whole of the thorax, or be large enough to enclose the entire body. 
 
 6. There is a tendency to acquire mouth parts at the expense of the 
 walking legs. 
 
 7. The appendages are often lost on the abdomen. 
 
 8. The eyes sometimes become stalked. 
 
 With these points in mind let us pass on to the classifica- 
 tion of the Crustacea. 
 
 Trilobita. This group of extinct arthropods was ex- 
 
58 
 
 GENERAL ZOOLOGY 
 
 tremely variable, the number of body segments ranging 
 from four to thirty-one. Antennae, compound eyes, and 
 biramous appendages were present. All trilobites were 
 marine and none have been alive for the past several 
 million years. Recent Crustacea are classified as follows : 
 
 DiHerogammarus Combarus Ca/linectes 
 
 FIG. 31. Examples of the Class Crustacea. 
 
 Subclass I. Entomostraca. Number of metameres variable; kidney 
 opens in base of maxillae; no gastric mill in stomach. 
 
 Order 1. PHYLLOPODA. With ten to thirty pairs of leaf -like 
 
 feet; a carapace often present. Examples: Eubranchipus, 
 
 Estheria (Fig. 31). 
 Order 2. CLADOCERA. Second antenna? large, biramous, and 
 
 used for swimming; body usually enclosed in a bivalve shell. 
 
 Examples: Daphnia, Bosmina (Fig. 31). 
 Order 3. OSTRACODA. Only seven pairs of appendages. Body 
 
 completely enclosed in a bivalve shell. Example: Cypris 
 
 (Fig. 32). 
 
CRUSTACEA 59 
 
 Order 4. COPEPODA. With large uniramous antennae which are 
 used for swimming; abdomen without metameric appendages. 
 Examples: Cyclops (Fig. 31), Diaptomus. 
 
 Order 5. CIRRIPEDIA. Fixed or parasitic; body enclosed in a 
 calcareous, non-metameric shell; barnacles. Example: 
 Balanus (Fig. 31). 
 
 Subclass 2. Malacostraca. Usually of large size; with five metameres 
 in the head, eight in the thorax, and six in the abdomen; with a 
 grinding apparatus, the gastric-mill, in the stomach. 
 
 Order 6. NEBALIACEA. Small shrimp-like, marine crustaceans, 
 with an extra metamere in the abdomen; carapace well 
 developed. 
 
 Order 7. ANASPIDACEA. With distinct thoracic segments, 
 stalked eyes, and no carapace. 
 
 Order 8. MYSIDACEA. Small shrimp-like crustaceans; with 
 biramous thoracic legs, and a large carapace. 
 
 Order 9. CUMACEA. Malacostraca without abdominal append- 
 ages, with a small carapace and four or five free thoracic 
 segments. 
 
 Order 10. TANAIDACEA. Malacostraca with all but two thoracic 
 segments free; respiratory chambers on sides of thorax. 
 
 Order 11. ISOPODA. Body usually broad and flat; no carapace; 
 seven free thoracic segments. Examples: Asellus, Porcellio 
 (Fig. 31). 
 
 Order 12. AMPHIPODA. Body usually laterally compressed; 
 with seven free thoracic segments; usually with three pairs 
 of appendages fitted for jumping at the posterior end. 
 Example: Dikerogammarus (Fig. 31). 
 
 Order 13. EUPHAUSIACEA. Thorax covered by carapace; with 
 respiratory chambers at sides; no thoracic legs serve as 
 mouth parts. 
 
 Order 14. DEC APOD A. With first three pairs of thoracic ap- 
 pendages specialized as mouth parts; ten walking legs; 
 carapace covers thorax; stalked eyes. 
 
 Suborder 1. Natantia. Slender, laterally compressed; ab- 
 domen well developed and provided with appendages. Ex- 
 amples: Cambarus (Fig. 31), lobster, shrimps, prawns. 
 Suborder 2. Reptantia. Flattened, short; abdomen small and 
 folded forward under thorax. Examples: Callinectes (Fig. 
 31), crabs. 
 
 Order 15. STOMATOPODA. Five pairs of thoracic limbs serve as 
 mouth parts; six biramous walking legs; carapace covers 
 only part of thorax. 
 
60 GENERAL ZOOLOGY 
 
 LAW OF BIOGENESIS 
 
 The Crustacea may well serve to illustrate one of the 
 great principles of zoology are particularly appropriate, 
 in fact, because the law in question was first discovered 
 while the classification of this class was being investigated. 
 The "Law of Biogenesis, " otherwise known as the " Re- 
 capitulation Theory," holds that ontogeny repeats phy- 
 logeny i.e., that each individual animal in its develop- 
 ment repeats the stages through which its race has passed 
 in its evolution. Huxley once put the idea briefly by 
 saying: " Every animal climb.s its own ancestral tree." 
 
 This law has been of great value to zoologists in helping 
 to work out the true relationships of animals. For example, 
 because man, in his early embryonic development, has gill- 
 like structures at the sides of his neck and a fish-like form, 
 it is believed that he came from what was once a fish-like 
 ancestor which lived in the water. Furthermore, it seems 
 reasonable to believe that all vertebrates came from a 
 common ancestry, because, up to a certain point, the de- 
 velopmental history of all is very similar. The embryonic 
 record is often dim, or warped. There are instances where 
 stages which should come later are sometimes pushed ahead, 
 and embryonic animals sometimes acquire new specializa- 
 tions which their remote ancestors did not possess, but the 
 great landmarks in the embryonic records generally keep 
 their resemblance to the racial, or phylogenetic, history so 
 that the story is readable. 
 
 The value of the Law of Biogenesis is well illustrated in 
 the Crustacea. The barnacles, for example, were until a 
 short time ago of very questionable relationships. Some 
 zoologists believed that they should be placed with the 
 molluscs because they had a hard shell, others that they 
 should be classed with the worms, but no one thought of 
 relating them to crustaceans. Then it was discovered that 
 many entomostracans hatch from the egg as a little six- 
 legged larva, the nauplius. This nauplius larva swims 
 
LAW OF BIOGENESIS 
 
 61 
 
 FIG. 32. Stages in the development of three 
 crustaceans illustrating the Law of Biogenesis, or 
 the Law of Recapitulation. On the first line three 
 eggs are shown. On the second the nauplei are rep- 
 resented which hatched from the respective eggs 
 above. The middle nauplius is a good example of 
 "acceleration in development," for it is already en- 
 closed in a little shell when it hatches. The larvae of 
 its remote ancestors probably had no such shell and 
 this feature, therefore, distorts the embryological 
 record by coming earlier than in the past. The third 
 line represents the adult stages of the first two crus- 
 taceans (A 3 , J3 3 ) and what was at one time probably the adult stage in the an- 
 cestors of the third (C 3 ). The lower figure shows that the free stage shown 
 in the line above has been succeeded by a fixed, or sessile, condition. This 
 last change is accompanied by some degeneration. A, Cyclops; B, Cypris; C t 
 Balanus. 
 
62 GENERAL ZOOLOGY 
 
 around in the water until its first moult, when it acquires 
 more legs and the form characteristic for its particular 
 species. After the nauplius stage some kinds of crusta- 
 ceans become Copepoda, others Ostracoda, etc., but all 
 agree pretty closely up to the time when their nauplius 
 skin is shed. When the embryology of a barnacle came to 
 be studied it hatched from the egg as a nauplius! Since 
 this fact was discovered zoologists have not questioned the 
 right of barnacles to be known as relatives of Cyclops, 
 Daphnia, and other crustaceans. The life history of a 
 barnacle indicates that its race developed to a free swim- 
 ming stage comparable to the adult forms of other entomos- 
 tracans; then took up a sessile existence, specialized along 
 new lines, and even degenerated somewhat because it led 
 an inactive life. Fig. 32 shows larval and adult stages of 
 the barnacle and two of its relatives. 
 
CHAPTER VI 
 
 PHYLUM ARTHROPODA, CLASSES ONYCO- 
 PHORA, MYRIAPODA, INSECTA 
 
 CLASS 2. ONYCOPHORA 
 
 The sole representatives of this class of arthropods are 
 the sixty-odd species of the genus Peripatus. Peripati 
 occur only in certain parts of the world Northern Africa, 
 Northern South America, Mexico, and some of the South 
 Pacific islands. These peculiar worm-like animals live in 
 rotten logs, in crevices in rocks, among fallen leaves, and 
 
 FIG. 33. Peripatus entangling a cockroach in sticky threads squirted from two 
 papillae beneath its head. 
 
 in similar situations. Their food consists of small insects, 
 spiders, etc. They entangle their prey in little sticky 
 threads which are ejected from glands on two papillae 
 beneath the head (Fig. 33). 
 
 The body of Peripatus is cylindrical and worm-like. It 
 has from seventeen to forty pairs of small legs, a pair of 
 jointed antennae, and an eye just beneath each antenna. 
 A single pair of appendages have been modified to serve 
 as mouth parts these act as jaws and tear the food to 
 pieces. Most species of Peripatus bring forth the young 
 alive. There is no metamorphosis the young resembling 
 the parents in their general structure. 
 
 63 
 
64 
 
 GENERAL ZOOLOGY 
 
 Peripatus is of great zoological interest because it is a 
 1 ' missing link." It has many points of marked resemblance 
 to annelid worms, and at the same time possesses character- 
 istics which make it unquestionably belong to the Phylum 
 Arthropoda. The annelidan affinities are shown by: (1) 
 the paired metameric nephridia (kidneys) ; (2) the presence 
 of cilia in the reproductive organs; and (3) the general 
 arrangement of the chief systems of organs. Arthropod 
 characteristics are: (1) appendages modified as jaws; (2) 
 a body cavity mostly converted into large blood sinuses; 
 (3) the presence of tracheae for breathing; and (4) the paired 
 jointed appendages. Peripatus closely resembles the hypo- 
 thetical ancestral arthropod described at the beginning of 
 Chapter 5 (Fig. 27). 
 
 CLASS 3. MYRIAPODA 
 
 This class includes the centipedes and millipeds (Fig. 
 34). These animals are readily distinguished from other 
 
 FIG. 34. At the left a centipede eating a fly; at the right millipedes resting and 
 
 eating a leaf. 
 
 arthropods by the following characteristics: (1) a distinct 
 head with one pair of antennae and simple eyes; (2) a long 
 body composed of many free similar segments ; (3) tracheae 
 for breathing; and (4) Malpighian tubules for excretory 
 organs. The myriapods are divided into three orders: 
 
 Order 1. Diplopoda. These are the millipeds. As the 
 name of the order indicates, they have two pairs of jointed 
 
. MYRIAPODA 65 
 
 legs on each segment; also, on the head, a pair of antennae, 
 a pair of mandibles, and one pair of maxillae. Millipeds 
 are usually cylindrical in form, but some species are flat. 
 They are slow-moving sluggish creatures whose food con- 
 sists entirely of vegetation. They often do damage to 
 crops or greenhouse plants. Julus and Polydesmus are 
 common genera. 
 
 Order. 2. Chilopoda. This order includes the centi- 
 pedes, which have only one pair of legs on each metamere. 
 The body is flat and has maxillipeds, which are a pair of 
 modified legs, in addition to the antennae, mandibles, and 
 maxillae mentioned as occurring in the last order. The 
 
 Heart Malpighian tubule 
 
 Optic gangl,on nteron 
 Antenna /Brain 
 
 35. A centipede with the body wall and appendages of the left side re- 
 moved to show the internal organs. Compare with Fig. 27. 
 
 maxillipeds in centipeds are poison claws. They have 
 little openings at their tips which connect with poison 
 glands within the body. The poison claws are used in 
 killing small insects and spiders for food. The large 
 tropical centipedes inflict painful bites, and have even been 
 known to kill children. 
 
 Order 3. Symphyla. These are small myriapods with 
 only twelve pairs of legs. They are rare, but are of some 
 theoretical interest because they resemble the most primi- 
 tive insects (Aptera) in the structure of their mouth parts 
 and in some other features. 
 
 Many of the myriapods remain with their eggs until they 
 hatch. Some species build little nests by hollowing out a 
 space in the ground or in a rotten log, or by plastering little 
 pellets of mud together to make a dome-shaped case. 
 
66 GENERAL ZOOLOGY 
 
 The Myriapoda (Fig. 35) show many points of progress 
 when compared with our hypothetical ancestral arthropod 
 (Fig. 27). The metameric kidneys have been lost and 
 replaced by Malpighian tubules; tracheae have been de- 
 veloped for breathing; the appendages are all uniramous 
 except one pair of mouth parts; and the different orders 
 show a progressive tendency to make walking legs into 
 mouth parts. 
 
 CLASS 4. INSECTA 
 
 The class Insecta, once known as the Hexapoda (six 
 legs), contains an enormous number of species exceeding 
 all the rest of the animal kingdom. The class is not only 
 large but versatile as well. Insects show all sorts of peculiar 
 adaptations for specialized modes of life. They are found 
 in all parts of the world and live in almost every conceiv- 
 able habitat. They vary in size from a twentieth of an 
 inch to nearly seven inches in length. 
 
 The body of an insect has three distinct regions: head, 
 thorax, and abdomen. The head bears a single pair of 
 antennae, usually two compound eyes, three simple eyes 
 or ocelli, and four different kinds of mouth parts the 
 labrum, which is single and not one of the series of matemeric 
 appendages; the mandibles, maxillae, and labium, which 
 are paired. The thorax is always composed of three seg- 
 ments prothorax, mesothorax, metathorax. Each seg- 
 ment is protected by four exoskeletal plates a dorsal 
 tergum, a ventral sternum, and two lateral pleura. Each 
 thoracic metamere bears a pair of walking legs, and each 
 of the last two frequently bears a pair of wings. The ab- 
 domen typically consists of eleven free segments which are 
 without appendages, except accessory reproductive organs 
 or a sting at the posterior end. 
 
 The mouth parts vary greatly but conform mostly to 
 two types: (1) for biting, as in a beetle; or (2) for sucking, 
 as in a bug. Some insects, however, like the honey bee, 
 may have very specialized mouth parts which may be used 
 
INSECTA 67 
 
 for both biting and sucking. The walking legs have five 
 parts: a proximal coxa, often fixed immovably to the 
 sternum to which it is attached; a- short trochanter; a long 
 femur; a slender tibia; and a jointed tarsus which is usually 
 provided with little hooks or pads at its free end. The legs 
 may be adapted in various ways for grasping, swimming, 
 digging, leaping, or other purposes. The wings arise as 
 outgrowths from the two posterior thoracic segments. 
 They are chiefly of two types: broad for sailing, as on a 
 butterfly; and narrow for rapid propulsion, as on a house 
 fly. They often bear scales or hairs. They may be thin 
 and membranous, thick and heavy for protection, or vary 
 in other ways. The little " veins" which traverse insects' 
 wings are not primarily for carrying blood, but are thicken- 
 ings serving as supporting or skeletal structures. 
 
 The inside of an insect's body is filled with digestive, 
 reproductive, respiratory, circulatory, and excretory organs. 
 The alimentary canal varies in its structure and extent 
 according to food habits. Vegetarians have longer ali- 
 mentary canals than other insects. The parts of the 
 digestive system are: a mouth, or buccal cavity; a slender 
 esophagus; a thin- walled crop; a glandular stomach from 
 which little pouches, or caeca, branch out; and a long 
 slender intestine. At the junction of the stomach and 
 intestine the slender Malpighian tubules discharge their 
 excretions into the alimentary canal. 
 
 The respiration of insects takes place through the tracheal 
 system. This is a very much ramified network of tubes 
 which carries oxygen to all parts of the body. The air 
 enters the tracheae through little openings, the spiracles, 
 along the sides of the abdomen and thorax. An insect 
 cannot be drowned by sticking his head in water; the 
 spiracles must be covered before it will die. Good flyers 
 have their tracheae expanded into air sacs, which make the 
 body light, but insects. which fly little have no such ex- 
 pansions (Fig. 36). 
 
 Insects usually hatch from eggs. As they grow the 
 
68 
 
 GENERAL ZOOLOGY 
 
 entire skin is shed at intervals. After each moult the body 
 enlarges rapidly for a short time, but when the exoskeleton 
 once hardens there is no more growing until the skin is shed 
 again. Insects show three conditions of larval life and 
 
 Fia. 36. Insect respiratory systems. A, a. flying insect with exparisions.or 
 air sacs, on the trachea! tubes; B, an insect which never flies; C, a portion of a 
 tracheal tube enlarged. 
 
 metamorphosis. Ametabolous insects have no metamor- 
 phosis. They are much like the adult in form when hatched 
 from the egg. Heterometabolous insects hatch as nymphs, 
 which are at first wingless, but have gradually larger wings 
 
INSECTA 69 
 
 after each moult, until the adult form is finally attained. 
 Holometabolous insects have a " complete " metamorphosis. 
 They hatch from the egg as a worm-like larva, which feeds 
 for a time; then goes into a resting, or pupal, stage during 
 which no food is taken. The larval structures are com- 
 pletely lost as the pupa is formed. When the pupa moults, 
 an adult insect, or imago, comes out. 
 
 According to recent usage, insects are classified into 
 nineteen orders. These will now be taken up in more or 
 less detail. 
 
 Order 1. Aptera (Fig. 37). These are little ametabol- 
 ous, wingless insects with biting mouth parts. Wingless- 
 
 Fio. 37. Insects belonging to the order Aptera. A, a collembolan; B, a- slicker, 
 or silver-fish; C, a spring-tail. 
 
 ness in this case represents a primitive condition. The 
 little slickers, silver-fish, or fish moths ; the spring-tails ; and 
 the snow-fleas are examples. Slickers eat the glazing on 
 paper and starch from clothes. They are frequently seen 
 in dwellings or factories. Spring-tails are expert jumpers, 
 being able to hop many times their own length. The snow- 
 fleas are sometimes so abundant as to give the snow a 
 characteristic color. 
 
 Order 2. Ephemerida (Fig. 38). The may-flies live 
 near water. They are heterometabolous insects with biting 
 mouth parts. Both the larval and adult may-fly are easily 
 recognized by the long setae which project from the end of 
 the abdomen. There are three of these on the larva, and 
 two or three on the adult. Eggs laid in the water hatch into 
 nymphs which live for about a year, feeding on aquatic life. 
 
70 
 
 GENERAL ZOOLOGY 
 
 A nymph when mature crawls out of the water, splits up 
 the back, and an imago emerges from the old skin. The 
 adult may-fly lives only a day or two and keeps fluttering 
 along the shore, looking for a mate or laying eggs. Its 
 mouth parts are so rudimentary that it is wholly unable to 
 eat. 
 
 Order 3. Odonata (Fig. 38). This order includes the 
 dragon-flies and damsel-flies. Both are largely aquatic, lay 
 
 ODONATA CPHCMfLRIDA TRICHOPTEHA PLECOPTERA 
 
 FIG. 38. Showing the immature and adult stages of the four chief orders of 
 
 aquatic insects. 
 
 their eggs in the water, and have a heterometabolous 
 metamorphosis. Damsel-fly larvse have three paddle-like 
 gills at the end of the abdomen. The adults are sly little 
 insects which fold their gauzy wings up over their backs 
 when at rest. Dragon-fly larvse have three short spines 
 at the end of the abdomen. They swim by drawing water 
 
INSECTA 
 
 71 
 
 in through the anus and then squirting the body forward. 
 The adult dragon-flies rest with their wings stretched out 
 on either side of the body. They are expert flyers and are 
 of benefit to man on account of the great numbers of mos- 
 quitoes they destroy. A dragon-fly catches all its prey 
 on the wing. Its enormous eyes cover the whole of the 
 
 FIG. 39. Termite nests. At the right is a section of a tree and a carton nest; 
 at the left a mound nest is shown. 
 
 head (30,000 facets are present in one species); its long 
 light wings enable it to fly with great swiftness in any direc- 
 tion. The larvsB of all Odonata have a peculiar grasping 
 pair of mandibles, which can be greatly extended to capture 
 food, or folded up like a hinge beneath the head when not 
 in use. 
 
 Order 4. Plecoptera (Fig. 38). The stone-flies are 
 heterometabolous, aquatic insects, with biting mouth parts. 
 
72 GENERAL ZOOLOGY 
 
 They get their name from the habit the larvae have of hiding 
 under stones. 
 
 Order 5. Isoptera (Fig. 39). The termites, or white 
 ants, have four leathery wings or are wingless, possess biting 
 mouth parts, and are heterometabolous. They are most 
 abundant in tropical countries where they do great damage 
 to timber and wooden structures. They never come out 
 in the light, but secretly dig burrows in wood until there 
 is nothing left but a thin outside shell which finally falls 
 into powder. Many species of termites make enormous 
 nests which contain thousands of individuals living as a 
 complex spcial community. Division of labor has been 
 carried so far that there are as many as eight different 
 castes in some species. These are females, males, workers, 
 soldiers, etc. The worker and soldiers castes are sexually 
 immature and may be either males or females. The 
 female breaks off her wings after mating so that she may 
 not stray from home. When mature she is a big helpless 
 egg-laying sac, and must even be fed and cleaned by her 
 nurses. She grows to be a thousand times as large as her 
 mate. 
 
 Orders 6, 7, 8, 9. Corrodentia, book-lice and bark-lice; 
 Mallophaga, biting bird lice; Thysanoptera, thrips; Euplex- 
 optera, earwigs. All these insects have biting mouth parts, 
 and are heterometabolous, except the Thysanoptera which 
 have mouth parts somewhat modified for sucking and a 
 peculiar metamorphosis in which there is a resting stage. 
 Different species of thrips are pests on onion, wheat, grass, 
 and fruit. 
 
 Order 10. Orthoptera (Fig. 40). The cockroaches, 
 walking-sticks, mantids, grasshoppers, locusts, katydids, 
 and crickets are included in this order. These insects all 
 have biting mouth parts and are heterometabolous. Most 
 of them have four wings; the anterior pair being straight 
 and leathery; the posterior pair, membranous and folding 
 like a fan. There are six important families: 
 
 Family 1. Blattidce; cockroaches. These insects have 
 
INSECTA 
 
 73 
 
 the legs all about the same size and fitted for running. 
 They are very swift and for the most part nocturnal. Their 
 natural home is in rotten stumps or under loose bark. 
 Around factories and apartment houses cockroaches often 
 cause a good deal of trouble. Sometimes they carry dis- 
 eases. A finely powdered mixture of equal parts of sugar 
 and borax will kill them. 
 
 Family 2. Acrididce; locusts, or " short-horned " grass- 
 hoppers. They have the hind legs greatly elongated for 
 
 FIG. 40. Representatives of the chief families of Orthoptera. From left to 
 right the insects are as follows: tree cricket (Oecanthidce) ; katydids, female and 
 male (Locustidce) ; mole-cricket, field cricket (Gryllidoe) ; grasshoppers^criduieE) ; 
 mantis and egg-mass (Mantidce) ; cockroach and egg case (Blattidce) ; walking- 
 stick (Phasmidce). 
 
 leaping. The name " short-horn " refers to the abbreviated 
 antennae, which are always shorter than the body. A rep- 
 resentative of this family will be discussed in detail in the 
 next two chapters. 
 
 Family 3. Locustidce; long-horned grasshoppers, katy- 
 dids and meadow grasshoppers. These orthopterans have 
 
72 GENERAL ZOOLOGY 
 
 They get their name from the habit the larvae have of hiding 
 under stones. 
 
 Order 5. Isoptera (Fig. 39). The termites, or white 
 ants, have four leathery wings or are wingless, possess biting 
 mouth parts, and are heterometabolous. They are most 
 abundant in tropical countries where they do great damage 
 to timber and wooden structures. They never come out 
 in the light, but secretly dig burrows in wood until there 
 is nothing left but a thin outside shell which finally falls 
 into powder. Many species of termites make enormous 
 nests which contain thousands of individuals living as a 
 complex spcial community. Division of labor has been 
 carried so far that there are as many as eight different 
 castes in some species. These are females, males, workers, 
 soldiers, etc. The worker and soldiers castes are sexually 
 immature and may be either males or females. The 
 female breaks off her wings after mating so that she may 
 not stray from home. When mature she is a big helpless 
 egg-laying sac, and must even be fed and cleaned by her 
 nurses. She grows to be a thousand times as large as her 
 mate. 
 
 Orders 6, 7, 8, 9. Corrodentia, book-lice and bark-lice; 
 Mallophaga, biting bird lice; Thysanoptera, thrips; Euplex- 
 optera, earwigs. All these insects have biting mouth parts, 
 and are heterometabolous, except the Thysanoptera which 
 have mouth parts somewhat modified for sucking and a 
 peculiar metamorphosis in which there is a resting stage. 
 Different species of thrips are pests on onion, wheat, grass, 
 and fruit. 
 
 Order 10. Orthoptera (Fig. 40). The cockroaches, 
 walking-sticks, mantids, grasshoppers, locusts, katydids, 
 and crickets are included in this order. These insects all 
 have biting mouth parts and are heterometabolous. Most 
 of them have four wings; the anterior pair being straight 
 and leathery; the posterior pair, membranous and folding 
 like a fan. There are six important families: 
 
 Family 1. Blattidce; cockroaches. These insects have 
 
INSECTA 73 
 
 the legs all about the same size and fitted for running. 
 They are very swift and for the most part nocturnal. Their 
 natural home is in rotten stumps or under loose bark. 
 Around factories and apartment houses cockroaches often 
 cause a good deal of trouble. Sometimes they carry dis- 
 eases. A finely powdered mixture of equal parts of sugar 
 and borax will kill them. 
 
 Family 2. Acrididce; locusts, or " short-horned " grass- 
 hoppers. They have the hind legs greatly elongated for 
 
 FIG. 40. Representatives of the chief families of Orthoptera. From left to 
 right the insects are as follows: tree cricket (Oecanthidai) ; katydids, female and 
 male (Locustidce) ; mole-cricket, field cricket (Gryllidce) ; grasshoppers(.Acrwh'<ieE) ; 
 mantis and egg-mass (Mantidce) ; cockroach and egg case (Blattidce) ; walking- 
 stick (Phasmidas) . 
 
 leaping. The name " short-horn " refers to the abbreviated 
 antennae, which are always shorter than the body. A rep- 
 resentative of this family will be discussed in detail in the 
 next two chapters. 
 
 Family 3. Locustidce; long-horned grasshoppers, katy- 
 dids and meadow grasshoppers. These orthopterans have 
 
74 GENERAL ZOOLOGY 
 
 the antennae longer than the body. Many show protective 
 resemblance in color or form to the leaves among which they 
 live. 
 
 Family 4. Gryllidce; crickets. The mole-crickets have 
 enormous front legs and spend their entire lives burrowing 
 underground; the true crickets are the common varieties 
 whose shrill chirps are heard in the fields or about houses; 
 the tree crickets live above ground in vegetation and have 
 very harsh, guttural voices. 
 
 Family 5. Mantidce; praying mantids, devil-horses. In 
 these insects the first pair of legs form prehensile organs 
 which are used in capturing insects, spiders, etc. Mantids 
 live in bushes or trees and are most abundant in the 
 warmer parts of the earth. 
 
 Family 6. Phasmidce; walking-sticks. These curious 
 orthopterans have small legs suited for walking. Their 
 form resembles the twigs on which they live. They are 
 very slow and depend upon their protective resemblance 
 to escape their enemies. If disturbed they often remain 
 motionless for hours at a time, lest they betray themselves. 
 Their food consists entirely of leaves. 
 
 The following key* will separate representatives of the 
 Orthoptera into their proper families: 
 
 1 (6) Legs equal or nearly equal in size; no sound-producing organs 
 
 (Suborder, Non-saltatoria), 2 
 
 2 (3) Body short, broad, oval, depressed; head almost wholly con- 
 
 cealed beneath the thorax; legs compressed, Family, Blattidce. 
 
 3 (2) Body elongate, narrow; head free, not covered by thorax; 
 
 legs slender and not all compressed 4 
 
 4 (5) Front pair of legs fitted for grasping Family, Mantidcs. 
 
 5 (4) Front pair of legs much like the others. . . .Family, Phasmidce. 
 
 6 (1) Hind legs fitted for leaping, or front legs greatly enlarged for 
 
 burrowing; organs for producing sound usually present 
 
 (Suborder, Saltatoria), 7 
 
 * In using the key the statement after a particular number is always 
 to be compared with that appearing after the number in parenthesis, which 
 occurs elsewhere in the key. For example: 1 is compared with 6, 2 with 3; 
 3 with 2, etc. 
 
INSECTA 75 
 
 7 (8) Antennae much shorter than the body; calling organs of male, 
 
 when present, on the hind femora and lower border of first 
 pair of wings; organs of hearing, when present, on the basal 
 segment of abdomen; ovipositor short .... Family, Acrididce. 
 
 8 (7) Antennae much longer than body, tapering; calling organs of 
 
 male, when present, on the dorsal surface of the first pair of 
 wings; organs of hearing, when present, on the tibiae of the 
 first pair of legs; ovipositor a long blade or needle 9 
 
 9 (10) First pair of wings slope down from the dorso-median line; 
 
 ovipositor compressed and sword-like .... Family, Locustidce. 
 10 (9) First pair of wings flat above, the sides bent abruptly down- 
 ward; ovipositor a cylindrical, straight, or slightly curved 
 needle, usually enlarged at the tip Family, Gryllidce. 
 
CHAPTER VII 
 
 PHYLUM ARTHROPODA, CLASS INSECTA, 
 ORDER ORTHOPTERA 
 
 THE KINDS OF GRASSHOPPERS 
 
 Grasshoppers, or short-horned locusts (Acrididce) , are 
 numerous the world over wherever appropriate conditions 
 are found. There are about 550 species in the United 
 States. Most of these do not range throughout the 
 country Indiana, for example, has only about 65 species 
 within its borders. With such diversity it is necessary 
 to have some means for dividing grasshoppers into groups 
 or clans. Zoologists have accordingly separated the Family 
 Acridida3 into four subfamilies, and these may be dif- 
 ferentiated by the following key: 
 
 1 (2) Size small; first segment of thorax prolonged backward so that 
 
 it reaches to, or even beyond, the posterior end of the abdomen; 
 first pair of wings rudimentary. The grouse locusts 
 
 Subfamily, Tettigiiice. 
 
 2 (1) Size larger; first segment of thorax never extending over the 
 
 abdomen; first pair of wings usually well developed as tegmina, 
 or wing-covers 3 
 
 3 (6) No spine or tubercle on the sternum between the front pair of 
 
 legs 4 
 
 4 (5) Front of the head, or face, making a very oblique angle with the 
 
 top of the head; wings never brightly colored and without a black 
 band Subfamily, Tryaxalinos. 
 
 5 (4) Face not forming a sharp angle with the top of the head; second 
 
 pair of wings usually with some yellow, red or black 
 
 Subfamily, (Edipodince. 
 
 6 (3) A distinct conical or cylindrical spine on the front margin of the 
 
 sternum between the first pair of legs Subfamily, Acridince. 
 
 The grouse-locusts (Tettigince) are the smallest grass- 
 
 76 
 
INSECTA 77 
 
 hoppers. They pass the winter as mature insects, hiding 
 beneath stumps, logs, or rubbish. On sunny days, they 
 sometimes venture out in great numbers, even in mid- 
 winter. On the first warm days of spring they can be 
 collected by hundreds from a grass-covered hillside hav- 
 ing a southern exposure, or from boggy places along the 
 margins of lakes and streams. They feed upon vegetable 
 mould, humus, lichens, mosses, tender sprouts, etc. 
 Grouse-locusts lay eggs underground in May, and the 
 young which hatch from them mature by autumn. 
 
 The TryaxalincB for the most part frequent the borders 
 of marshes and damp prairie meadows. Their noiseless 
 flight and quiet colors render them rather inconspicuous. 
 They pass the winter in the egg stage, mature during the 
 summer, and die in the autumn. Many species have the 
 wings short and imperfectly developed. 
 
 The subfamily (Edipodince includes many locusts that 
 are familiar to everyone. In all these insects the body and 
 outer wings are dull brown or gray. Such somber colors 
 blend with the dusty roads and bare patches which they 
 frequent. When in flight, however, the bright colors of 
 the inner wings are displayed. The (Edipodince sing on 
 the wing making a peculiar crackling sound. They 
 " crackle" and display their bright colors to attract mates, 
 and such activities make them rather conspicuous. The 
 Carolina Locust, Dissostera Carolina, is the common large 
 species along roadways, its inner wings are black with a 
 broad yellow border. A few of the grasshoppers in this 
 family pass the winter as half-grown " nymphs," but most 
 species winter over in the egg stage. 
 
 Most of our common small grasshoppers belong to the 
 family Acridince. They vary much in size and general 
 appearance. Among them are found our most injurious- 
 Orthoptera for they commonly feed on vegetation, such as 
 farmers' crops. Most species are dull olive-brown, or 
 green, frequently with stripes or spots, and none have 
 brightly colored wings. Members of the same species may 
 
78 GENERAL ZOOLOGY 
 
 vary in color in different localities or at different seasons 
 of the year. The genus Melanopus is the largest one in 
 this group. Its representatives are to be found in grass, 
 which is their chief food. The males of the Acridince 
 usually sing when at rest and most of them rarely produce 
 sounds at all. In order to get a concrete idea of the daily 
 life of the grasshopper, we will now consider the activities 
 of the red-legged locust. 
 
 Wheeler says: "An organism is a complex, definitely 
 coordinated and, therefore, individualized system of activi- 
 ties, which are primarily directed to obtaining and assimi- 
 lating substances from an environment, to producing other 
 similar systems, known as offsprings, and to protecting the 
 system itself and usually also its offspring from disturbances 
 emanating from the environment. The three fundamental 
 activities enumerated in this definition, namely nutrition, 
 reproduction and protection, seem to have their inception 
 in what we know, from exclusively subjective experience, 
 as feelings of hunger, affection and fear respectively." 
 The activities of the red-legged locust may properly be 
 considered, therefore, under the topics: self-maintenance, 
 self-protection, and race preservation. 
 
 You may wonder why anyone should study a grasshopper, 
 or why the writer should think it would be profitable for 
 you to do so. There are many reasons why it is desirable. 
 It may be used to illustrate many general zoological facts 
 and principles, with particular application to insect prob- 
 lems, and you may perhaps be glad after all that we did 
 not choose elephants, or rattle snakes for illustrative 
 material. If we are going to look into the daily life of 
 the grasshopper, however even pry into his pantry, cup- 
 board, sleeping apartments, courting places, and other most 
 private haunts you must try to put yourself in friend 
 grasshopper's place, and realize to the extent of your ability 
 what -his sensations, wants, and difficulties are. If we know 
 a great deal about one animal, we will be prepared to make 
 more rapid progress with others. 
 
INSECTA 79 
 
 THE RED-LEGGED LOCUST, OR GRASSHOPPER 
 Melanopus femur-rubrum DeGeer 
 
 Self -maintenance. The grasshopper eats grass or almost 
 any other vegetation. Its food is, therefore, abundant 
 and easily found. If hungry, it gives a few hops this way 
 and that, and finally lands in the midst of plenty. Once 
 established on a blade of grass, its jaws start swinging side- 
 ways, and it chews away until satisfied. The strong 
 mandibles, or jaws, bite off the grass. The other mouth 
 parts help manipulate the food and pass it on into the 
 esophagus, much as the lips and tongue help the teeth in 
 handling our food. 
 
 The grasshopper is well provided with sense organs 
 which may help it in finding and testing its food (Fig. 41). 
 The antennae are supplied with tactile bristles for feeling 
 and olfactory pits for smelling. The sense of touch is 
 very acute but smell is less developed than in insects like 
 flies, which find their food by its odor. In addition to the 
 olfactory pits of the antennae, others occur on the body 
 between the bases of the legs. On some of the mouth 
 parts there are gustatory pits. These are organs of taste 
 which test the palatability of the food before it enters the 
 mouth. 
 
 There are five eyes on the head. The front bears three 
 simple eyes, or ocelli ; two near the bases of the antennae and 
 one near the center of the forehead. At the sides of the 
 head are the great oval compound eyes, made up of 
 thousands of six-sided ommatidia. Each ommatidium is 
 a complete organ of vision, with a lens, pigment, a sensitive 
 nerve ending, and nerve fiber connecting with the optic 
 ganglion and the brain. A compound eye may see either 
 a mosaic or an apposition image. In the first type there 
 is a picture of the surroundings but it is broken up into a 
 great number of small pieces, each of which is perceived by 
 one ommatidium. In an apposition image the view in 
 each ommatidium overlaps those adjacent to it so that 
 
82 GENERAL ZOOLOGY 
 
 Back flow of the blood is prevented by a series of little 
 valves which allow only movement toward the head. After 
 the blood traverses the sinuses (where it bathes all the 
 internal organs, so that they may absorb nourishment from 
 it and shed excretions into it), it flows into the pericardial 
 chamber and enters the heart through several pairs of 
 lateral ostia. If more food is digested and absorbed than 
 can be utilized, it is converted into fatty substances and 
 stored in the two elongated "fat bodies" on either side 
 of the heart. 
 
 The blood of a grasshopper, or. of any tracheate arthro- 
 pod, does not carry oxygen or carbon dioxide as it does in 
 most animals. The trachea form separate channels for 
 carrying air and the respiratory system is, therefore, en- 
 tirely independent of the circulatory system. This is an 
 admirable scheme, for each system is free to carry on its 
 own work. It has the disadvantage, or course, of requiring 
 two separate systems of conducting tubes in all parts of the 
 body. The grasshopper's tracheal system has good-sized 
 air-sacs (Fig. 36), which indicate that it is well fitted for 
 flight.. When a grasshopper breathes rapidly the abdomen 
 is rhythmically shortened and lengthened. By such move- 
 ments air is forced in and out of the spiracles on the sides 
 of the thorax and abdomen. A tracheal tube will not col- 
 lapse, for the walls have an elastic strand in them which, 
 though not continuous, is closely coiled like a wire spring 
 and keeps them expanded. The tracheal tubes divide into 
 very fine branches which go to every part of the body. 
 They extend to the tips of the legs, are closely wrapped 
 about all the internal organs and even penetrate into them. 
 Oxygen is thus always at hand to help in metabolism. 
 
 The excretions' resulting from metabolic processes in the 
 grasshopper's body are eliminated through Malpighian 
 tubules which open into the intestine (midgut) just behind 
 the stomach. These tubules are closed at the free end. 
 They are very slender and swing freely about in all direc- 
 tions in the blood sinuses. Waste products are absorbed 
 
IXSECTA 83 
 
 through their thin walls and discharged into the intestine. 
 These waste products, which result from the breaking down 
 of protein compounds (protoplasm), take the form of urea 
 or of uric acid. Uric acid is a crystalline substance not 
 soluble in water. Grasshoppers that live in very dry places, 
 where they need all their water, have uric acid as their 
 excretion product. Those which live in moist places 
 excrete urea, which is dissolved in water and eliminated as 
 urine. 
 
CHAPTER VIII 
 
 PHYLUM ARTHROPODA, CLASS INSECTA, ORDER 
 ORTHOPTERA 
 
 THE RED-LEGGED LOCUST (Continued) 
 
 Self -protection. A grasshopper not only needs to secure 
 food but must also escape from lurking enemies and other 
 dangers if it is to be successful. Each obstacle must be 
 met or avoided in a different way. The grasshopper, 
 therefore, has many adaptations for enabling it to slip out 
 of difficulties which might, in the absence of special meas- 
 ures, prove its undoing. The flight of red-legged grass- 
 hoppers is comparatively slow, but so erratic that birds 
 have some difficulty in catching these insects on the wing. 
 The grasshopper's method of escape from a pursuing enemy 
 is to give a hop or make a short flight, then rest quietly for 
 a time, and, if again threatened with immediate danger, 
 to make another quick change of locality. Such tactics 
 are continued until the grasshopper either escapes or is 
 captured. An expert hunter, like a turkey, catches the 
 grasshopper at the moment it alights, before. there is time 
 to start off again. Some species of grasshoppers are so 
 swift, however, that they easily escape capture unless taken 
 unawares. 
 
 When a grasshopper rests in its natural haunts, it is 
 difficult to see, for each species is decked out to match its 
 surroundings. There may be green to simulate the grass, 
 dark bands to resemble the shadows among the vegetation, 
 or an arrangement of colors to obliterate the outline of the 
 grasshopper's body so as to make it look like something else. 
 It is, of course, a great advantage to be protectively colored, 
 and the grasshopper's attitudes make its resemblance to 
 
 84 
 
INSECTA 
 
 85 
 
 its surroundings more complete. When at rest the body is 
 held parallel with a blade of grass so as to be easily over- 
 looked. Often a grasshopper that has been sitting in some 
 careless attitude may be seen to change its position when 
 danger approaches. The body is whirled around so as to 
 lie along a grass blade in a crouching position ready, set 
 for a jump at a moment's notice. Our red-legged locust 
 
 FIG. 42. The activities of grasshoppers. A, being captured by a robber 
 fly; B, flying; C, dead with fungus disease; D, caught by a spider; E, laying eggs 
 F, captured by a digger wasp;G, H, 7, nymphs. 
 
 has a touch of yellow and brown so that it may go unseen 
 among the dead grass. When caught napping in the green 
 foliage, it at once falls head over heels to the ground where 
 it is more likely to escape notice. Some relatives of 'the 
 grasshopper show extreme cases of protective resemblance. 
 Certain katydids, and " leaf -insects" have in their bodies 
 reproduced the color and form of the leaves they frequent, 
 
86 GENERAL ZOOLOGY 
 
 even to the picturing of midribs, veins, and other fine 
 details. 
 
 Suppose, though, that our stupid grasshopper's simple 
 wiles are of no avail, and that it is caught by an enemy. 
 It does not give up without a struggle kicks, and strains 
 to the best of its powers in efforts to escape. If it is held 
 by only one leg this appendage is kicked free from his body 
 and gladly left in the enemy's hands. It is a small matter 
 for a young grasshopper to lose a leg; somewhat inconven- 
 ient for a time, perhaps, but no serious consequences result. 
 A new leg is formed beneath the exoskeleton and becomes 
 functional at the next moult. An adult grasshopper, 
 however, cannot grow new legs. 
 
 Often when captured, in the excitement of the resulting 
 struggle, a grasshopper forms great quantities of the brown 
 " molasses. " or crop secretion, which drools from the mouth. 
 Such secretory activity is somewhat comparable to the 
 formation of " cold sweat " on the skin of man during. certain 
 emotional states. In the present instance, the molasses 
 sometimes serves a useful purpose by making the grass- 
 hopper distasteful to the animal holding it captive. 
 
 Grasshoppers are comparatively soft-bodied and of good 
 flavor. They are hence sought for food by a great number 
 of animals, even including man in some parts of the world. 
 They are not even safe when they settle down to rest at 
 the close of a day, for the night-prowling skunk esteems 
 grasshoppers a great delicacy, and seeks them out during 
 the night. In his work on the Orthoptera of Indiana, 
 Blatchley lists forty-nine birds which feed upon grasshoppers 
 in that state. Chief among these are hawks, blackbirds, 
 crows, bluejays, prairie chickens, mocking birds, and blue- 
 birds. Many snakes feed very largely on grasshoppers, and 
 their sly habits make them hard to avoid. Toads, frogs, 
 and some fish also prey upon these insects when the oppor- 
 tunity offers. Moles and shrews eat the eggs laid by grass- 
 hoppers underground. 
 
INSECTA 87 
 
 There are many arthropods which use grasshoppers for 
 food (Fig. 42). The large orb- weaving spiders spread their 
 snares in the low vegetation. Small mites lurk beneath 
 leaves, waiting to attach themselves to some unwary 
 grasshopper and suck his blood. Fierce robber-flies dart 
 over the fields ; ever ready to pounce down and carry a 
 grasshopper to some retreat to be devoured at leisure. The 
 wasps and beetles are also important enemies among the 
 insects. Some wasps store their nests with nothing but 
 grasshoppers. 
 
 All the enemies mentioned up to this time seek to devour 
 the grasshopper, piecemeal or in toto. We have now to con- 
 sider certain animals which invade the interior of the body 
 and live as parasites. 
 
 Often on a hot day you may see a lazy grasshopper flap 
 slowly along, changing his feeding ground. In the midst of 
 his course he is pursued and touched for an instant by a 
 swift little fly, which then goes about its business. This is 
 a tachina-fly, and it needs only a fraction of a second to 
 poke an egg between the segments of the grasshopper's 
 abdomen. The egg hatches into a maggot which straight- 
 way begins to bore his way into and through the interior 
 of the grasshopper, feeding as it goes. The vital organs 
 are left until the last, so the grasshopper drags out a miser- 
 able existence as a living hotel for this voracious larva. 
 Some flesh-flies also lay their eggs in grasshoppers when they 
 have a chance. Certain of the horsehair worms likewise 
 live for a part of their life cycles in these abundant insects. 
 
 The grasshopper, like man, is subject to certain diseases 
 which make it sick or even cause death. These are, in 
 addition to the large parasites already mentioned, mostly 
 caused by bacteria and parasitic fungi. Such pathogenic 
 organisms are more abundant, and hence more easily 
 acquired, during damp weather. Wet seasons are, there- 
 fore, undesirable for the grasshopper but good for the 
 farmer. Parasitic plants, such as bacteria, are able to 
 enter the body more readily if there is a wound on the 
 
88 GENERAL ZOOLOGY 
 
 surface, so that they can get through the exoskeleton, 
 which usually gives protection to the internal organs. 
 
 Perhaps enough has been said to show that the grass- 
 hopper pays the penalty for being abundant and, as in- 
 sects go, to a reasonable degree successful. It is beset by 
 a host of enemies which in various ways try to make an 
 end of it. If we have "put ourselves in the grasshopper's 
 place," we may now feel a certain sympathy for the 
 creature but there is another side to its case. If grass- 
 hoppers have a good season, with abundant food, few 
 enemies and favorable meterological conditions, it will 
 be a bad year for crops. In many parts of the United States 
 in fact, the grasshopper is successful enough to be a con- 
 tinual menace, and in some seasons becomes a veritable 
 calamity. This has led man to devise means for his own 
 protection, and these have been developed along four lines: 
 (1) traps, (2) poisonous sprays, (3) inoculation with diseases 
 or the introduction of natural enemies, and (4) poison 
 baits. 
 
 On certain crops, grasshoppers may be killed by using 
 sprays (arsenate of lead, Paris green, kerosene, etc.). 
 Their method of feeding renders them readily susceptible 
 to poisons sprayed on the outside of plants. Oils, like 
 kerosene, enter the tracheal tubes and kill in that way. 
 Such methods are not, however, readily practised on a 
 comprehensive scale. In large fields trapping is often 
 resorted to as a means of control. One method is to set 
 large cans of water and kerosene below the surface of the 
 ground in places where the insects are most abundant. 
 In hopping about the grasshoppers fall into the traps and 
 are killed. The hopperdozer (Fig. 43) is a movable trap 
 which is driven over infected fields. The grasshoppers 
 hop or fly as it comes near, fall into the kerosene pan, and 
 die. 
 
 In the way of living enemies, a flock of turkeys will do 
 as efficient work as any remedy. Other measures consist 
 in the introduction of parasitic flies, or in the liberation 
 
INSECTA 
 
 89 
 
 of grasshoppers already infected with contagious diseases. 
 Several years ago a plague of locusts was successfully fought 
 in Argentina by liberating a fungous disease. More 
 recently in New Mexico a fly, Sarcophaga kellyi, "was 
 found to be by far the most important factor in control." 
 Some wild birds (horned lark, meadow lark, sparrow hawk, 
 kildeer, quail) also served as loyal allies of man on this 
 occasion. 
 
 The great "plagues" of grasshoppers in the United States 
 have in most cases been due to the ravages of the migratory 
 locust, Melanopus spretus, which travels in enormous flocks 
 
 FIG. 43. A hopperdozer. Kerosene is placed in the trough which is then 
 dragged over a field. 
 
 through the high prairies of Western Kansas, Nebraska, and 
 Eastern Colorado. In the plague of 1913, another species, 
 Dissostera longipennis, was so abundant that it repeatedly 
 stopped trains by massing on the railroad tracks. It 
 rendered an area of 500 square miles unfit for grazing. 
 Grasshoppers not only harm man by feeding on crops, 
 but do injury by spreading the spores of various plant 
 diseases. 
 
 If a grasshopper successfully escapes all its enemies, it 
 still cannot rest in idle ease, but must make continual 
 adjustments to the changing conditions in its surroundings. 
 During rains the big head and roof-like wings protect it 
 
90 GENERAL ZOOLOGY 
 
 somewhat, but wet weather is bad for grasshoppers for 
 then diseases multiply and other difficulties arise. A few 
 grasshoppers pass the winter as adults, or half-grown 
 nymphs. The red-legged locust, however, gives up the 
 struggle in the autumn, but before dying lays a mass of 
 eggs which hatch in the following spring. This brings 
 us to the last aspect of the grasshopper's life race 
 preservation. 
 
 Race Preservation. Grasshoppers are of separate sexes, 
 males and females. A female is readily distinguished ex- 
 ternally by the ovipositor, or " egg-layer," at the tip of the 
 abdomen. This is composed of two short curved horny 
 plates, which cover the opening for the genital organs. 
 The internal reproductive organs in a female grasshopper's 
 abdomen consist of a series of long tubes modified in dif- 
 ferent regions to form a pair of ovaries, oviducts, shell 
 glands, and a seminal receptacle. The ovaries are glands 
 which give off living egg cells. Each consists of a number 
 of little tubes which connect with the oviduct. A male has 
 in his abdomen a pair of testes which produce sperm cells, 
 and two long twisted vasa deferentia, or sperm ducts. 
 Extending at the posterior end are a pair of cerci for 
 introducing sperm into the female. 
 
 If red-legged locusts are to eat grass year after year there 
 must be a continual series of new generations. These are 
 produced in only one way from eggs. A female grass- 
 hopper can grow eggs by herself and even lay them in some 
 cases, but the eggs will not hatch out young grasshoppers 
 unless they have been fertilized. " Fertilization " consists 
 primarily in the union of a sperm cell nucleus with that in 
 an egg cell. In order to insure fertilization the two sexes 
 must be brought together. 
 
 The coming together of the male and female grasshopper 
 is not altogether a matter of accident. The male takes the 
 initiative. If you sit quietly by a dusty road on a summer 
 day, you cannot fail to observe the Carolina locusts. The 
 males at intervals rise from the ground, flap their wings 
 
INSECTA 91 
 
 for a minute, and then settle down again. The "flapping" 
 is to notify females that an amorous male is abroad. It 
 displays the yellow and black of the under wings and the 
 sound produced has a certain pitch which is characteristic 
 for each species in the family to which the Carolina locust 
 belongs. If there is an unfertilized female near, she comes 
 to the male and he pokes a little packet 'of sperm into 
 her seminal receptacle with his cerci. She is now pre- 
 pared to lay fertile eggs. The red-legged locust does not 
 make such a splendid display as its larger relative, just 
 described. However, it sits in the grass and sings a feeble 
 song which is easily recognized by females of its own 
 species. 
 
 The bright " recognition " colors, the sound-producing 
 organs, and ears, are of little or no value in procuring food 
 or for self-protection, but are adaptations for mating! So 
 grasshoppers use many means to insure fertile eggs, and the 
 continuance of the race. 
 
 After a female grasshopper has been " fertilized" the 
 sperm cells move to the eggs and enter them, the egg and 
 sperm nuclei fusing in the cytoplasm of the egg cell. Thus 
 a single cell is formed by the union of the egg and sperm 
 cells. This new cell is the zygote. It has all the character- 
 istics of youth, as described in Chapter III, and is the 
 beginning of a new individual. 
 
 Soon after the zygote has been formed it begins to seg- 
 ment, or undergo mitotic cell division (Fig. 25). First the 
 nucleus divides several times and the new nuclei arrange 
 themselves near the outside of the egg ; the cytoplasm about 
 each nucleus becomes separate from the rest and a number 
 of little cells are thus formed just beneath the egg-shell 
 (Fig. 44, A, B, C). Development takes place on the out- 
 side of the egg because the yolk, or food material fills the 
 interior. As the cells continue to multiply a little embryo 
 is soon formed on one side of the "egg" (D), and this later 
 is enveloped by two protective membranes which are known 
 as the amnion and serosa (E, F). Finally the yolk is all 
 
92 
 
 GENERAL ZOOLOGY 
 
 FIG. 44. Development of a grasshopper's egg; A, an unsegmented egg con- 
 taining a single nucleus near the center; B, an egg in which the original nucleus 
 has divided many times and the daughter nuclei are arranged around the out- 
 side of the egg; C, a later stage in which the yolk material is completely enclosed 
 by cells; D, the thickening on one side indicates the beginning of an embryo; 
 E, embryo partly overgrown by amnion; E', cross section of same; F, amnion 
 and serosa completely covering embryo; F', cross section of same;G, insect with 
 appendages forming. 
 
INSECTA 
 
 93 
 
 used up and 'the young grasshopper is ready to hatch (G). 
 Soon after fertilization the female grasshopper drills a 
 hole in the ground with her abdomen and deposits thirty 
 or forty eggs, which remain covered until spring. Most of 
 the development within the egg-shell therefore takes place 
 after the eggs are laid. In spring, when the ground is 
 warm enough, the egg-shells break and little wingless 
 nymphs work their way to the surface. The young grass- 
 hoppers begin to feed at once, but cannot grow on account 
 of their hard exoskeletons. This difficulty is overcome by 
 a periodical casting of the outer skin (Fig. 45). After such 
 a moult a grasshopper is soft for a short time and grows 
 
 D 
 
 FIG. 45. A grasshopper nymph shedding its skin. (After Packard.) In E the 
 adult which emerged is drying its wings; in F, it is ready to fly away. 
 
 very rapidly. Then the exoskeleton hardens again and 
 there can be no more growth until the next moult. After 
 the first moult a tiny pair of wings appear. These increase 
 in size each time the skin is shed, but do not become func- 
 tional until the last moult, when complete maturity is 
 attained. After becoming mature the grasshopper seeks a 
 mate, starts off a new generation, and dies. Thus the 
 cycle goes round. The grasshopper must strain every 
 faculty to keep up its race. Its arduous struggle has but 
 one end to produce offspring and die. 
 
 But is this all? The characteristics of animals are 
 handed down with great fidelity generation after genera- 
 tion. Heredity is jealous of any change. But as the ages 
 
94 GENERAL ZOOLOGY 
 
 go by a race improves a little from time to time, if it 
 struggles. This is an unvarying biological law struggle 
 and trying bring improvement; lack of struggling and trying 
 bring retrogression and degeneracy. Has the grasshopper 
 improved? If we compare a mature grasshopper with its 
 remote ancestors (Figs. 27, 28) there is no gainsaying that it 
 has gone ahead. It has lost the awkward arrangement of 
 legs on each segment of the body and condensed them on 
 the three thoracic segments so that it can turn quickly and 
 easily. The legs are specialized for certain purposes ; wings 
 have developed, improvements have been made in methods 
 of breathing, eating, etc. The ancestral grasshoppers did 
 their work better from day to day, and the race has 
 improved! 
 
 Now, how much has our grasshopper attained? How 
 much ability does it display in adjusting itself to its 
 surroundings? Its structure is highly specialized and 
 admirably adjusted to the life it lives, but its mind is of 
 rather low order. Most of its activities are reflexes or 
 tropisms i.e., they are repeated each time an individual is 
 subjected to the same set of external conditions. A grass- 
 hopper eats when it is hungry, rests when satiated, breeds 
 when ripe, and carries on many other routine activities 
 but does it reason? We must answer, no, for the most part. 
 A grasshopper can make simple adjustments in its behavior 
 to adapt itself to minor changes in its environment, but its 
 reactions are usually stereotyped. If you put one in a 
 tumbler, it bumps its head over and over again in efforts 
 to escape and there is no "intelligent" effort to seek an 
 exit. Yet, notwithstanding the grasshopper's present lowly 
 mental state, we do not know what it may attain in the 
 future. In its discrimination of bright colors and specific 
 songs we can see the first glimmerings of appreciation, 
 satisfaction, pain, and pleasure. 
 
CHAPTER IX 
 
 PHYLUM ARTHROPODA, CLASS INSECTA 
 
 (Continued) 
 
 Order 11. Hemiptera. This order includes the "bugs" 
 proper, and no other insects should be called by that name. 
 Bugs are heterometabolous, have sucking mouth parts, and 
 may possess two pair of wings or be without any. There 
 are three suborders: 
 
 Suborder 1. Parasita. This group comprises the lice, 
 which are wingless and parasitic. Lice do a small amount 
 
 A B C 
 
 FIG. 46. Lice. A, eggs of head louse attached to a hair; B, head louse; C, 
 
 body louse. 
 
 of damage by making punctures and sucking blood; 
 but are dreaded chiefly because they may inoculate man 
 with the germs of certain diseases when they pierce the skin. 
 In most parts of the world there are three species which 
 attack man and many more which prey upon other animals. 
 Pediculus humanus is the head louse (Fig. 46). It is 
 found the world over and its color varies somewhat accord- 
 
 95 
 
96 GENERAL ZOOLOGY 
 
 ing to the race upon which it is found. It is very common 
 in many tropical countries where the natives may often 
 be seen searching through each others heads for the para- 
 sites. The female head louse lays about fifty eggs, attached 
 to hairs. Pediculus corporis, the body louse, is larger than 
 the head louse. It is gray in color and hence is known in 
 some localities as the "gray-back." This louse causes 
 much irritation, crawling over the body and inflicting 
 painful bites. The eggs are laid in the seams of garments. 
 The young lice or the adults may be active in discarded 
 clothing. The crab-louse, Phthirius inguinalis, is found 
 among the hairs of the pubic region, eyelashes, eyebrows, 
 and sometimes on other parts of the body where it causes 
 irritation, eruption, and discoloration of the skin. It 
 attaches its eggs to hairs. 
 
 Though fleas are the chief carriers of the bubonic plague, 
 lice may transmit the disease and have also been suspected 
 of carrying leprosy. Lice are of chief interest, however, as 
 the proven carriers of typhus, a fatal febrile infection of 
 unknown cause. Body lice of course thrive most when 
 people are filthy. The terrible outbreaks of typhus fever 
 in Mexico and Serbia, following the advent of war, were due 
 to poverty and uncleanliness among a rather ignorant 
 population. The chief means used in fighting these great 
 epidemics were simple sanitary measures which would kill 
 or prevent the breeding of lice i.e., boiling clothes regularly, 
 fumigation of public conveyances, and the use of blue 
 ointment, paraffin oil, or kerosene on the surface of the body. 
 
 Suborder 2. Homoptera (Fig. 47). Hemipterous in- 
 sects with four similar membranous wings; the mouth 
 parts form a jointed beak which arises from posterior ventral 
 part of the head. Many insects of great economic impor- 
 tance belong here scale insects, plant lice, cicadas, tree- 
 hoppers, spittle insects, etc. All these obtain their food 
 by burying their beaks in plants and sucking the sap. The 
 three following families are of particular importance: 
 
 Aphidce, plant lice or aphids. These little insects are 
 
INSECTA 
 
 97 
 
 all of small size, and often occur in great numbers on 
 plants. In spring and during the summer certain females, 
 the "stem mothers," bring forth living young which have 
 developed from unfertilized eggs. In the autumn male 
 individuals appear, and fertilized eggs are laid which pass 
 through the winter. Several different kinds of individuals 
 
 FIG. 47. Homoptcra. At the left are plant-lice, or aphids, one being de- 
 voured by a lady-bird beetle (not Homoptera) ; above are leaf-hoppers and 
 "brownie bugs," or tree-hoppers; on the tree trunk is a cicada, or seventeen- 
 year locust, which has just emerged from its nymphal skin, and two species of 
 scale insects. 
 
 are often present in the same species: winged, wingless, 
 woolly, smooth, large, small, etc. The grape-phylloxera 
 is a destructive species which perforates the roots of plants 
 and starts diseases; green-fly on wheat, and the woolly 
 apple aphis are also important. 
 
 Coccidce, scale insects. These are particularly destruc- 
 
98 GENERAL ZOOLOGY 
 
 tive to trees and fruit. They are always of minute size 
 but may be very abundant. The male is winged and 
 moves about freely. The female buries her beak in a 
 plant when young and never moves again. She has no 
 wings, and her body is covered by a secreted " scale." 
 She lays eggs which hatch beneath the scale. The young 
 females settle down near their mother, hence scale insects 
 spread rather slowly. The most important species eco- 
 nomically are the San Jose scale, introduced originally into 
 California but now found in most parts of the United States, 
 the cottony cushion scale, and the oyster scale. 
 
 Cicadidce, the cicadas. The seventeen-year " locust" is 
 the best known representative of this family. The adults 
 sing discordant notes in trees during late summer and, after 
 mating, lay eggs in the twigs of trees. The larvae hatch 
 in about six weeks, drop to the ground, and live under- 
 ground for nearly seventeen years as ugly nymphs. The 
 nymph burrows about in the soil, feeding on roots and 
 humus. In the summer of the seventeenth year it crawls 
 out of the earth, climbs upon some plant, and an adult 
 cicada emerges from its skin (Fig. 47). 
 
 Other insects belonging to this suborder are the tree- 
 hoppers, leaf-hoppers, lantern-flies, white-flies, spittle in- 
 sects, etc. The tree-hoppers and leaf-hoppers have peculiar 
 triangular or pointed heads. The spittle insects are found 
 within a mass of froth on low vegetation. 
 
 Suborder 3. Heteroptera, the True Bugs (Fig. 48). 
 In this order the fore and hind wings differ; the former 
 being thick in front and thin behind; the latter, entirely 
 membranous. The peculiar fore-wings give the name 
 (Hemiptera half wings) to this whole order of insects, 
 though they are of course not characteristic of the first 
 two suborders. Several insects of great economic impor- 
 tance belong to this group, and many are common " water- 
 bugs." 
 
 Among the aquatic Heteroptera are many with interest- 
 ing habits. All breathe air and must therefore have special 
 
INSECTA 99 
 
 means for respiration. Corixa is the water-boatman 
 which has the hind legs flattened to form two great oars. 
 When swimming under water it carries air between the 
 fine hairs which cover the body and beneath the wings, 
 hence is able to stay submerged for a long time. Noto- 
 necta, the back-swimmer, is something like Corixa, but 
 usually swims with its ventral surface up. The water- 
 scorpions (Nepa, Ranatra) 
 have long slender breath- 
 ing tubes for taking air 
 from the surface of the 
 water; Nepa is flat, Ra- 
 natra is long and slender. 
 Belostoma is the giant 
 water-bug or 
 light" bug, which often 
 flies over the land at night. 
 Its small relative, Zaitha, 
 
 FIG. 48. Heteroptera. From left to right the insects are the water-boatman 
 (Corixa) ; back-swimmer (Notonecla) ; water-stridcrs (Gerris) ; a water-scorpion 
 (Nepa) catching a minnow; a male water-bug (Zaitha) carrying eggs on his back; 
 chinch-bugs; flower- bug; assassin-bug; squash-bug. 
 
 is of interest on account of its peculiar breeding habits. 
 The female sticks her eggs on the back of a male, and he 
 is obliged to carry them around till they hatch. The 
 marsh-treaders and water-striders (Gerris, etc.) run over 
 the surface of the water with great skill. They are kept 
 from sinking through the surface film by the little hairs on 
 the legs which cannot be wet. All the aquatic bugs are 
 
100 GENERAL ZOOLOGY 
 
 predaceous, feeding on insects, worms, small fish, and tad- 
 poles. 
 
 The terrestrial bugs in this suborder are common every- 
 where. The flower-bugs are small in size but often very 
 numerous on plants, where they lurk to catch minute 
 insects. The large assassin-bugs feed mostly on insects. 
 They pierce them with their strong beaks and suck the 
 juices. Some species, such as the one most often known 
 as a "kissing-bug," Reduvius personatus, are able to inflict 
 painful bites on man. The ambush bugs are colored like 
 flowers, and this enables them to capture small insects which 
 come to the blossoms they frequent. There are over 250 
 species of leaf bugs (Capsidce) in the United States, and many 
 are garden pests, though some are predaceous. The squash- 
 bug, Anasa trislis, is well hated by every 
 gardener, being a persistent enemy of 
 squashes. The chinch-bug, Blissus leucop- 
 terus, occurs throughout the United States 
 and in parts of Canada. It is particularly 
 abundant in dry seasons. The annual loss 
 in the United States due to this insect has 
 been estimated at $20,000,000. The stink- 
 
 FIG. 40. The , ., . j , , , . , 
 
 bed-bug. bugs are easily recognized by their char- 
 acteristic odor. Some species feed on 
 plants, others on insects. 
 
 The bed-bug, Cimex lectularius (Fig. 49) is parasitic and 
 has the wings represented by very small scales. It lays its 
 eggs in cracks, nail holes, mattresses, under wall paper, 
 and in other suitable places. It is entirely nocturnal, 
 feeding while its victims slumber. Killing a bed-bug by 
 hand is no easy matter. The body is so flat that it can 
 stand considerable pressure without being injured. In 
 India and certain other warm countries the bed-bug car- 
 ries an important human disease known as kala-azar. 
 
 Order 12. Neuroptera (Fig. 50). This is a small order 
 which, however, contains some very interesting insects. 
 All have four membranous wings with many "veins," 
 
INSECTS - ,,,' - .101 
 
 biting mouth parts, and a complete metamorphosis (holo- 
 metabolous). The ant-lions' are the larvae of a neurop- 
 terous insect, Myrmelion. They live in little burrows 
 which open at the bottom of conical pits. When a small 
 insect starts to slide down the side of the pit, they throw 
 sand at it. The victim finally reaches the formidable jaws 
 waiting at the bottom of the pit. The aphis-lion hatches 
 
 FIG. 50. Neuroptera. At the left is the lace wing with its eggs and nymph 
 (aphis-lion) ; in the center the ant-lion and the insect which emerges from it are 
 shown; at the right is the dobson-fly and its larva. 
 
 from curious little stalked eggs, and spends its larval life 
 searching over plants for aphids, which constitute its chief 
 food. It thus does much good to man. The adult of the 
 aphis-lion is the beautiful little lace-winged fly, Chrysopha. 
 The dobson-fly, or hellgrammite, Corydalis cornuta, is a 
 large insect (four inches across with spread wings) with 
 
102 GENERAL ZOOLOGY 
 
 vicious-looking jaws. The larva is known by many names 
 (dobson, crawler, amly, conniption bug, clipper, water 
 grampus, goggle goy, bogart, helldevil, flip-flop, etc.), and 
 is often used by fishermen for bait. It is an inch or more 
 in length, has three pairs of legs and a fringe of slender gills 
 along either side of the abdomen. 
 
 Order 13. Mecoptera. The scorpion-flies possess wings 
 similar to the Neuroptera; the head is prolonged ventrally 
 into a long beak which has biting jaws at its tip; metamor- 
 phosis is holometabolous. The common name refers to 
 the structure present in some species at the end of the male 
 abdomen which somewhat resembles the sting of a scorpion. 
 
 Order 14. Trichoptera, Caddis-flies (Fig. 38). The 
 larvae of the insects in this order are the caddis-" worms " 
 which make little tubes or cases to protect the body. The 
 tubes are usually carried about with the larvae, but are 
 sometimes fastened to rocks or stones. They are character- 
 istic for each species, in regard to both materials and archi- 
 tecture. Some species make their cases entirely of snail 
 shells, others use sticks, leaves, sand, etc. Adult caddis- 
 flies have four membranous wings, covered with little hairs. 
 They live only a short time (long enough to mate and lay 
 eggs) but often occur in great numbers near bodies of 
 water where they passed their larval life. The mouth parts 
 of caddis-flies are rudimentary, as in may-flies. 
 
 Order 15. Lepidoptera (Fig. 51). Here belong the 
 butterflies, skippers, and moths. These insects have four 
 wings covered with scales; the mouth parts form a long 
 sucking tube; all are holometabolous. 
 
 The larvae of lepidopterous insects are caterpillars, which 
 often do damage by gnawing vegetation. The adults feed 
 largely on nectar, sucked out of flowers by means of the 
 long proboscis. When not in use the proboscis is coiled up 
 beneath the head. There are over 7000 species of Lepidop- 
 tera in the United States, and many are of great economic 
 importance. This vast array is divided into two sub- 
 orders : 
 
INSECTA 
 
 103 
 
 Suborder 1. Rhopalocera, butterflies and skippers, in 
 which there is a knob or swelling at the tip of each antenna. 
 Skippers have also, in addition to the knob, a terminal 
 recurved point. 
 
 FIG. 51. Lepidoptera. At the left a cecropia moth with its eggs, larva, and 
 cocoon; at the right a monarch butterfly with eggs, larva, and chrysalis. 
 
 Suborder 2. Heterocera, moths, which have no knobs 
 at the ends of the antennae. 
 
 The colors of butterflies and moths are often very beauti- 
 
104 GENERAL ZOOLOGY 
 
 ful. They are produced by the little scales which cover the 
 wings and other parts of the body. They may be due to 
 the presence of specific pigments which give a particular 
 tint to the surface, or they may be " refraction colors" 
 caused by the spreading of the light from the many small 
 flat surfaces presented by the scales. Reds, yellows, and 
 browns are usually pigment colors, while greens and blues 
 are more often due to refraction. 
 
 The development of Jepidopterous insects (Fig. 51) may 
 well illustrate the holometabolous type; the monarch 
 butterfly and cecropia moth serving as examples. The 
 monarch lays its eggs in little groups on the underside of 
 milkweed leaves. These eggs produce minute larvae which 
 at once begin to feed on the leaf where they hatch. The 
 larvae grow rapidly, moulting several times, and when 
 mature measure an inch or more in length. They then 
 attach themselves in a suitable place, moult the larval skins, 
 and become naked pupae, or chrysalids, which hang sus- 
 pended until they in turn moult to become imagoes, or adult 
 butterflies. The cecropia moth lays its eggs in the spring 
 on willows, and the larvae which hatch feed voraciously on 
 the leaves. When grown each of them spins a cocoon about 
 itself and passes through the winter attached to some 
 branch, safe inside its silken covering. In the spring an 
 adult moth emerges from the pupal skin and forces its way 
 out of the cocoon. The imago of this moth never eats and 
 its mouth parts are rudimentary. Its chief work in life is 
 to find a mate and start a new generation. 
 
 There are many butterflies of particular interest. 
 Among the most beautiful are the swallowtails (Family 
 Papilionidce) which have slender appendages from the 
 posterior portion of the wings. The mourning-cloak 
 butterfly (Euvanessa antiopa) is one of the harbingers of 
 spring in most parts of the United States. Its larva often 
 does damage to poplar and willow trees. The " monarch" 
 (Anosia plexippus) and ''viceroy" (Basilarchia archippus) 
 furnish an excellent case of mimicry. The monarch is dis- 
 
INSECTA 105 
 
 tasteful to birds; its red color gives notice of its undesira- 
 bility, and it generally is let alone. The viceroy lives 
 on the monarch's " reputation. " It is apparently of good 
 flavor, for it is readily eaten by birds if its color is rubbed 
 off. Its color and general appearance, however, are very 
 much like those of the monarch, and it is consequently 
 avoided by predaceous animals. Probably the commonest 
 butterfly in the United States is the cabbage butterfly, 
 Pieris rapce, which is white with black spots on its wings. 
 Its larva is the green caterpillar which is often so destruc- 
 tive to cabbages. 
 
 Moths are of much more economic importance than 
 butterflies. Many species do injury and some are highly 
 beneficial. Among destructive larvae may be mentioned: 
 the web worm, on trees and shrubs, tent caterpillars (trees), 
 cotton worm, tobacco worm, army worm (garden crops), 
 boll worm (cotton), canker worm (fruit trees), codling 
 worm (apple), and the larvae of the gypsy (trees), tussock 
 (trees), clothes, flour, and grain moths. The larvae of the 
 brown-tailed moth has lately attracted considerable atten- 
 tion in the Eastern United States. A person who accident- 
 ally touches one of the caterpillars may have poisonous 
 hairs imbedded in the skin, and a festering eruption results. 
 
 The silkworm, Bombyx mori, stands preeminent among 
 beneficial insects. This larva is fed on mulberry leaves and 
 after attaining its growth spins the cocoon in which it 
 becomes a pupa. This cocoon is the only source of com- 
 mercial silk. The silk industry originated in China but 
 the propagation of silk worms is now carried on in most 
 parts of the world where conditions are suitable. To ob- 
 tain silk, the cocoons are plunged into hot water and un- 
 wound, thus unraveling the fine threads which originated 
 in the spinning glands of the silk moth larva. Silk indus- 
 tries in the United States amount to about $100,000,000 a 
 year. 
 
CHAPTER X 
 
 PHYLUM ARTHROPODA, CLASS INSECTA 
 
 (Continued) 
 
 Order 16. Diptera, Flies. This great order includes the 
 insects with two wings (the flies, mosquitoes, gnats, etc.) 
 and a few degenerate species which have no wings. The 
 hind wings are usually represented by a pair of halteres, or 
 poisers, the mouth parts are fitted for sucking or piercing, 
 and the metamorphosis is complete. More than forty 
 thousand species of Diptera have been described and prob- 
 ably 300,000 more are as yet unknown. Howard says: 
 "Not only have the true flies a superiority in point of 
 numbers, but entomologists are concluding that they prob- 
 ably stand at the head of the insect system in point of 
 evolution, that is to say, they are the most specialized 
 of insects. " They are also of great importance as disease 
 carriers. There are two suborders: 
 
 Suborder 1. Diptera genuina; true flies. 
 
 Section 1. Nematocera (Fig. 52); long-horned flies: mosquitoes, 
 
 crane flies, gall-gnats, midges, black-flies. 
 Section 2. Brachycera (Fig. 53); short-horned flies: horse-flies, 
 
 bee-flies, robber-flies, house-flies, bot-flies, flower-flies. 
 Suborder 2. Pupipara; "ticks" and sucking lice. All parasitic. 
 
 Crane flies (Tipulidce) look like big mosquitoes. They 
 are commonly seen in late summer in pastures and woods. 
 The larvae of most species live in the earth but some exist 
 in water, in rotten wood, or even upon the leaves of plants. 
 
 The mosquitoes (Culicidce) are of particular interest to 
 man. They not only cause inconvenience and irritation 
 by their bites, but also carry some of the most important 
 of human diseases. The eggs of most mosquitoes are laid 
 on water (Fig. 52). They hatch into little "wigglers," 
 
 106 
 
INSECTA 
 
 107 
 
 which after about a week are in turn transformed into awk- 
 ward pupae. Though the larva and pupa live in water, 
 they breathe air and must therefore come to the surface 
 from time to time. The adult mosquito lives a longer or 
 shorter time, depending on the amount of food it is able to 
 secure and how soon it finds a mate. "Some species are able, 
 under favorable conditions, to pass through their entire life 
 cycle in a week (egg, one day; larva, four; pupa, two; adult, 
 
 FIG. 52. Diptera, Nematocera. Crane fly and larva; mosquito with 
 larva, and pupa; willow-cone gall fly; midges with larva and pupa. 
 
 variable). Mosquitoes are very prolific breeders; it is 
 possible for one pair to produce two hundred million in 
 four months. An adult mosquito may live for five months, 
 perhaps longer. It is thus able to exist long after breed- 
 ing ponds have dried up, and can hibernate during the 
 winter. Male mosquitoes do not suck blood. They usu- 
 ally subsist on plant juices, and will sip water, wine, etc. 
 
108 GENERAL ZOOLOGY 
 
 A female may also eat plant juices, but requires a meal of 
 blood before she can ripen her eggs. 
 
 Among important mosquito-borne diseases are malaria, 
 yellow fever, filaria, and dengue fever. Malaria is caused 
 by parasitic animals (Plasmodium) which live in the red 
 blood corpuscles of man and in mosquitoes. It will be 
 considered in detail in Chapter XII; it suffices to say here 
 that it is carried only by mosquitoes of the family Ano- 
 phelince. Man can get malarial fever only through the bite 
 of such a mosquito. The genus Stegomyia is the sole car- 
 rier of yellow fever. Filaria is a parasite worm (page 187), 
 causing the horrible disease known as elephantiasis, in 
 which parts of the body become enormously enlarged. 
 This malady is also inoculated hypodermically into the 
 bodies of unsuspecting persons by certain mosquitoes while 
 sucking the blood. The commonest mosquito in the United 
 States, Culex, carries bird malaria but does not transmit 
 any human diseases. 
 
 The best way to treat mosquito-borne diseases is by 
 preventive measures i.e., better not to have the disease 
 than to try to get \yell after it has been acquired. This 
 means fighting mosquitoes, and in warm countries where 
 such diseases are prevalent, every intelligently managed 
 economic project has a staff of men who devote their whole 
 time to general sanitation. The building of the Panama 
 Canal depended as much upon the efficient work of doctors 
 and zoologists as it did upon the efforts of engineers, steam 
 shovels, and laborers. Mosquito larvse are eliminated from 
 particular districts by putting oil on the water; by intro- 
 ducing natural enemies of the larvse, such as sticklebacks, 
 top-minnows,, and other fish; and by the elimination of 
 breeding places (filling in or draining swamps, covering 
 reservoirs, etc.). The adults are kept from houses by 
 fumigation, screening, locally applied " dopes," etc. 
 
 The midges (Chironomidce) are of great economic impor- 
 tance, chiefly because they occur in such numbers as to 
 furnish an easily obtained food for many animals. The 
 
INSECTA 109 
 
 larvae are aquatic and some are known as " blood- worms" 
 on account of their color. Fishes commonly feed on them 
 and on midge pupae. Many birds, insects, and spiders 
 eat the adult midges, which often occur in great swarms, 
 covering trees, buildings, and everything else over large 
 areas. Such swarms are more apt to come in the spring 
 than at other seasons of the year. Though some of the 
 midges resemble mosquitoes in general appearance, most 
 species are perfectly harmless. The larvae live mostly on 
 aquatic vegetation and the adults usually do not suck 
 blood. The minute "punkie" or "no-see-um," of the 
 northern woods is an exception, as those who have been 
 punctured by these minute pests can testify. 
 
 The gall-gnats (Cecidomyiidce) resemble mosquitoes in 
 appearance but do not bite. Their larvae differ consider- 
 ably in their habits, but those of most species form galls 
 on the twigs or leaves of plants. The most notable species 
 of the family is the Hessian fly, Cecidomyia destructor; 
 which does many millions of dollars worth of damage annu- 
 ally to the wheat crop of the United States. Another com- 
 mon species forms curious galls on willows, which resemble 
 pine cones in shape (Fig. 52). Others live on the golden 
 rod, sunflower, aster, and other plants. The little black- 
 flies, sand flies, or buffalo-gnats (Simuliidce) are stout, 
 hump-backed Diptera which are great pests in many parts 
 of the world. Their larvae live in swift flowing streams. 
 The adults are extremely blood-thirsty; cases are known 
 in which cattle, and once even a man, were killed by the 
 bites of these little insects. 
 
 Among the short-horned flies (Brachycera, Fig. 53) 
 the horse-flies (Tabanidce) may first be considered. These 
 insects are also known as gad-flies, deer-flies, breeze-flies, 
 etc. They are active, strong-flying creatures. The females 
 live by sucking blood from warm-blooded animals, but the 
 males are harmless and get their sustenance from plant 
 juices. Tabanids usually lay their eggs above water on 
 cat-tails, rushes, and other aquatic plants. The larvae- are 
 
110 
 
 GENERAL ZOOLOGY 
 
 carnivorous and live either in or near the water. The bee- 
 flies (Bombyliidce) superficially resemble bees, having 
 spotted or banded wings and hairy bodies. They frequent 
 flowers and their long proboscis enables them to get nectar 
 and pollen. Their larvae are often parasitic on noxious 
 insects, such as caterpillars and grasshoppers, and hence are 
 beneficial to man. The robber-flies (Asilidae, Fig. 53) 
 are veritable hawks among insects. They are slender- 
 bodied creatures which fly with great agility, often captur- 
 ing insects larger than themselves on the wing. Though 
 
 
 FIG. 53. Diptera, Bracyccra. Flower-fly, drone-fly and its "rat-tail" larvae, 
 robber-fly, flesh-flics and maggots on fish, horse fly with larva and eggs. 
 
 generally beneficial, they sometimes do injury to bee- 
 keepers and are known in some parts of the country as 
 " bee-killers." The bot-flies (GEstridce) are parasitic and 
 have unique habits. The larvae live in the bodies of living 
 animals; the common one in the horse being found in the 
 stomach; those of man, the cow, and rabbit beneath the 
 skin. The tachina flies (Tachinidce) do much good by 
 destroying caterpillars and other insects. They lay their 
 
INSECTA 
 
 111 
 
 eggs in the bodies of their hosts and the larvae live as in- 
 ternal parasites. The flesh-flies or blow-flies (Sarcopha- 
 gidce) are commonly found about decaying meat. The 
 maggots burrowing through their food; the adults eat and 
 lay eggs on the outside. The larvae are sometimes found 
 in living animals, including man. The flower-flies (Syr- 
 
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 Fia. 54. Showing the relations of flies to disease and a cartoon from a current 
 
 newspaper. 
 
 phidce) frequently are abundant in the United States. 
 They fly about on bright days and rest at other times in 
 flowers or among leaves. Their food varies greatly in the 
 larval state, but as adults most species subsist on pollen 
 and nectar. 
 
 The house-flies (Muscidce, Fig. 54) are probably of more 
 
112 GENERAL ZOOLOGY 
 
 importance to man than any other group of insects. The 
 commonest species are the house-fly, Musca domestica, and 
 the stable-fly, Stomoxys calcitrans. The horn-fly of cattle, 
 Hcematobia serrata, is also well known to farmers. Many 
 flies belonging to this family are of value as scavengers, 
 destroying decaying matter through their great numbers 
 and rapid breeding. The horn-fly and stable-fly produce 
 great losses in the dairy and stock industries by their 
 attacks on cattle. The common house-fly, and its near 
 relatives, do untold injury to man. The facility with 
 which these insects take to civilized life and their uncleanly 
 habits make them ideal carriers of disease germs. The 
 house-fly commonly breeds in the excreta of diseased men or 
 other animals, and its roving habits make it a continual 
 source of contamination. Many bacteria will pass through 
 its alimentary canal without losing their virulence. Thus 
 a house-fly not only goes about with dirty body fouling 
 everything it touches, but. is continually defecating and 
 spitting out germ cultures as it goes. Typhoid fever, 
 diarrhoea, and cholera are commonly carried by house- 
 flies. Tuberculosis, infantile paralysis, anthrax, and yaws 
 may spread in the same way. Perhaps syphilis, eye dis- 
 eases, leprosy, diphtheria, small pox, plague, and tropical 
 sores may also be carried by flies in some cases (Fig. 54). 
 Small wonder that the house-fly is the most dreaded hanger- 
 on of civilization and that many communities have taken 
 active measures to keep it down. 
 
 Order 17. Siphonaptera, Fleas (Fig. 55). These in- 
 sects show many adaptations to parasitic life on mammals. 
 They are greatly compressed, so that they are not easily 
 squeezed to death and may easily ramble about among 
 hairs. The mouth parts are suitable for sucking blood. 
 There are no wings, but the powers of jumping are re- 
 markably developed. A flea is thus able, if it drops off 
 one animal, to hop on another when it comes near. Fleas 
 lay their eggs in cracks in floors and similar situations. 
 The larvae eat sweepings, or almost any sort of waste 
 
INSECTA 
 
 113 
 
 material, such as may accumulate about the abodes of 
 animals. Adult fleas feed on various warm-blooded ani- 
 mals. Dogs, cats, rats, men and other animals have 
 characteristic species associated with them. Most spe- 
 cies of fleas prefer a particular host/ but will feed on others 
 if hungry. 
 
 Fleas have lately excited interest among medical men 
 because they have been found to be carriers of several 
 diseases. The rat flea, Xenophylla cheops, carries the 
 bubonic plague. This dreadful disease has several times 
 become established on the borders of the United States, 
 being introduced on vessels from foreign countries. Pre- 
 ventative measures consist in the destruction of rats and 
 
 Fio. 55. A flea with its eggs, larva, and cocoon. 
 
 other rodents, placing metal "rat guards" on the cables of 
 ships while in port, fumigation, etc. Where the plague has 
 become established, regular campaigns must be waged. 
 In California the government has recently hired squads 
 of men to destroy rats and ground squirrels. In New 
 Orleans the houses in certain parts of the city were com- 
 pletely destroyed and replaced by " rat-proof" buildings. 
 Order 18. Coleoptera, Beetles (Fig. 56). This order 
 approaches the Diptera and Lepidoptera in numbers. 
 There are about twelve thousand species in the United 
 States. Beetles have four pairs of wings, the first pair 
 forming hard sheaths, or elytra, which cover the mem- 
 
114 
 
 GENERAL ZOOLOGY 
 
 branous hind wings. Their mouth parts are fitted for 
 biting, and metamorphosis is complete. There are eight 
 suborders : 
 
 Suborder 1. Adephaga. The beetles in this group are 
 all carnivorous. Two important families live on land and 
 the other two are aquatic. The tiger-beetles (Cincindelidce) 
 are bright-colored active creatures which seek their prey 
 
 "'- 
 
 & -^ - 
 
 FIG. 56. Coleoptera. June-beetles, their grub and pupa; potato-beetle; 
 lady-bird; rove -beetle; tumble "bug"; plant slugs; click-beetle; predaceous 
 ground-beetles; snout-beetles; weevils. 
 
 in open places, such as roadways and sandy beaches. 
 The predaceous ground-beetles (Carabidce) are usually 
 black or brown. They search the surface of the soil for 
 caterpillars and other small insects which they attack with 
 great vigor. The predaceous diving-beetles (Dytiscidce) 
 are found among aquatic vegetation where they catch 
 various small animals. The whirligig-beetles (Gyrinidce) 
 make curious gyrating movements on the surface of the 
 
INSECTA 115 
 
 ponds or rivers. Their food consists mostly of small 
 insects which fall into the water. 
 
 Suborder 2. Clavicornia. The club-horned beetles have 
 the antennae swollen at the distal ends. Their habits 
 vary greatly. The water-scavengers (Hydrophilidce), rove- 
 beetles (Staphylindce) , and burying-beetles (Silphidce) do 
 great good by disposing of dead animals. The grain 
 beetles (Cucujidce) are injurious in granaries. The larder- 
 beetles (Dermestidce) are great pests in museums, factories, 
 etc. They eat all sorts of organic substances, but are 
 particularly fond of dry animal remains, such as insects and 
 furs. 
 
 Suborder 3. Serricornia. The saw-horned beetles have 
 serrated antennae. Here belong the metallic wood borers 
 (Bubestridce) which injure trees and logs; the click-beetles 
 (Elateridce), so-called because they can flip themselves over 
 when turned on their backs; the fireflies and soldier-beetles 
 (Lampyridce) ; and others. 
 
 Suborder 4. Lamellicornia. The blade-horned beetles 
 have the distal ends of their antennae flattened. The stag- 
 beetles (Lucanidce) are probably beneficial, as they de- 
 stroy other insects. Their name refers to the peculiar 
 antler-like growths on the heads of the males of some 
 species. The family Scarabceidce contains a number of 
 beetles which fall into two groups as regards habits. The 
 scavenger-beetles eat decaying matter. The tumble-bug 
 is an example. It places its eggs in a ball of manure, which 
 it rolls to some hole and buries. The Sacred Scarabaeus 
 of Egypt belongs in this family. The leaf-chafers are 
 injurious vegetarians; the common June-bug, Lachnos- 
 terna fusca, being an example. This beetle is a biennial. 
 It spends over a year in the ground as a white grub, and 
 hence appears only every third year. 
 
 Suborder 5. Phytophaga. The plant-eating beetles in- 
 clude the long-horned wood-borers (Cerambycidce) and the 
 bean weevils (Bruchidce). The larvae of these insects are 
 able to bore their way through very resistant materials. 
 
116 GENERAL ZOOLOGY 
 
 The most important family is the Chrysomelidce which 
 contains most of the common beetle pests the potato- 
 bug, elm-leaf beetle, cucumber-beetle, etc. 
 
 Suborder 6. Trimera. This order contains only one 
 important family, the CoccinellidoB y or lady-bird beetles. 
 These little insects both as larvae and adults, do great 
 good to man by feeding on aphids and scale-insects. 
 
 Suborder 7. Heteromera. The darkling and blister- 
 beetles are of considerable economic importance. The 
 former (Tenebrionidce) include Tenebrio molitor, the larva 
 of which is the " meal- worm" so common about flour and 
 grain bins. One of the blister-beetles (Meloidce) when 
 dried and pulverized, has the peculiar property of raising 
 blisters on the human skin, and is used by physicians for 
 that purpose. 
 
 Suborder 8. Rhynchophora. The snout-beetles differ 
 from all others in having the head prolonged into a beak, 
 with tiny mouth parts at its tip. The bark-beetles (Sco- 
 lytidce) are most injurious of all insects to forest trees, 
 causing $100,000,000 worth of damage annually in the 
 United States. The larvae make tunnels beneath the bark 
 of living or dead trees and thus open the way for rot- and 
 decay-fungi. The curculios and weevils damage fruits 
 and grains. 
 
 Order 19. Hymenoptera (Fig. 57). To this order belong 
 the gall-flies, ichneumon-flies, horn-tails, saw-flies, wasps, 
 bees, and ants. These insects possess four membranous 
 wings with few veins; their mouth parts are fitted for both 
 biting and sucking; the female often has a long ovipositor 
 so modified as to form a sting; the metamorphosis is 
 complete. Many of these are of unusual importance to 
 man. There are about 7500 species in North America. 
 Only the most important families can be mentioned. 
 
 The saw-flies (Tenthredinidce) are so named on account 
 of the saw-like ovipositor possessed by the females. The 
 adults are rather inconspicuous, but many of the larvae 
 are well known as rose-, currant-, pear-, larch-, and willow- 
 
INSECTA 
 
 117 
 
 slugs. The horn-tails (Siricidce) are related to the saw- 
 flies. The females have a long hard ovipositor at the end 
 of the abdomen with which they drill holes in trees. The 
 larvae burrow in the heart wood and grow into white grubs. 
 The gall-flies (Cynipidce) lay eggs in living buds, leaves, 
 or stems, and their larvae develop inside galls which develop 
 on the plants attacked. Though such abnormal growths 
 are found on many plants they are most common on the 
 
 FIG. 57. Hymenoptera. Chalcid-fly and cocoons on caterpillar; bees and 
 wasps; ichneumon-flies; horn-tail larva. 
 
 oak, rose, and hickory. All the gall-flies are of small size, 
 the largest being only a third of an inch long. The galls 
 produced by different species are quite characteristic. 
 They present a great variety of forms rosettes, stars, 
 spheres, and other shapes. The growth of a gall is due to 
 the irritation from the developing larva within. Some 
 gall-flies do not form galls of their own, but lay their eggs 
 in the growths produced by other species. 
 
118 GENERAL ZOOLOGY 
 
 The chalcid-flies (Chalcididce) do great good to man by 
 laying their eggs in insects, particularly caterpillars. 
 The eggs hatch in or on the host and the larvae burrow about 
 among the internal organs. Sometimes in the summer one 
 may find a dead caterpillar covered with little white 
 cocoons. These contain the pupae of chalcids which 
 lived within the caterpillar as larvae. 
 
 The ichneumon-flies (Ichneumonidce) are also parasitic 
 on caterpillars and other insects. There are many spe- 
 cies and most of them attack only one host. Thalessa 
 lunator (Fig. 57) is an interesting parasite on the larvae 
 of the pigeon horn-tail, Tremex columba. These larvae 
 are found in the heart wood of trees. The female Thalessa 
 bores into a tree with her slender ovipositor and lays her 
 eggs in them. 
 
 There are several families of wasps. The most primi- 
 tive species are diggers, carpenters, or masons. Some 
 tunnel in the ground, others make galleries in wood, or 
 plaster pellets of mud together to form nests. Such 
 cavities are stored with spiders, caterpillars, and other 
 insects. An egg is deposited with accumulated food and 
 each cavity is sealed up. The sting of a wasp contains a 
 poison which paralyzes the nervous system of its prey. 
 The insects placed in the cells with the eggs are not killed, 
 but only paralyzed. They thus remain in "cold storage" 
 until the wasp's larva is ready to devour them. The social 
 wasps (Vespidce) live in colonies. They do not seal up the 
 egg with stored food, but care for and feed the larva soon 
 after it hatches. The greater part of each colony dies 
 when winter comes. Single fertile females hibernate in 
 rotten stumps, in barns, under logs, and in similar situa- 
 tions. In spring each builds a little nest and begins to lay 
 eggs. As soon as the young wasps emerge from the pupal 
 condition they begin to help enlarge the nest, gather food, 
 and care for the young. The original queen, the males, 
 and the "workers" (which are sexually immature females) 
 all die in the autumn. Before the colony breaks up, how- 
 
INSECTA 119 
 
 ever, the males fertilize young females so as to enable them 
 to start colonies for the next year. The two common 
 ' ' paper- wasps " in the United States are Vespa and Polistes. 
 Their nests are made of chewed vegetable fibers. 
 
 The bees are included in one great family, Apidce. As 
 among the wasps there are solitary and social species. 
 There are also burro wers, carpenters, leaf-cutters, etc. 
 Bees may have short or long tongues and the food varies 
 accordingly. The bumble bee is like the social wasps, in 
 that most of the colony dies with the advent of winter. 
 The honey-bee, however, has a perennial community which 
 gives off new colonies by "swarming." When swarm- 
 ing takes place a queen rushes from her parent hive, fol- 
 lowed by most of the workers and the drones, or males. 
 She is fertilized but once; while making this flight, and is 
 then able to lay thousands of fertile eggs. Scouts go out 
 from the swarm and locate a good place for a new colony. 
 The queen never leaves the new home, but spends her time 
 going about laying eggs in the comb which the workers 
 construct. She is fed and cared for by the workers, but 
 usually dies after three or four years and is replaced by a 
 young queen. The bee-industries in the United States 
 are of considerable importance. About seventeen million 
 dollars worth of honey and wax are produced annually. 
 
 The ants (Formieidce, Fig. 58) are remarkable for their 
 development of social instincts. They have gone far 
 ahead of the bees in this respect, and even excel the ter- 
 mites. The queens start the colonies, as among other 
 social Hymenoptera, and the workers are all sterile females. 
 Polymorphism reaches great extremes in all the castes. 
 The queens and males may be large or small, and with or 
 without wings. The worker caste may have several 
 varieties, such as large and small workers, soldiers, door- 
 keepers, crop harvesters, and nurses. 
 
 Ants carry on many complicated activities which involve 
 the cooperation of all members of the colony. The leaf- 
 cutters of tropical America build large nests in forests. 
 
120 GENERAL ZOOLOGY 
 
 From these the workers, under protection of soldiers, go 
 forth in great armies along beaten paths. They seek out 
 particular plants from which the leaves and stems are 
 methodically snipped off. The booty is carried back to 
 the nest, taken underground, and chewed into fine bits 
 by another caste. The soft finely divided mass is then 
 spread out in underground beds and planted with a pecu- 
 liar fungus which is the sole food of leaf cutters. The 
 particular species of fungus used by the ants has never 
 been found except in their nests. It is, therefore, as much 
 
 Fio. 68. An ant's nest showing queen, workers, soldiers, eggs, larvae, pupae, 
 
 and beetle guests. 
 
 a "tame" or cultivated crop as the cabbages and potatoes 
 of men. When a young queen is ready to start a new 
 colony she takes a little pellet from the fungus bed and 
 places it in a little pocket at the back of her mouth. She 
 then goes to some favorable locality, digs a little burrow in 
 the ground, cuts and chews a few leaves, and makes a little 
 bed. The pellet is taken from her mouth and planted on 
 the bed. All these operations take a month or more and 
 during this time the queen takes no food. She now lays 
 eggs and cares for the larvae which hatch from them. As 
 
INSECTA 121 
 
 soon as the youngsters mature, however, she does no more 
 work, but is fed and kept clean by them. 
 
 The large red mound-building ant, Formica rufa, found 
 in various parts of the United States, commonly enslaves 
 other species. A suitable nest is located in advance and 
 the whole colony raids it killing the soldiers and carrying 
 away the workers, larvse and pupae. Some tropical ants 
 are so dependent upon slaves that they are not even able 
 to feed themselves. The harvesting ants collect certain 
 seeds which they husk, dry, and store in little underground 
 granaries. 
 
 The complex communities of ants all center about the 
 queen. She is always guarded, cleaned, and fed with great 
 care. She must be kept in good condition, for the con- 
 tinued existence of the colony depends upon her ability to 
 lay eggs. The males are of no service to the community 
 except to fertilize the queen. The workers carry on all 
 general activities. They gather food and feed each other, 
 fight, dig, gnaw, clean, nurse, etc. The instinct to re- 
 gurgitate food from the crop to feed other ants is strongly 
 developed and is probably largely responsible for the 
 extreme development of castes among ants. It makes it 
 possible for one caste to do all the food collecting and feed- 
 ing, while other individuals become specialized for other 
 occupations. 
 
 Yet, notwithstanding the complex activities of ants, 
 they have little " intelligence." They follow their in- 
 stincts slavishly and are not able to solve the simplest 
 problems. Different individuals of a colony recognize each 
 other largely by odor. Experiments have been performed 
 in which strange ants were smeared over with the extract 
 from the bodies of certain individuals from a particular 
 colony. When these disguised individuals were put in the 
 nest from which their odor came, they \vere received as 
 friends. In another experiment an ant was taken from 
 a nest and its odor was washed off. On being returned to 
 its own nest, it was seized and torn to pieces. 
 
122 GENERAL ZOOLOGY 
 
 This blind following of reflexes and instincts makes ants 
 much subject to deception by other animals. Such ras- 
 cals live as social parasites in ant communities and are not 
 ejected though they do no work because they have the 
 proper odor or some other quality which takes advantage 
 of the ant's simple instincts. More than fifteen hundred 
 species of ants' guests, or myrmeeophiles, are known. 
 These include beetles, isopods, and other arthropods. 
 Many of them are not known to occur except in ant's 
 nests (Fig. 58). 
 
CHAPTER XI 
 
 PHYLUM ARTHROPODA, CLASS ARACHNIDA 
 
 The arachnids differ from all other arthropods in having 
 the following combination of characters: (1) no antennae; 
 (2) no true jaws, but only a pair of claw-like appendages 
 on either side of the mouth; (3) the head and thorax are 
 combined to form a cephalothorax, which bears the four 
 walking legs and is usually distinct from the abdomen. 
 There are twelve orders in the Class Arachnida but only 
 four will be considered. The other eight contain animals 
 which are rare, minute in size, or confined to tropical 
 countries. 
 
 ORDER 1. ARANEIDA, SPIDERS 
 
 The true spiders (Fig. 59) have two body regions the 
 cephalothorax and abdomen. The cephalothorax is rec- 
 tangular and flattened; the abdomen is unsegmented, soft, 
 and rounded. The spiders are, therefore, readily distin- 
 guished from the other common arachnids. The ticks and 
 mites have no division of the body into regions; in the 
 scorpions the body is flattened and the abdomen is very 
 slender; and in the phalangids, or "daddy-long-legs," the 
 appendages are slender and the body small. 
 
 The class Arachnida received its name on account of the 
 spinning ability of the spiders. It is related in ancient 
 mythology that Arachne was a mighty spinner; so good, in 
 fact, that she became conceited and boasted that she could 
 beat Minerva herself. That great goddess, after spinning 
 and winning a match with the foolish Arachne, turned her 
 into a spider. And thus, says the legend, spiders had their 
 beginning. Modern zoology has kept Arachne's memory 
 green by naming spiders and related arthropods for her, 
 
 123 
 
124 
 
 GENERAL ZOOLOGY 
 
 but only the true spiders are able to spin. In order to give 
 some idea of the daily life of a spider, the activities of the 
 common garden spider will now be considered. 
 
 THE GARDEN SPIDER, Miranda aurantia Lucas 
 
 Self -maintenance. The golden garden spider (Fig. 59) 
 spreads its beautiful orbs on low shrubs, bushes, or even 
 among tall grasses. These symmetrical webs are traps 
 
 FIG. 59. Spiders. Orb weavers and cocoon; ground spider carrying its co- 
 coon; jumping spider; young individual ready to "balloon"; a tent weaver; a 
 wasp flying to its nest with a spider; crab-spider lurking in a flower. 
 
 set to catch prey. Their building and maintenance re- 
 quire a great deal of work. . They are made from " spider- 
 web," or silk, which is elaborated by little glands within 
 the spider's abdomen and discharged through the six 
 finger-like spinnerets at the posterior end. The web is a 
 viscid fluid within the body, but hardens into threads on 
 
ARACHNIDA 125 
 
 exposure to the atmosphere. Spider's silk is very strong, 
 and yet flexible. It is used by man for several purposes 
 as cross lines in surveyor's instruments, to swing the 
 pendulums of very delicate and accurate clocks. 
 
 It is very interesting to watch an orb- weaver spin its 
 web. First it climbs to some favorable spot and sends a 
 floating strand of silk out on the wind. If the other end 
 catches, the spider runs along the line several times, 
 strengthening the first thread by adding other strands; 
 then goes to the middle of its line, attaches a new strand, 
 and drops to the ground, thus forming a new line from the 
 middle of the first to the ground. By letting go new 
 threads, and dropping down, and scrambling up, a couple 
 of dozen lines are stretched. These meet at a point which 
 is to be the center of the orb. After these " radii" are 
 ready, a continuous spiral is spun from the center outward 
 which forms a series of concentric cross connections between 
 them. An orb-weaver is able to spin two kinds of silk, 
 one of which is covered with little viscid droplets. Finally, 
 two viscid threads are put on the radii between the con- 
 centric curves of the spiral. 
 
 The snare is now ready, and the spider takes position at 
 the center, ready to dart out in any direction. If an awk- 
 ward grasshopper or a careless fly blunders into the web, 
 the viscid threads stick to it until the spider can run out 
 and quiet the struggling victim by swathing it in silk. A 
 spider, crouching at the center of its web, is able to see 
 well in all directions, for there are eight little eyes on the 
 top and front of its head. It is able to observe and pursue 
 its prey with precision. The claws at the tips of the legs 
 are admirably adjusted for running over the web and 
 handling silk threads. Any animal captured is seized in 
 the jaws and the secretion from the poison claws at their 
 tips soon quiets it. 
 
 A spider has a small mouth and cannot eat large 
 pieces of food. It sucks the juices of its victims and casts 
 their shriveled bodies out of its web. At the end of the 
 
126 GENERAL ZOOLOGY 
 
 esophagus there is a muscular sucking stomach which pumps 
 the nutritive fluids from the prey. The food is digested 
 to some extent in the large true stomach, and then passes 
 on into the intestine to be acted upon further and absorbed. 
 There are five large sacs, or caeca, connected with the stom- 
 ach. These serve to increase its digestive and absorp- 
 tive surface. Digestion is also assisted in the intestine by 
 a number of pouches which branch out to such an extent 
 that they fill most of the space within the abdomen. 
 
 After being absorbed the food is taken into the circulatory 
 system ; passes through sinuses and veins to the pericardial 
 chamber. A spider's heart is wholly within the abdomen 
 and is fastened to the walls of the chamber in which it 
 lies by little elastic threads. These pull it out into an ex- 
 panded condition after each contraction. While expanding 
 the heart fills with blood through three pairs of lateral 
 slits, or ostia. During contraction the blood is forced out 
 through the arteries to all parts of the body. Backflow 
 through the ostia is prevented by valves. Most of the 
 blood coming to the heart has been purified by passing 
 through the "leaves" of the book lungs, which are the 
 respiratory organs of spiders. There is a pair of openings 
 on the ventral side of the spider's abdomen, which lead 
 into little chambers within the body in which the lung- 
 books lie. Air is drawn in through the openings and 
 passes between the leaves of the book, thus giving the blood 
 within a chance to give up waste gases and acquire oxygen 
 through the thin epidermal covering. The excretory 
 organs of spiders consist of slender Malpighian tubules. 
 Their free ends are closed and lie among the internal 
 organs. Their proximal ends open into the intestine, and 
 there discharge absorbed waste products. 
 
 Self -protection. Spiders, like most predaceous animals, 
 are solitary in their habits. They usually sit quietly in 
 their webs waiting for prey. If danger threatens, a spider 
 quickly seeks some retreat or drops to the ground. Before 
 it goes, however, a thread is attached and a silken strand, 
 
ARACHNIDA 127 
 
 the ground line, is always passed out as the spider flees. 
 After danger is past, the spider is able to return quickly 
 and easily to its web by taking in the ground line. 
 
 The chief enemies of spiders are the mud-dauber wasps. 
 One of these agile insects is able to pull an orb-weaver 
 from its web and fly away without being entangled. The 
 luckless victim of the raid is rendered torpid by stinging 
 and used to provision the little earthen cells in which the 
 wasp's eggs are laid. The spider guards against the wasp's 
 attacks by spinning an irregular tangle of threads on all 
 sides of its orb. These outlying lines impeded the prog- 
 ress of winged aggressors, or by their vibrations at least 
 give warning that an attack is impending. 
 
 Spiders are subject to certain parasites and diseases 
 which sap their vitality or destroy them outright. Such 
 afflictions are more prevalent during damp weather. For 
 'many reasons rain is one of the spider's chief obstacles to 
 success in life. Raindrops or heavy dews break down the 
 web and on this account continual repairs are necessary. 
 
 Race Preservation. Spiders are of two sexes, male or 
 female. The latter is larger and spins the more perfect 
 webs. When mature the little male frequents the webs of 
 females in search of a mate (Fig. 59). The males of some 
 species go through various fantastic courting activities, 
 such as dancing, posturing, and posing before the female. 
 Being a chosen suitor is a somewhat doubtful honor, how- 
 ever, for the female often eats the male, sometimes even 
 before she has been fertilized. 
 
 After a female has received sperm cells for fertilizing 
 her eggs, she usually makes elaborate preparations for 
 cocooning. Some species of spiders carry their cocoons 
 about in their jaws, or attach them to their abdomens; 
 others hide them away under stones or logs. The orb- 
 weaver goes to some sheltered nook in low vegetation and 
 fastens her cocoon to the plants (Fig. 59). The gauzy 
 structure is pear-shaped and hangs free from surrounding 
 objects, suspended by silken threads. In the center is a 
 
128 GENERAL ZOOLOGY 
 
 mass of eggs closely swathed in soft " woolly" silk and 
 around this are two or three closely woven envelopes for 
 protection. A mother spider usually dies soon after her 
 cocoon is finished, but the young spiders, hatching within 
 the envelopes she has furnished, are able to endure the 
 winter and in the spring shift for themselves. The young 
 of some spiders hatch in the spring, others in the autumn. 
 Development within the egg usually requires only a few 
 days but spiderlings may remain inside the cocoon for 
 months. 
 
 On a warm spring or autumn day you will often find 
 every tree and fence bearing long streamers of " cob webs." 
 These are the " balloons" of the young spiders. A co- 
 coon may produce fifty young and it would not be desira- 
 ble if they should all try to build orbs in the same bush. 
 This difficulty is avoided by the ballooning instincts of the 
 young. Soon after hatching they climb to some elevated 
 situation, each lets out a long streamer of silk, and then 
 jumps off into space (Fig. 59). The floating strands of 
 silk are carried by the wind and the spiders may travel 
 long distances. Thus each finds favorable hunting grounds, 
 remote from numerous brothers and sisters. Spiders pass 
 through a series of moults but there is no metamorphosis. 
 The young are much like adults in their general form. 
 
 GENERAL REMARKS ON SPIDERS 
 
 If spiders are compared with their remote ancestors 
 (Figs. 27, 28) it is apparent that they are highly specialized. 
 The race has acquired many improvements in structures 
 and methods. Though traces of segmentation are dis- 
 cernible in all the body regions, there are no free body 
 segments. To secure greater compactness and rigidity 
 all the segments have been united; the head and thorax 
 have also fused to form a single region, the cephalothorax. 
 The awkward abdominal appendages have been completely 
 lost, the heart has become restricted to the abdomen, and 
 there are many other evidences of racial specialization. 
 
ARACHNIDA 129 
 
 It is remarkable with what tenacity different groups of 
 animals cling to the " styles" laid down for them by nature. 
 There is no particular reason why an arthropod should 
 have three or four pairs of legs yet, there is no adult 
 insect which does not have three, and no arachnid without 
 four. Spiders all have spinning glands on the abdomen; 
 insects possess such organs only during larval stages. 
 Many insects have wings; no arachnids bear such append- 
 ages. The exact ancestry of spiders is somewhat un- 
 certain. The earliest spider-like animals known lived in the 
 shallow waters along the seashore. The fact that some 
 arachnids (ticks, mites) have larvae with three pairs of 
 legs, however, has led some men to believe that spiders 
 descended from insect stock. 
 
 In making adjustments to their surroundings, spiders 
 display great acuteness in the development of their sense 
 organs. They can perceive objects at a distance and jump 
 at them with great speed and accuracy. They have many 
 structural adaptations which enable them to vary their 
 activities with changing conditions of life. But their 
 mental powers are not highly developed. Their usual 
 activities are simple movements or series of movements, 
 which are repeated over and over if a certain set of condi- 
 tions is repeated. Adjustments involving mental processes 
 are simple. Peckham tells of a spider in his garden which 
 dropped off its web when a tuning fork was sounded. It 
 fell to the ground five or six times each day and then re- 
 sponded no more to the sound. It kept this up for fifteen 
 days and then ceased to respond at all. Such behavior 
 shows some ability to learn and remember. The processes 
 involved are very simple, however, and do not allow us to 
 assume that the mental powers of the spider rank very 
 high among those of animals in general. 
 
 There are a number of different kinds of spiders and 
 many differ greatly from the orb-weavers in their habits. 
 The following are among the most important families: 
 Thomisidce, crab-spiders, spin no webs, but lurk in flowers 
 
130 GENERAL ZOOLOGY 
 
 to capture their prey; many are colored so as to closely 
 resemble the flowers in which they usually hide. The 
 Attidce, or jumping spiders, are very stocky. They do not 
 spin webs but hunt over the surfaces of plants, buildings, 
 etc. The Drassidce, ground-spiders, and Lycosidce, running- 
 spiders, also spin very little, but seek small animals on the 
 ground or among vegetation. Among the web-makers 
 there are four important families. The Agelenidce are 
 the tube-weavers or tent-weavers, which make flat snares 
 in the grass (Fig. 59). There is usually a little tube at 
 one side in which the spider lurks. The Lyniphiadce 
 build flat irregular webs; the Epeiridce make round orbs; 
 and the Therididce build irregular webs in the spaces among 
 vegetation. 
 
 ORDER 2. SCORPIONIDEA 
 
 Scorpions (Fig. 60) are confined to the warmer parts of 
 the earth. They are readily distinguished from other 
 arachnids by the character of the abdomen, which is very 
 slender behind and bears a poisonous sting at its tip. The 
 sting is used for paralyzing small animals captured for 
 food, but may also serve as a weapon of defense. Scor- 
 pions breathe through lung books, like spiders. They have 
 many curious mating habits. Some species dance; in 
 many the females carry their young about on their backs. 
 When a scorpion stings a human being there are usually no 
 serious consequences. The resulting irritation, pain, and 
 swelling as a rule cease in a day or two, but in some in- 
 stances even death may result. Stings are more serious 
 for children than adults. 
 
 ORDER 3. PHALANGIDA 
 
 The phalangids (Fig. 60) are commonly known as 
 " daddy-long-legs " or " harvest-men. 7 ' They always have 
 very long slender legs, the abdomen is distinctly segmented, 
 and there is no slender " waist" between the cephalothorax 
 
ARACHNIDA 131 
 
 and abdomen, as in true spiders. Phalangids are nocturnal. 
 They lurk in quiet nooks during the day and come out at 
 night to feed on small insects and other animals. Their 
 food habits make them beneficial to man. 
 
 Fia. 60. Scorpion carrying young and killing a spider; phalangid eating a fly. 
 
 ORDER 4. ACARINA 
 
 This order includes the mites and ticks (Fig. 61). These 
 arachnids are wholly without external evidences of seg- 
 mentation. The body is not even divided into regions, 
 the head, thorax, and abdomen being fused into one mass. 
 Many of the ticks and mites are parasitic in their .habits 
 and some carry diseases. All are able to go for long periods 
 of time without food, sometimes for as much as three or 
 four years. 
 
 The mites are all of small size and their bodies are never 
 greatly swollen. There are several important species in 
 the United States. The poultry mite, Dermanyssus gallince 
 sucks the blood of fowls and annually causes great losses. 
 The chiggers, or harvest mites, Trombididce, are common 
 on low vegetation and hence often get on men working in 
 fields. As six-legged larvae, they commonly attack the 
 skin and cause severe irritation. Preventative measures 
 consist in sprinkling powdered sulphur or naphelene in the 
 underclothing. Bites are treated with ammonia or weak 
 carbolic solution. The disease known as scabies or seven- 
 
132 
 
 GENERAL ZOOLOGY 
 
 year itch, is caused by a mite, Sarcoptes scabei, which lives 
 in little tunnels in the skin. Another mite, Demodex 
 folliculorum, commonly lives in the oil glands of the skin, 
 particularly those on the nose and face, but apparently 
 does no harm. The scab-parasite, Psoroptes communis, 
 attacks sheep, cattle, and horses, causing sores on the 
 skin. 
 
 FIG. 61. Mites and ticks. A, B, nymph and adult of chigger, or harvest mite; 
 C, the follicle mite, Demodex; D-G, ticks: male, female, larva and nymph. 
 
 Ticks are larger than mites and adult females become 
 greatly swollen, some reaching the size of a lima bean. 
 The cattle tick, Margaropus annulatus, is the best known 
 representative in this country. An adult female of this 
 species, after filling herself with blood, drops to the ground, 
 and lays several thousands of eggs. As soon as the six- 
 legged "seed ticks" hatch, they climb up to the top of the 
 nearest grass and await the passing of some animal which 
 
ARACHNIDA 133 
 
 may serve as a host. If need be, they remain thus on the 
 alert for several months, patiently Baiting through wind, 
 sun, and rain. If the larva succeeds in attaching itself to 
 a cow or some other animal, it feeds, and then moults to 
 become an eight-legged nymph. The nymph takes another 
 meal and sheds its skin to emerge as an adult tick. There 
 are a number of species of ticks in the United States. 
 They attack man, poultry, and other animals. 
 
 Though ticks may cause some trouble and inconvenience 
 by sucking blood, their chief claim to economic importance 
 lies in their ability to transmit diseases. The cattle tick 
 carries Texas fever. This disease is caused by a proto- 
 zoan parasite, Babesia bigemina, which not only flourishes 
 inside a tick in the blood sucked from an infected animal, 
 but may also be transmitted by a female tick to her off- 
 spring. The national government has recently worked 
 out methods for the eradication of Texas fever. One of 
 the best is the rotation of cattle through several pastures 
 so as to free them from ticks. The disease has caused losses 
 amounting to $100,000,000 in a single year in the United 
 States. Another tick, Dermacentor andersoni, is prevalent 
 in certain parts of the northwestern United States. It 
 carries and transmits the spotted fever, or mountain fever, 
 a fatal disease. In West Africa ticks are very numer- 
 ous and carry most of the prevalent fevers affecting man 
 and other animals. Several cases have been recorded in 
 America in which paralysis occurred in persons bitten by 
 ticks. 
 
CHAPTER XII 
 PHYLUM PROTOZOA 
 
 Protozoa are single-celled animals. In many species 
 great numbers of cells are grouped together to form colonies, 
 but in such cases there is no division of labor between the 
 cells; tissues are not formed, as in true many-celled animals. 
 A protozoon, then, has all the characteristics of single cells 
 as described in Chapter IV. Each protozoon, though con- 
 sisting of but a single cell, is able to feed, escape dangers, 
 reproduce, and carry on all the activities characteristic of 
 living things (Chapter III). Most protozoans are very 
 small and can usually not be seen without the aid of a 
 microscope. But what they lack in size is made up in num- 
 bers, and there is no bit of natural water or soil without 
 them. They often swarm by myriads in fields, lakes, rivers, 
 puddles, eave troughs, cisterns, water pipes, and other wet 
 places. Many species live as accidental guests or as para- 
 sites in the bodies of many-celled animals, some even caus- 
 ing important diseases of man and the domestic animals. 
 
 There are four classes of Protozoa, arranged primarily 
 according to methods of locomotion. They are as follows: 
 
 1. Sarcodina (Rhizopoda). Protozoa in which locomotion is by 
 means of rather blunt protoplasmic projections called pseudopodia. 
 
 2. Mastigophophora. Protozoa which have whip-like flagella for 
 locomotion. 
 
 3. Sporozoa. Parasitic protozoans possessing no special locomotor 
 organs, and in which there is at times a formation of many spores for 
 reproductive purposes. 
 
 4. Infusoria. Protozoa which have the body covered with many 
 hair-like cilia for locomotion. 
 
 134 
 
PROTOZOA 135 
 
 CLASS I. SARCODINA 
 
 The Sarcodina are naked cells (Fig. 62, A-D). Many 
 have shells secreted around their little bodies, but such 
 accretions are not actually a part of the protoplasm. The 
 protoplasm of these animals is, then, without much speciali- 
 zation, but it may form elaborate structures on the outside. 
 The amoeba will be considered as a representative of the 
 Sarcodina. 
 
 THE AMCEBA 
 
 Self-maintenance. There are a number of different 
 kinds of amoebas and their structure and habitats vary 
 considerably. The species chosen for the subject of this 
 discussion, Amoeba proteus (Fig. 62, A), is found in water, in 
 the brown coating of microscopic plants such as accumulates 
 in water troughs and among aquatic vegetation. Its food 
 consists of minute plants and animals which are engulfed 
 by the soft body. There is no mouth or definite place for 
 the entrance of food. The body consists, like all cells, of a 
 central nucleus surrounded by cytoplasm. The cytoplasm 
 is of about the consistency of the white of an egg, and shows 
 a differentiation into two regions: an outer denser portion, 
 the ectosarc, and an inner granular portion, the endosarc. 
 When an amoeba engulfs its food the ectosarc is first turned 
 in to form a little pocket, then surrounds the food com- 
 pletely. The obj ect eaten goes into the endosarc, surrounded 
 by a little water and the ectosarc which came in as it was 
 being engulfed. The watery secretion formed around the 
 particle makes a "food vacuole," and contains certain sub- 
 stances which dissolve the food so that it may be gradually 
 absorbed by the surrounding endosarc. If there are sub- 
 stances in the food which cannot be digested and assimi- 
 lated, they are dropped out from the body anywhere, as the 
 amoeba moves along. In spite of the extreme simplicity of 
 the amoeba's body, it is able to discriminate between objects 
 fit for food and those which are unsuitable. It rejects sand 
 
136 
 
 GENERAL ZOOLOGY 
 
 grains or other worthless particles, but persistently follows 
 moving animals or plants which may serve for food. Such 
 behavior shows that the amoeba has a simple " chemical 
 sense/' or what corresponds to taste and smell in higher 
 animals. 
 
 After food has been absorbed, it flows about in the endo- 
 plasm so that all parts are supplied with nourishment. 
 
 FIG. 62. Protozoa. A-D, Class Sarcodinia; E-G, Class Mastigophora; 
 H-K, Class Infusoria. A, Amoeba; B, Arcella; C, Diffiugia; D, Actinosphoerium; 
 E, Mastigamoeba; F, Euglena, G, Volvox; H, Paramcecium; I, Vorticella; J, 
 Stentor; K, Stylonychia. 
 
 The amceba is less than a thousandth of an inch in length 
 and therefore does not need special adaptations for respira- 
 tion. Waste gases are shed into the surrounding water and 
 oxygen is readily absorbed. Waste products resulting 
 from metabolism are dissolved in the water fluids within 
 the body and flow into a little reservoir, the contractile 
 
PROTOZOA 137 
 
 vacuole, which pulsates regularly and empties its accumu- 
 lated wastes through several pores opening through the 
 ectosarc. 
 
 Self -protection. The amoeba is so small that it has few 
 enemies . It could no t run away very successfully if pursued, 
 for it moves very slowly by projecting little soft finger-like 
 pseudopods on which it walks or flows along. Sometimes 
 one amceba will chase another and eat it. 
 
 The worst dangers for amcebas, however, are the changes 
 in physical conditions which may come with the drying up 
 of the water in which they live, or with the changing seasons. 
 If such unfavorable changes do not come too suddenly an 
 amoeba secretes a resistant covering about its body and 
 remains in a quiescent state, biding its time until conditions 
 are again favorable. Such a resting amceba is said to be 
 encysted, and is called an amceba cyst. 
 
 The amceba lives from day to day so as to escape ordi- 
 nary dangers as far as possible. It is wholly without any- 
 thing which may be interpreted as an eye, yet is sensitive to 
 light. This sensitiveness is of use to the amceba in enabling 
 it to avoid the direct rays of the sun, which would be injuri- 
 ous, and allows it always to creep away from the light into 
 dark crevices where it finds food and shelter. The amceba 
 is therefore protected from harm because it seldom comes 
 out into the open. 
 
 Race Preservation. For an amoeba, the formation of 
 new individuals is an easy matter. One individual has only 
 to undergo cell-division to produce two, and if each of the 
 " offspring" produced in this way continues to divide at in- 
 tervals, the final total will be enormous. When an amoeba 
 divides, the body forms two cells of equal size and hence 
 the process is known as binary fission. Such fission may 
 continue to be the only means of multiplication for several 
 hundreds of generations, but if continued too long the last 
 generations lack vitality and the stock finally dies out. An 
 amceba, therefore, does not pass through its life cycle as an 
 individual. Starting with a "young" individual it divides 
 
138 GENERAL ZOOLOGY 
 
 in the succeeding generations to form other amoebas and 
 after a time maturity, old age, and death are attained. 
 Any one individual is young, or mature, or old, through all 
 its " life." 
 
 If binary fission were the only process concerned in the 
 reproduction of amoebas, they would probably all have died 
 of old age long ago. But there is another way in which in- 
 dividuals may be produced. This is by conjugation. At 
 intervals two amoebas come together and, after a prepara- 
 tory or ripening process, exchange some nuclear material. 
 When they separate again they are not the same, for each 
 contains part of the other. There is also another and more 
 important change resulting from conjugation. After it is 
 completed each of the conjugants is young, no matter what 
 its condition was before. An amoeba may be mature or in 
 the last stages of old age, and after conjugation be rejuve- 
 nated, starting anew with all the vigor of youth. 
 
 When compared with other rhizopods (Fig. 62), the 
 amoeba shows great simplicity of structure. It is a 
 bit of naked protoplasm with a nucleus and a contrac- 
 tile vacuole. Some of its relatives have little shells about 
 the body. Arcella (B) has a hemispherical shell composed 
 of chitin; Difflugia (C) encloses itself by sticking sand 
 granules together to make a beautiful little case. In one 
 order of rhizopods, the Foraminifera, a calcareous shell 
 is secreted within the cytoplasm of the cell. These shells 
 are of various forms in different species like rods, wheels, 
 strings of beads, etc. The largest protozoan known is a 
 fossil foraminifer, Nummulites, which is disc-shaped and 
 sometimes' has a diameter of about two inches. Chalk 
 deposits like those at Austin, Texas, are made up mostly 
 of the shells of Foraminifera. The rhizopods in the order 
 Heliozoa (D) do not have a shell but possess very slender 
 pseudopodia which radiate from the body and have per- 
 manent rod-like supports in them. The Radiolaria are 
 marine rhizopods which have slender pseudopodia like 
 
PROTOZOA 139 
 
 those of the Heliozoa, and also have elaborate basket-like 
 internal shells of silica. 
 
 Some amcebas cause important diseases of man and other 
 animals. Entamceba histolytica burrows in the wall of the 
 intestine, swallows red blood corpuscles, and is the cause 
 of dysentery ; Entamceba gengevalis is associated with pyhor- 
 rhea alveolaris, a very prevalent disease of the teeth. 
 
 CLASS II. MASTIGOPHORA 
 
 Mastigophorans (Fig. 62, E-G) all have one or more 
 flagella for locomotion, and on this account are commonly 
 known as flagellates. A flagellum is a. slender hair-like 
 process which pulls the cell along by lashing about. In 
 addition to this locomotor appendage, the Mastigophora 
 are generally to be distinguished from the Rhizopoda by 
 the presence of a membrane, or pellicle. This is a thin 
 outer covering which, though it may be pliable, gives the 
 body a more definite form. The simplest mastigophorans, 
 however, lack such a covering. Mastigamceba (Fig. 62, E), 
 for example, is much like an amceba with a single long 
 flagellum. Its body is naked, and it can crawl about by 
 protruding pseudopodia, but it may also swim actively 
 through the water by moving its flagellum. 
 
 Many mastigophorans show relationships to plants. All 
 green plants contain chlorophyll, a substance which is 
 able in the presence of sunlight to elaborate organic com- 
 pounds (starch, etc.) from simple substances like carbon 
 dioxide and water. Such " manufacturing " is known as 
 photosynthesis. Many of the protozoans in this class 
 contain the green chlorophyll and are able to carry on 
 photosynthesis like that in plants. Another indication of 
 relationship between mastigophorans and plants is the fact 
 that the motile stages (swarm spores) of a number of the 
 simpler plants are very much like free living mastigophorans 
 which do not come from plants. We have at one extreme, 
 mastigophorans which are clearly animals, and at the other, 
 
140 GENERAL ZOOLOGY 
 
 flagellated organisms which are clearly plants. On account 
 of such mixed relationships some biologists make a group, 
 Flagellata, which contains both plants and animals, and 
 which is believed to have given rise to plants by emphasizing 
 certain qualities, and to animals by stressing others. Ac- 
 cording to this view the flagellates were the first living 
 things and hence stand at the base of the evolutionary 
 series. A form like the Amceba may, therefore, not be primi- 
 tive, even though it is simpler in structure than any other 
 animal, but perhaps may have arisen from the more complex 
 flagellates by the loss of some structures. Mastigamceba 
 in such an evolutionary history might serve as a " missing 
 link/' 
 
 Some flagellates almost become metazoans. There are 
 many species in which colonies are formed by cells remaining 
 united after fission. In such colonies the number of cells 
 may be variable or very constant. In certain species there 
 are always four, eight, sixteen, or thirty-two cells. Others 
 have a very large number; a colony of Volvox (Fig. 62, G), 
 for example, may be made of 12,000 cells. In colonies of 
 flagellates there is often a division of labor between the cells 
 some carry on locomotion and feeding, others serve for 
 reproduction only. There is thus a differentiation into 
 somatic (body) and germ (reproductive) cells. In Volvox 
 and some other colonies there is a further division of labor 
 in the germ cells; some developing into large ova (eggs) and 
 others into small sperm cells. This, however, is the limit 
 of cell-specialization in flagellates, or in protozoans, and the 
 colonies cannot properly be classed as many-celled animals 
 because the somatic cells are all alike i.e., there are no 
 tissues. 
 
 There are a number of flagellates which cause serious 
 diseases. Different species of the genus Trypanosoma 
 cause sleeping sickness in man, surra and dourine in horses, 
 nagana in cattle, etc. The Texas fever which is prevalent 
 in cattle throughout the southern United States is caused 
 
PROTOZOA 141 
 
 by a flagellate. There are several species which live in the 
 alimentary canal of man and other animals. 
 
 CLASS III. SPOROZOA 
 
 This group of protozoans contains only species which are 
 entirely parasitic, and they may live in representatives of 
 any phylum of the animal kingdom. There are some which 
 even dwell in the bodies of other protozoans. As would be 
 expected from their parasitic habits, many sporozoans 
 cause diseases in the animals they inhabit. The sporozoan 
 disease of most interest to man is malarial fever, prevalent 
 in all the warm regions of the earth. 
 
 1. Day 
 
 Fever day 
 2. Day 
 
 3. Day 
 
 Fever e/atf 
 4. Day 
 
 \ 
 
 \ 
 
 I 
 
 \ 
 
 A B C D A( B' C' D' /I" 
 
 FIG. 63. A temperature chart of a man sick with malarial fever, and (below) 
 red blood corpuscles containing the parasites causing the disease. Note that 
 there is high fever when an infected corpuscle ruptures, thus freeing spores, 
 poison, and black pigment. A, A', A", young Plasmodium in corpuscle; B, B', 
 well grown; C, C', nearly mature; D, D', ripe spores escaping. 
 
 Malaria is caused by parasites of the genus Plasmodium 
 and there are three different species which cause character- 
 istic fevers. Plasmodium vivax causes " benign tertian 
 malaria" (i.e., mild malarial fever in which there is fever 
 every third day). This parasite can enter a man only when 
 a certain mosquito bearing the disease bites him, and it then 
 migrates from the insect's proboscis directly into the blood. 
 Once in the blood it enters a red corpuscle and grows for 
 
142 GENERAL ZOOLOGY 
 
 about a day and a half, then divides into several little 
 spores (Fig. 63). The infected corpuscle finally bursts open, 
 setting the spores and a poison free in the blood. The 
 poison causes the febrile condition which a patient shows. 
 The little spores enter new corpuscles, grow for two days, 
 and then release another crop of spores. There is thus a 
 periodic recurrence of fever, every third day, which is corre- 
 lated with a certain stage in the development of the parasite. 
 ; Three things are necessary before malaria can become 
 established in a region: (1) infected people, (2) anophelene 
 mosquitoes, and (3) warm climate. A man may carry the 
 parasites and serve as a " reservoir" long after he has 
 ceased to have fever. The mosquito cannot transmit the 
 disease as soon as it draws blood from an infected person, 
 but must " incubate" the parasite for about twelve days. 
 During this incubation period the germ cells of the parasite 
 conjugate and young spores are produced which take up 
 their position in the mosquito's salivary glands, ready to 
 enter the next man who is bitten. 
 
 CLASS IV. INFUSORIA 
 
 In this class of protozoans cilia are the characteristic 
 organs of locomotion (Fig. 62, H-K). Cilia are much like 
 flagella, but are smaller and always occur together in large 
 numbers. There is always a cell-membrane or pellicle 
 covering the body of a ciliate and two nuclei are usually 
 present. The larger, or trophonucleus, is concerned with 
 ordinary metabolic activities; the smaller micronucleus is 
 usually active only during fission or conjugation. The cells 
 of some ciliates are the most complicated in the animal 
 kingdom. It will be well, therefore, to consider the activi- 
 ties of a typical representative in some detail in order that it 
 may be compared with the amoeba (page 135), which is one 
 of the simplest of "cells. For this purpose the "bell animal- 
 cule" Vorticella, is chosen. 
 
PROTOZOA 143 
 
 Vorticelia Campanula Ehrenberg 
 
 Self -maintenance. Vorticelia (Fig. 62, I) is to be found 
 in fresh- water ponds and ditches attached to some object by 
 a slender stalk. The specific name, campanula, refers to the 
 shape of the body, which resembles a dinner-bell with a very 
 long handle. Around the distal margin of the bell there is a 
 row of cilia. These by their continual beating create a 
 vorfcex in which the water is drawn toward the mouth 
 hence the generic name, Vorticelia. 
 
 This little protozoan feeds on minute particles brought 
 to it in the currents created by the cilia. If unsuccessful 
 when drawing water from one source, it jerks back by con- 
 tracting and coiling the stalk, and then stretches out in a 
 new direction, thus increasing its chances of finding organ- 
 isms on which it may feed. There are little contractile 
 strands passing from one end of the stalk to the other, and 
 these are just as much specialized for contraction as the 
 fibrils in the muscle cells of a vertebrate. Food particles 
 enter the mouth after passing through a short gullet, and are 
 enclosed in a little food vacuole before beginning to circulate 
 in the endoplasm. Undigested food passes out through a 
 definite opening in the pellicle. There is a contractile 
 vacuole near the distal end of the bell, which serves for ex- 
 creting metabolic waste products. The trophonucleus in 
 Vorticelia is large and U-shaped. It thus extends through 
 a considerable part of the interior and can readily control 
 metabolism. 
 
 Self -protection. Vorticelia has few enemies. There are 
 some sporozoans which live within it as internal parasites. 
 Some protozoans and larger animals feed upon it. It cannot 
 run away when set upon; its only method of escape is to 
 shorten the stalk as much as possible and remain quiet until 
 danger is past. 
 
 If the conditions of life are unfavorable in a certain place, 
 a Vorticelia may break loose from its stalk and swim to some 
 better spot, where it forms an attachment and grows a new 
 
144 GENERAL ZOOLOGY 
 
 stalk. If conditions become unfavorable everywhere, a 
 wall is formed around the body, and encystment takes place. 
 Vorticella can then remain in a quiescent condition until 
 better times come. 
 
 Race Preservation. Vorticella can produce new individu- 
 als by fission. The trophonuclei and micronuclei elongate 
 and divide, the bell becomes wide, and a constriction ap- 
 pears at the distal end which splits the cell longitudinally 
 into two parts, each with a pair of nuclei. Two individuals 
 are thus formed, attached to one stalk. One of them soon 
 forms a second band of cilia about its body, breaks loose and 
 swims away to settle down and form a stalk of its own. Vor- 
 ticella may multiply for many generations by such binary 
 fission but will finally die of old age unless conjugation takes 
 place. 
 
 For conjugation two kinds of individuals are formed, 
 macrogametes and microgametes. The former are stalked 
 individuals differing neither in size nor in any other obvious 
 feature from an ordinary Vorticella. The microgametes, 
 as their name implies, are much smaller. They are also 
 peculiar in possessing a posterior circle of cilia about the 
 body and in being without a stalk. A microgamete when 
 set free from the "parent" (by cell-division), swims about 
 until it finds a macrogamete and then conjugates. Con- 
 jugation begins at daybreak and lasts thirteen or fourteen 
 hours. The cytoplasm of the macrogamete fuses with that 
 of the microgamete. The trophonuclei in both cells then 
 degenerate and take no part in what follows. The micro- 
 nucleus in each cell, after dividing several times in order to 
 cast out some of the nuclear material, is ripe, and then fuses 
 with a nucleus formed from the micronucleus originally in 
 the other cell. Thus a single new cell, the zygote, origi- 
 nates from the macrogamete and the microgamete, which 
 were two separate cells at the beginning of conjugation. 
 
 When compared with those in the amoeba, the conju- 
 gating cells in Vorticella show progress along the line of sex 
 differentiation. The small motile microgamete has male 
 
PROTOZOA 145 
 
 characteristics; the larger stationary macrogamete is the 
 female. Vorticella is not as specialized in this particular 
 as some of the flagellates and sporozoans (Volvox, page 140; 
 Plasmodium, page 141). 
 
 GENERAL FACTS ABOUT PROTOZOA 
 
 Protozoa are remarkable among animals because their 
 entire bodies consist of single cells. Some of them are the 
 most complex animal cells known, others are the most 
 simple. Metabolism in many protozoans is extremely rapid. 
 One species, Didinium nasutum, can devour other proto- 
 zoans that are ten times its own size, the body becoming 
 stretched so that it forms a thin covering for the food, yet in 
 an hour or two it is ready to eat another meal of similar 
 size. 
 
 It is often said that "the Protozoa are immortal," and 
 this may indeed be true of most species. When a protozoan 
 cell is about to die of old age, it may become young again by 
 conjugation or by fusing with another cell, but must, of 
 course, sacrifice its identity by so doing. This is not true, 
 however, of protozoans in which there is a differentiation 
 into somatic (body) and germ (reproductive) cells, (Volvox, 
 Plasmodium). In these, as in many-celled animals, the 
 "body" must die and only the germ cells may help to form 
 a new generation. The statements in regard to the necessity 
 for conjugation in order to attain youth would probably not 
 be accepted by Professor L. L. Woodruff, who has reared a 
 ciliate (Paramcecium) for more than five thousand genera- 
 tions without conjugation. In a recent paper (1916) he 
 says: "an individual of Paramoecium is self-sufficient to 
 reproduce indefinitely without recourse to conjugation." 
 If Woodruff's point of view is accepted, it may be necessary 
 to revise completely the current ideas in regard to the pro- 
 duction of youth, or it may be found that only a few proto- 
 zoans are able to multiply indefinitely by fission without 
 dying of old age. Perhaps the latter alternative may be 
 looked upon as the more probable for among many-celled 
 
146 GENERAL ZOOLOGY 
 
 animals there are only a very few cases of parthenogenesis, 
 or development of ova without fertilization, and consider- 
 ing the whole animal kingdom there are very few cases in 
 which new individuals do not originate from zygotes. 
 Another view regarding the effects of conjugation has been 
 advocated by a number of zoologists, i.e., " it is more prob- 
 able that conjugation results in increased powers of adapta- 
 bility than in rejuvenescence." 
 
 If protozoans are always single cells, it is but natural to 
 ask what their psychic capabilities may be. Can they 
 profit by experience? Have they memory? In connection 
 with the discussion of Amoeba and Vorticella it has already 
 been shown that protozoans can vary their behavior some- 
 what to meet changes in surrounding conditions, but for 
 the most part their activities are simple and are repeated 
 over and over if stimulating conditions are repeated. A 
 free-swimming individual will start in a certain direction 
 and keep going until it finds food (or a mate, if the breed- 
 ing instincts are active). If some unfavorable stimulus is 
 encountered, however, it backs up a little and starts off 
 in a new direction. If conditions are again unfavorable, 
 there is more backing and trying, until a proper course is 
 found. The life of an active protozoan, then, is a series 
 of continual testings of the surroundings ; success is attained 
 by holding to what promises to be good and avoiding what 
 appears to be bad. There is no reasoning, no planning 
 or forethought. 
 
 The Protozoa present one other point of general interest 
 they were doubtless the ancestors of the Metazoa. Many 
 protozoans have almost become many-celled animals, in 
 fact, it is hard sometimes to draw the line between single 
 and many-celled forms, but none have quite attained 
 the goal. Most of them have taken the wrong road by 
 specializing the structures of single cells, instead of form- 
 ing a group of cells and dividing the work to be done among 
 them. Perhaps many-celled animals have arisen from more 
 than one group of protozoans. The possible steps by which 
 the Metazoa arose will be considered in the next chapter. 
 
CHAPTER XIII 
 
 THE ORIGIN AND CHARACTERISTICS OF THE 
 METAZOA 
 
 ORIGIN OF MANY-CELLED ANIMALS 
 
 There is every reason to believe that Metazoa arose from 
 Protozoa; i.e., that single-celled animals gave rise to many- 
 celled animals. There is no fossil record of such a trans- 
 formation because the earliest geological strata on the earth 
 have all been modified so that their records are destroyed. 
 The first records show single-celled animals living with 
 many-celled types, as they do today. Yet the develop- 
 ment of every metazoan starts with a single cell and the 
 law of biogenesis (Chapter V), therefore, indicates that the 
 origin of Metazoa was from single cells. Furthermore, 
 all the processes that take place in the development of a 
 metazoan, up to the formation of tissues, are duplicated 
 in protozoans. There is a general direction which all cell 
 associations take, and metazoans have simply gone farther 
 than protozoans. 
 
 With these things in mind, it is possible to give a synop- 
 sis of the probable steps in the origin of many-celled animals. 
 First, groups of cells arose as a result of the ordinary multi- 
 plicative processes in protozoans binary fission. The cells 
 after dividing did not separate but remained united. After 
 a time such behavior became the regular procedure in cer- 
 tain strains of protozoans and colonies of cells always re- 
 sulted from cell-division. This gave opportunity for the 
 next step division of labor between the cells. Some were 
 set aside to carry on cell multiplication and reproduction, 
 others looked after the nutritive and protective functions. 
 This specialization was a convenience for the whole colony, 
 for each individual cell had fewer things to attend to. This 
 
 147 
 
148 GENERAL ZOOLOGY 
 
 differentiation into soma and germ had, and still has, one 
 disadvantage, however all the somatic cells must die when 
 the body is worn out. 
 
 After germ and somatic cells were established another 
 improvement came in, by dividing the labor between the 
 sexual cells. As stated in the last chapter a young indi- 
 vidual is produced by the fusion of two sexual cells to form a 
 single cell. It is desirable, if two sex cells are going to fuse, 
 to have them able to move so that they can more readily 
 get together. It is also desirable that they contain reserve 
 energy, so that development may go on after fusion has 
 taken place, during the time when the usual activities are 
 more or less disturbed. Locomotion and storage of nour- 
 ishment are therefore two essential qualities in germ cells. 
 It is not very feasible, however, to have a cell store large 
 quantities of food and at the same time be able to move 
 about readily. This difficulty has been met by the develop- 
 ment of two kinds of germ cells: (1) ova, which are large, 
 immobile, and contain much nourishment ; and (2) sperma- 
 tozoa, which are minute and very active. 
 
 In a number of groups of protozoans colonies of cells 
 have become differentiated to the extent described up to 
 this point, but only true metazoans have differentiated 
 the somatic cells so that tissues are formed. Tissues are, 
 therefore, the only criterion which will separate one group 
 from the other. If, in the future, some colony of protozoans 
 develops a division of labor in its somatic cells, so that one 
 group attends to one function and another group to some 
 other activity, we will then be obliged to classify it as a 
 many-celled animal. 
 
 TISSUES AND ORGANS 
 
 A tissue is a group of similar cells w T hich carries on some 
 particular function in a metazoan's body. Tissues have 
 developed chiefly along four lines, and each of the chief 
 types has arisen by emphasizing some quality present in all 
 protoplasm. The groups are as follows: 
 
TISSUES AND ORGANS 149 
 
 1. Epithelial tissue consists of thin sheets of cells which 
 usually cover surfaces. The external covering of meta- 
 zoans consists of epithelial tissues and their products; the 
 linings of various internal cavities and the membranes which 
 connect different organs are usually epithelial. 
 
 2. Muscular tissue has specialized by developing its 
 power of contractility. The simplest muscular tissues 
 are made up of elongated single cells, but the more elabor- 
 ate types, like the striated muscles of arthropods and 
 vertebrates, are extremely complex and have various special 
 adaptations for making them contract more effectively. 
 There is one point to remember in regard to muscular 
 tissue it can only do work when contracting. 
 
 3. Nervous tissue is composed of cells which have greatly 
 developed their powers of conducting and modifying im- 
 pulses. Most nerve cells have long fibers leading out from 
 the cell-body and these carry nervous impulses from place 
 to. place. Ganglia are places in nervous tissues where 
 there are more cells then fibers, and they serve as " relay 
 stations" for modifying or originating impulses. Nerves 
 are portions of nervous tissue which contain few or no cell- 
 bodies, but have many fibers. 
 
 4. Sustentative, or Connective tissue, is quite variable but 
 is always characterized by the presence of a large amount of 
 intercellular substance. This material between the cells 
 may be bone (in osseous tissue), fibers (fibrous tissue), 
 cartilage (cartilaginous tissue), plasma (blood), or other 
 matter. 
 
 In the bodies of most many-celled animals division of 
 labor has been carried beyond the formation of tissues. 
 Usually organs are formed by grouping various tissues to- 
 gether for carrying on particular labors. The heart, for 
 example, is an organ for pumping blood, and is composed 
 of muscular, epithelial, fibrous and nervous tissue. The 
 hand is an organ for prehension and offense; it is made up 
 of bone, cartilage, muscle, nerve, epithelium, etc. Organs 
 in the more complex animals are in turn grouped into 
 
150 GENERAL ZOOLOGY 
 
 great systems which are chiefly as follows: digestive, circu- 
 latory, respiratory, excretory, muscular, skeletal, nervous, 
 reproductive. 
 
 REPRODUCTION IN METAZOA 
 
 It has been said that all Metazoa start as single cells, and 
 in general this statement is true, but there are many excep- 
 tions in particular cases. In fact, it may be said that there 
 are two methods of reproduction sexual and asexual. 
 The former always involves the union of an ovum and a 
 spermatozoon to produce a new and young individual. 
 Asexual means without sex, and there are various ways in 
 which new individuals may be formed without the usual 
 fusion of germ cells. Some sponges may be cut up in pieces 
 and propagated as slips, like plants; others give off groups of 
 cells as buds, and these grow into new individuals, still 
 others have the bulk of the body decay at certain seasons, 
 but at such times groups of cells in the interior form a re- 
 sistant coating, live until they can have opportunity to grow 
 again, and then form a complete new sponge. Various 
 polyps and worms are able to produce new individuals by 
 budding off portions of the body or by breaking it in two. 
 In addition to usual asexual methods of reproduction, 
 there are among metazoans some modifications of sexual 
 processes. Many-celled animals show two conditions in 
 regard to sex they may be hermaphroditic or dioecious. 
 In the former case an animal has both male and female 
 organs in its body, and in rare instances is able to fertilize 
 itself. It can thus sometimes produce offspring without 
 the presence of another individual. There is another 
 modification of sexual reproduction shown in partheno- 
 genesis. An ovum will usually not develop unless it has 
 united with a sperm cell but there are a few animals in 
 which such union may not be necessary, and ova therefore 
 develop without fertilization. However, in most cases, 
 perhaps in all, parthenogenetic reproduction cannot go on 
 
ONTOGENY OF METAZOA 151 
 
 indefinitely, but must give place occasional!}' to sexual 
 processes, and the great majority of animals cannot re- 
 produce parthenogenetically at all. 
 
 In spite of the various ways in which individuals may 
 arise, the development of any particular metazoan from a 
 fertilized ovum usually shows a rather definite sequence of 
 events, and this series of stages is practically the same in 
 all the different phyla, except one. Before passing on to 
 the phyla of metazoans, it will be proper therefore to 
 consider a typical developmental history. 
 
 ONTOGENY OF A TYPICAL METAZOAN 
 
 1. Maturation (Fig. 64, A, B). A many-celled animal 
 starts as a single cell, the zygote, formed by the union of the 
 egg cell, or ovum, with a sperm cell, or spermatozoon. Be- 
 fore a zygote can be formed, however, there must be cer- 
 tain preparatory or ripening changes in both the egg and 
 sperm, and such changes are known as maturation. The 
 egg cell during its maturation undergoes mitotic cell divi- 
 sion twice; but at each division (&, c), instead of two cells of 
 equal size being formed, one cell is very large and the other 
 minute. The smaller cell is known as a " polar body," 
 and is to be looked upon as a rudimentary egg which has 
 been sacrificed in order to leave the functional egg as much 
 nourishment as possible. The maturation of the sperm 
 cell (B) consists also of two successive cell divisions (b, c), 
 but in this case four functional spermatozoa of equal size 
 are formed (d), each bearing a little tail for swimming. 
 
 But, you ask, why not have the egg and sperm fuse at 
 once without ripening why have maturation at all? 
 There is apparently but one important thing involved. 
 During the two cell divisions chromatin material is elimi- 
 nated and the number of chromosomes is reduced to one- 
 half the somatic number. If the somatic cells in a par- 
 ticular animal showed ten chromosomes during mitosis, 
 a ripe germ cell would have only five. 
 
152 
 
 @-p-g' 
 
 GENERAL ZOOLOGY 
 
 X 
 
 H / J 
 
 FIG. 64. Scheme for the developmental stages of a metazoan. A, B, mat- 
 uration of egg and sperm cells a, first division; 6, two cells resulting from 
 division; c, cells after next division; d, ripe egg or sperm cell. C, zygote formed 
 by union of ripe egg and sperm, now dividing. D, cleavage stages. E, section 
 through blastula stage the stippled cells represent the portion to become ento- 
 derm. F, section during gastrula formation; x, cleavage cavity. G, section 
 through mature gastrula; ec. ectoderm; en, entoderm; y, archenteron, or primitive 
 digestive cavity. H, young metazoan with tubular digestive system. H-K, 
 stages in the formation of the coelom or body cavity; the lower figure of each 
 stage represents a cross section of the one above; co, coelom. 
 
ONTOGENY OF METAZOA 153 
 
 2. Fertilization, or the Formation of the Zygote (C). 
 After maturation, the sperm cells move to the eggs, if 
 conditions permit, and fuse with them. Only one sperm 
 cell unites with each egg, though many may try. The 
 sperm cell is always much smaller than the egg. Sometimes 
 its bulk is only one five-hundred thousandth that of 'the 
 ovum with which it unites. It usually pierces the cyto- 
 plasm of the egg for a short distance. Before fertilization 
 takes place both the germ cells have half the somatic 
 number of chromosomes. When the sperm enters the egg, 
 it brings chromatin and chromosomes equal to those in 
 the egg, and the somatic condition is thus restored in the 
 zygote. 
 
 3. Cleavage. Soon after the sperm cell has entered the 
 egg chromosomes are formed in both egg and sperm 
 nuclei, a spindle is originated from the centrosomes of the 
 sperm nucleus, and the chromosomes take position around 
 its equator (C). These chromosomes are half from the 
 sperm cell and half from the egg. The usual steps in mito- 
 sis (page 40) are now completed arid the " fertilized egg" 
 or zygote, thus divides into two cells. This division is 
 known as the first cleavage. There follow two-, four-, 
 eight-, sixteen-, thirty-two-celled " cleavage stages" (D); 
 and others involving even larger numbers of cells. After a 
 time a little ball of cells, the morula, is formed and this 
 later becomes hollow by having a water-filled " cleavage 
 cavity" (x) form in the middle. Such a ball of cells with 
 a central cavity is called a blastula (E). It soon changes 
 to a gastrula by the turning in of one side. 
 
 4. Gastrulation; the Formation of the Germ Layers. 
 When the side of the hollow spheroidal blastula is turned in, 
 the cleavage cavity (x) is encroached upon and is finally 
 completely obliterated because the turned in portion 
 comes in contact with the epithelium on the other side of 
 the original sphere (F, G). The result is a concave hemi- 
 sphere made up of two layers. The epithelium covering 
 the outside is the ectoderm (ec); the cavity within is the 
 
154 GENERAL ZOOLOGY 
 
 archenteron (?/), or primitive digestive cavity, which is 
 lined by the inner epithelium or entoderm (en). The 
 ectoderm and the entoderm are the primary germ layers, 
 and the metazoan is now a gastrula (G). 
 
 Some animals (Fig. 68) are adults in the gastrula stage 
 and their development goes no farther. They remain in a 
 diploblastic, or two-layered, condition all their lives and 
 the archenteron serves their simple digestive needs. Most 
 metazoans, however, have a third layer, the mesoderm, 
 formed between the ectoderm and entoderm, and also have 
 modifications of the archenteron so that, instead of having 
 only one opening for the ingestion and rejection of food, 
 a mouth and anus develop. During the gastrula stage, 
 or soon after, metazoans take on the distinctive character- 
 istics of the phylum to which they belong. A coelenterate 
 remains a gastrula but forms tentacles, stinging cells, and 
 other structures proper for its adult life. Most flatworms 
 develop a great mass of mesodermal cells but retain the 
 original opening into the archeneteron for feeding. More 
 highly specialized animals develop far beyond the gastrula 
 stage; the three germ layers becoming greatly modified 
 by thickenings, the formation of cavities, foldings, and in 
 other ways. 
 
 5. The Formation of the Coelom, or Body-cavity. After 
 passing through the gastrula stage a coslomate animal de- 
 velops a tubular digestive system, the original entrance 
 into the archenteron serving as one opening and a new 
 aperture forming by breaking through at the opposite 
 end (Fig. 64, H). Although there is some variation in 
 the way the coelom arises in different groups of animals, the 
 following description will serve to illustrate the general 
 method. Pairs of little pockets form along the sides of the 
 alimentary canal and push out against the body wall. 
 The cavities in these later lose their connection with the 
 alimentary canal (taking the form of closed sacs), then 
 enlarge greatly so as to envelop all the internal organs 
 (J), and fuse together (K). Thus a large space, the coelom 
 
ONTOGENY OF METAZOA 155 
 
 (co) is formed between the body wall and that of the 
 digestive tube. Organs which develop later as outgrowths 
 from the alimentary canal must push aside the membrane 
 lining the coelom, and on this account the membrane en- 
 velops all the internal organs closely. For example, the 
 peritoneum, or lining of the ccelom, surrounds all the ab- 
 dominal organs in a man's body. This makes it difficult 
 to perform surgical operations, for, if bacteria gain access 
 to the ccelom, they will cause peritonitis (inflammation of 
 the peritoneum) and the infection will wander about among 
 the internal organs, usually causing death. 
 
 6. The Formation of Systems of Organs. The germ layers 
 play rather definite roles in subsequent development. The 
 nervous tissues and the outer integument, or skin, always 
 arise from the ectoderm. The entoderm forms the lin- 
 ing for the middle portion of the alimentary canal, or midgut. 
 Other tissues and systems of organs take their origin 
 chiefly from the mesoderm. The greater part of the body 
 is therefore formed, in most animals, from the middle germ 
 layer. 
 
 This brief synopsis indicates the chief steps in metazoan 
 development : Maturation, fertilization, cleavage, gastrula- 
 tion, and ccelom formation. The first three stages occur 
 in protozoans as well as metazoans, but it is only in many- 
 celled animals that tissue differentiation takes place after 
 the formation of the primary germ layers by gastrulation. 
 As has been stated, some metazoans never get much beyond 
 the gastrula stage, but even those that pass on to more 
 specialized conditions (ccelom, paired appendages, chorda, 
 etc.) follow the regular route through the stages of matura- 
 tion, fertilization, cleavage, and gastrulation. This per- 
 sistent adherence to one conventional type of develop- 
 ment is believed to indicate genetic relationship among 
 animals. According to the Law of Biogenesis, therefore, 
 metazoans arose from protozoans; all were at one time (and 
 some still are) gastrulas, and many have through speciali- 
 zation gone farther. The racial development of all the 
 
156 GENERAL ZOOLOGY 
 
 phyla of animals was, therefore, probably the same up to 
 the gastrula stage (with one exception, and this will be 
 considered in the next chapter). 
 
 Aside from the development of the ccelom, the most 
 important advances in the recent racial history of animals 
 are: (1) The ascendency of the bilateral type of symmetry. 
 (2) The development of paired appendages. (3) The more 
 "progressive" bilaterally symmetrical animals are all 
 metameric, and the simpler metameric types consist of a 
 similar series of segments from the anterior to the posterior 
 end. This is homonomous metamerism, such as is present 
 in the earthworm. (4) In the most specialized phyla 
 of metameric animals, however, there is more importance 
 given to the segments toward the anterior end (i.e.j there is 
 progressive cephalization), and this often results in the 
 formation of a distinct head which bears important sense 
 organs. Appendages develop first, both phylogenetically 
 and ontogenetically,* as mere outgrowths or flaps from the 
 body and may later become jointed or modified in various 
 ways. 
 
 * Phylogeny is racial development; ontogeny is individual development. 
 
CHAPTER XIV 
 PHYLUM PORIFERA SPONGES 
 
 Porifera means " pore-bearers, " and the sponges could 
 not have been better named. The body of any sort of a 
 sponge is full of little canals which open through minute 
 apertures over the whole outside surface (Fig. 65). On 
 the interior the canals connect with larger channels which 
 
 A B 
 
 FIG. 65. Structure of a simple sponge. A, half of an animal split length- 
 wise to show the central longitudinal canal and the lateral canals (p) leading into 
 it. B, enlarged section of a sponge's body wall showing different types of cells; 
 a, amoebocyte; 6, undifferentiated cell; c, collared flagellate cell, or choanocyte; 
 e, epidermis on outside of body; s, spicule within the cell that formed it. 
 
 lead again to the outside. Certain portions of the internal 
 canals are lined with peculiar cells, the choanocytes, which 
 each bear a flagellum and a flexible collar surrounding it 
 (B, c). The flagella on all of these cells wave constantly 
 and create currents in the canals. Most sponges have a 
 
 157 
 
158 GENERAL ZOOLOGY 
 
 skeleton, which may be composed of separate spicules of 
 lime or silica, or consist of a horny network. In their man- 
 ner of growth sponges are much like plants, living attached 
 to some object and varying their branches to suit the 
 surroundings. The natural history of a simple representa- 
 tive of the Porifera will give some idea of the general ac- 
 tivities of these peculiar metazoans. 
 
 A SIMPLE SPONGE 
 
 Self -maintenance. A sponge obtains food from the 
 water continually passing through its canals (Fig 65). 
 Little plants and animals are sucked into the pores from the 
 surrounding water, are drawn inside the collars of the choa- 
 nocytes or engulfed by other cells as they pass through the 
 canals. There is no digestive system, or digestive cavity, 
 no mouth or anus. A cell lining a canal takes food into 
 its cytoplasm much as an amoeba does; digestion takes 
 place only within the cells. The choanocytes and some 
 other cells lining the canals are, therefore, the only ones 
 able to feed, and they must pass on nourishment to the 
 rest of the sponge body. There is no circulatory system to 
 distribute food; transfer takes place through accidental 
 spaces between cells and by gradually soaking from one 
 cell into the next. Excretion occurs through exposed sur- 
 faces, and is greatly assisted by wandering amoeboid cells 
 which engulf waste particles and migrate outside the sponge's 
 body with them. 
 
 Self -protection. Sponges are not very palatable morsels 
 for other animals, and hence have few enemies that seek to 
 devour them. They are usually protected by the sharp 
 spicules in all parts of their bodies. The spicules are ac- 
 cretions formed by cells which die after the work is completed 
 (Fig. 65, B, c). They not only protect the body, but also 
 help to support it. 
 
 Sponges grow attached and hence cannot migrate to new 
 habitats if their surroundings become unfavorable. They 
 
PHYLUM PORIFERA SPONGES 159 
 
 must be able to resist injurious conditions, or die. A large 
 part of the body may disintegrate when the water is foul or 
 when food fails; but if a few cells are left, they will grow into 
 a complete new sponge, just as a tree may send up a shoot 
 from its stump after being cut down. Sponges, then, 
 usually have little power of resistance but possess remark- 
 able ability to regenerate lost parts. Professor H. V. 
 Wilson has recently succeeded in growing complete sponges 
 from minute pieces which had been strained through a fine 
 sieve. The bodies of fresh-water sponges disintegrate in the 
 autumn, but certain groups of cells, the gemmules, form 
 resistant coats about themselves, remain dormant until 
 spring, and then grow into new sponges. 
 
 Race Preservation. As has just been stated, sponges 
 have remarkable powers of regeneration. Many species are 
 even able to throw off parts of the body which grow into 
 complete individuals. In a large sponge it is often impos- 
 sible to determine how many individuals are present for the 
 canals are interwoven and cross-connected inextricably. 
 Sponges may be easily separated into parts without serious 
 injury and different individuals are as readily fused together. 
 All the methods of reproduction thus far described are 
 strictly asexual there is no fusion of sexual cells to form a 
 zygote, no maturation, fertilization, or cleavage. Sexual 
 reproduction does occur in sponges, however, and most 
 individuals start in the beginning from a fertilized egg. 
 
 The ova and spermatozoa of sponges ripen during typical 
 maturation stages, as described in the last chapter, and 
 fertilization takes place as in other metazoans. Though 
 the cleavage stages are somewhat peculiar, they are much 
 like those in other many-celled animals. At- this point, 
 however, the resemblance between sponge development and 
 that of other Metazoa ceases, for no gastrula is formed. In 
 other words all metazoans, except sponges, pass through 
 maturation, fertilization, cleavage, and gastrula stages; the 
 sponges pass through the first three, but form no gastrula. 
 This is taken to mean that all many-celled animals except 
 
160 GENERAL ZOOLOGY 
 
 sponges are related, and that the Porifera arose independ- 
 ently from the Protozoa. The Metazoa are, therefore, 
 divided into two groups (page 24) : the Enterozoa, in- 
 cluding most of the phyla; and the Parazoa, containing only 
 the sponges. 
 
 These theoretical matters regarding classification are not 
 to the point, however, and we will return to the develop- 
 ment of the sponge. A ciliated larva is formed within a 
 special cavity, or brood pouch, after cleavage inside the 
 body of its parent. The larva when set free leaves its 
 parent's body in the water currents and swims about 
 actively until it finds some suitable place for attachment; 
 then settles down, loses its ciliated covering, develops pores 
 through the tissues, and grows into a mature sponge. 
 
 In its mature state a sponge is barely across the line be- 
 tween the Protozoa and the Metazoa. It has tissues, but 
 that is about all that can be granted it ! There is little or 
 no correlation between the different parts of the body, and 
 there are no organs. Each tissue does its work more or less 
 automatically and is rather independent of others. The 
 flagellated choanocytes beat continually until they die, and 
 will always make currents if the canals are open. If some 
 injurious substance is entering with the water, it can be 
 excluded only by closing the openings, and such closure is 
 brought about by the direct action of the substance on the 
 little sphincters, or rings of muscular tissue, surrounding 
 the openings. If a certain part of a sponge's body is cut, it 
 may contract and adjacent pore openings may close, but 
 there is no transmission of the disturbance to other parts of 
 the body. The sponge is, therefore, a rather poorly organ- 
 ized community of tissues. Muscular and supporting 
 tissues are present but there are no nerves, nor are there 
 evidences of nervous transmission. This .is an interesting 
 fact from a psychological point of view for it indicates that 
 the nervous system in animals generally has arisen after 
 various effectors (muscles, glands, etc.), and has secondarily 
 come to control them. A sponge has effectors (parts of the 
 
PHYLUM PORIFERA SPONGES 161 
 
 body capable of doing work), but has no nervous mechanism 
 to control them and they respond when stimulated directly. 
 In more specialized animals the nervous system has more or 
 less complete control of effectors the receptors (eye, ear, 
 etc.) receive stimuli and send impulses to the central nervous 
 system, which in turn sets off appropriate effectors. 
 
 SPONGE FISHERY 
 
 The total value of the sponge fisheries of the world 
 amounts to about $3,500,000 annually. In the year 1907 
 sponges were imported to the United States to the value of 
 $488,426 and exports during the same year amounted to 
 $144,354. In 1908, on the Florida fishing grounds alone, 
 622,489 pounds of sponges were captured and sold for 
 $548,876. The growing importance of the sponge fisheries 
 has led the federal government to make studies of the best 
 methods of propagation and fishing. 
 
 It is possible to raise sponges from slips, like plants, and 
 this has been made use of in a commercial way. Selected 
 individuals of good quality are cut up and the pieces placed 
 on the tips of sharp stakes attached to a frame. The frame 
 is lowered to the bottom of the ocean, left for a year or two, 
 and raised again. If all has gone well, the " slips " have 
 grown to marketable size and are of good shape (Fig. 66). 
 It is possible to graft fragments of fine sponges on the at- 
 tached growing bases of Bother indviduals of poor quality. 
 The two sponges fuse and a vigorous growth of the desirable 
 portipn is thus obtained. 
 
 Sponges are usually collected by hooking or diving. In 
 the first method one man rows a boat and another uses a 
 rake-like hook to tear the sponges from their attachments. 
 In the second, divers go down and cut the sponges off the 
 bottom by hand. Along the shores of the United States 
 fishers usually, wear diving suits, but in the Mediterranean 
 and other seas they are more often nude. In some places 
 dredges are used in capturing sponges, but they are un- 
 
162 
 
 GENERAL ZOOLOGY 
 
 desirable because they destroy great numbers of young 
 individuals. Sponges soon die on the deck of a vessel, and 
 are allowed to rot for a day or two. They are then hung 
 over the side of the ship or placed in " crawls, " or cages, in 
 the water alongshore to macerate until the flesh is very soft. 
 
 FIG. 66. Showing sponges grown from "slips" on spikes set in cement blocks 
 and left for a time on the bottom of the ocean. (From Moore.) 
 
 When thoroughly rotted, the sponges are beaten and 
 squeezed under water. They are then strung in bunches 
 and auctioned off to buyers on the wharves. In the packing 
 houses the sponges are trimmed, cleaned, sorted according 
 to size and quality, and sometimes bleached. They are 
 then ready for the consumers. 
 
CHAPTER XV 
 
 PHYLUM CCELENTERATA; PHYLUM CTE- 
 NOPHORA 
 
 The coelenterates are all modified gastrulas which are 
 radially symmetrical, and possess peculiar " stinging cells," 
 or nematocysts. They are all aquatic, and most of them 
 are marine, only three species occurring in fresh water. 
 Well-known examples of the phylum are polyps, such as 
 corals and hydras, and the jellyfishes, or medusae. In many 
 species there is an alternation of generations a polyp 
 stage giving rise to a medusa, and it in turn to a polyp 
 again. There are three classes: Hydrozoa, Scyphozoa, 
 and Anthozoa. 
 
 CLASS 1. HYDROZOA 
 
 On account of the bush-like forms often assumed by the 
 colonies of polyps when growing alongshore, the coelen- 
 terates in this class were long known as " zoophytes" 
 or ' ' animal plants." A ' ' bush " may bear hundreds or even 
 thousands of little polyps at the tips of the branches, and 
 all are connected through the twigs with the main stalk of 
 the colony which is attached to some solid object. Though 
 polyps are the most prevalent types of animals in this 
 class, minute medusae are usually produced at intervals. 
 All medusae are shaped more or less like an umbrella and 
 those of the Hydrozoa are easily distinguished from others 
 by the presence of a velum a shelf-like structure pro- 
 jecting inward around the entire margin of the umbrella 
 (Fig. 68, v). Along the Atlantic coast of the United States 
 a little hydrozoan grows very commonly on docks, piles, 
 and seaweeds. This is Obelia geniculata (Linnaeus), a 
 Colonial Hydroid, which may well serve as an example of 
 
 163 
 
164 
 
 GENERAL ZOOLOGY 
 
 this class. This hydroid always grows as a branching 
 colony (Fig. 67). All individuals are intimately associ- 
 ated with each other and the food taken by one animal 
 may pass down the hollow stalk to nourish other members 
 of the colony. Such cooperative activity between different 
 
 FIG. 67. Obelia, a colonial hydrozoan. A, a small colony somewhat enlarged; 
 B, portion of same greatly enlarged; 6, medusa buds; c, coenosarc, or soft flesh; 
 e, old branch from which an individual has broken off; g, gonangium, or repro- 
 ductive individual ; h, a mature hydranth ; or feeding individual ; j, joints to permit 
 bending of hydranth on its stem; ra, young medusa swimming out of gonangium; 
 p, perisarc; y, young hydranth. 
 
 individuals makes division of labor possible and in Obelia 
 differentiation of functions has gone so far that some mem- 
 bers of the colony carry on reproduction and cannot feed, 
 others only feed and cannot reproduce. There are, then, 
 
PHYLUM CCELENTERATA 
 
 165 
 
 two kinds of individuals at the ends of the branches: 
 feeding hydranths (h)' and gonangia (g) . Obelia is therefore 
 an example of polymorphism, a condition in which a single 
 species appears in more than one form. 
 
 Po/yp 
 
 Medusa 
 
 FIG. OS. Indicating the fundamental structural similarity between gastrula, 
 polyp, and medusa. The polyp is a gastrula with tentacles; the medusa is like 
 a polyp that has grown upward around the tentacles in the region of x. a, 
 archenteron, or primitive digestive cavity; ra, mouth. 
 
 It has been said that all ccelenterates are modified gas- 
 trulas, and this will hold true for both hydranths and 
 gonangia. In a hydranth a circlet of tentacles has grown 
 about the gastrula 's mouth, which is simply the original 
 opening into the archenteron (Fig. 64, G). The gonangium 
 
166 
 
 GENERAL ZOOLOGY 
 
 has developed no tentacles, but has little knobby buds 
 along its sides. Both sorts of individuals have only two 
 layers (ectoderm and entoderm) in the body wall. The 
 hydranth is a polyp (Fig. 68) which differs but little from a 
 gastrula, except for the connections it has with the other 
 individuals in a colony. 
 
 Self -maintenance. The Obelia hydranths are the only 
 individuals in a colony capable of feeding. Each one 
 catches little animals in its tentacles and pokes them 
 through its mouth into the simple digestive cavity. There 
 
 FIG. 69. A cnidoblast, or cell containing a nematocyst. A, with nematocyst 
 coiled; B, nematocyst discharged; c, cnidocil, or trigger. 
 
 is no opportunity to move about to look for food, hence 
 Obelia always grows where there are currents and waves, so 
 that food may be brought to it. If the animals captured are 
 too lively and struggle about, they are stung by the nema- 
 tocysts which cover the tentacles. The nematocysts 
 of ccelenterates (Fig. 69) contain a poison which soon 
 kills small animals, so that they can be swallowed without 
 trouble. 
 
 After the food enters the enteron, or digestive cavity 
 (Fig. 68, a) it is kept circulating by the movement of little 
 flagella on certain cells in the epithelial lining (= ento- 
 
PHYLUM CCELENTERATA 167 
 
 derm). Other cells pour out secretions which soften and 
 liquefy the food. If there are parts which cannot be 
 digested they must be thrown out through the mouth for 
 there is no other opening into the enteron, except the canal 
 which leads down the stalk to connect with other indi- 
 viduals. The digested food is absorbed by the entoderm 
 and part of it is there assimilated, but some soaks on 
 through to nourish the ectoderm on the outside of the 
 body. The layers of Obelia's body wall are so thin that 
 there is no need for any separate circulatory or excretory 
 systems. 
 
 Self-protection. An Obelia colony is invested over its 
 entire surface by the peritheca, a tough horny secretion 
 which forms a branched system of tubes and protects the 
 soft tissues within. This exoskeleton has little openings 
 at the tips of the branches so that the hydranths and 
 gonangia may communicate with the exterior. Each hy- 
 dranth is enclosed by a little cup, or hydrotheca (Fig. 67, ti) 
 into which it may withdraw when threatened with danger. 
 The peritheca also keeps the delicate body from drying out, 
 if it is by chance left exposed at low tide. If. this protective 
 covering were too rigid, it would be a burden and is there- 
 fore jointed (j) near the base of each branch so that 
 movement is permitted. 
 
 The nematocysts (Fig. 69) help to protect Obelia from 
 small creatures which might otherwise prey upon it,. but 
 some animals appear to be immune to their feeble stings. 
 Certain crabs use Obelia and other hydroids as a favorite 
 food. Some snails which have no shells not only devour the 
 hydroids, but do so without discharging the nematocysts; 
 and some species are even able to pass them out to their 
 own backs where they use them " second-hand " to sting 
 their enemies. 
 
 The attachment of Obelia to some solid object protects 
 it from certain dangers. It cannot be swept up on the 
 beach by storms and is never carried out to sea, but it 
 thus sacrifices its ability to search about for food. 
 
168 GENERAL ZOOLOGY 
 
 Race Preservation. Obelia has a rather complicated life 
 cycle because there is always an alternation of polyp and 
 medusa. The polyp generation reproduces by asexual 
 methods. It starts as one individual which produces a great 
 number of others by budding ; some remain attached to the 
 colony and others are set free. The medusa produces 
 zygotes which grow into young and vigorous polyps. Such 
 an alternation of a generation which reproduces asexually 
 by budding with another generation which produces new 
 individuals by sexual methods is known as metagenesis. 
 
 Metagenesis in Obelia takes the following sequence (Fig. 
 71): The zygote (A) develops in the way described for a 
 typical metazoan, passing through blastula and gastrula 
 stages, and finally forms a ciliated larva (B) which swims 
 about for a time, then attaches itself to some object along 
 the shore. The larva is transformed into a little polyp 
 (C), which soon starts to feed and forms a peritheca about 
 itself. As it grows the polyp buds off hydranths and 
 gonangia until a great branching colony is formed (D). 
 
 When the gonangia on a colony are mature, they are 
 continually forming medusa buds (Fig. 67, &). These 
 ripen into little medusse (Fig. 71, E) which break loose and 
 swim out through the opening at the end of the gonangium. 
 A medusa swims by pulsating its umbrella, and is able to 
 feed on small animals. It swims feebly, however, and is 
 often swept a long way by ocean currents. Though some 
 individuals may die before the ends of their journeys, such 
 traveling is of benefit in general, for Obelia is thus able to 
 become established in new localities. Each medusa is 
 either a male or female. The females give off zygotes 
 formed by the union of egg and sperm cells and the cycle 
 thus begins again. 
 
 In metagenesis, then, as illustrated by Obelia, there is 
 always an alternation of an asexual polyp generation with 
 a sexual medusa generation. Polyps cannot form other 
 polyps directly except by budding and medusae must 
 always produce germ cells which grow into polyps. At first 
 
PHYLUM CCELENTERATA 169 
 
 glance one generation is quite different in structure from 
 the other, but if some details are ignored, striking simi- 
 larities between polyp and medusa are apparent. Fig. 68 
 shows that a simple polyp may readily be transformed to 
 a medusa by two changes an additional ring of tentacles 
 must grow out at x t and the part of the polyp bearing the 
 mouth and tentacles will have to be pushed back into a cup 
 formed by an outgrowth at the bases of the new set of 
 tentacles. 
 
 If we compare Obelia with the sponge described in the last 
 chapter, it is at once apparent that we are dealing with 
 a more progressive animal. The sponge excels Obelia 
 only in its great elaboration of tissues. Though a hydroid 
 has its tissues rather poorly defined, it shows progress in 
 other ways: there is a definite digestive cavity; the ten- 
 tacles are organs, for they consist of different tissues which 
 cooperate to capture food and poke it into the enteron. 
 There is coordination between different parts of the body 
 in a hydroid, whereas a sponge must have each of its tis- 
 sues activated by a direct stimulus. If meat juice is 
 squirted in the water near an Obelia, the tentacles wave 
 toward the source of the extract and the mouth is also 
 bent in the same direction. If the tip of a tentacle is in- 
 jured the whole hydranth withdraws within the protection 
 of the peritheca. A sponge is a rather loosely coordinated 
 federation of tissues; a hydroid is a well-organized metazoan . 
 with a definite digestive cavity and simple organs. 
 
 Obelia may be looked upon as an average hydroid. Other 
 representatives vary greatly in the degree to which poly- 
 morphism and metagenesis have been developed. The 
 fresh-water Hydra (Fig. 68, Polyp) shows a very simple 
 condition. It consists of a simple polyp which may at times 
 form new individuals by budding and at other times give 
 off eggs or spermatozoa. The zygotes grow into polyps and 
 there is thus no metagenesis and no polymorphism. The 
 Portuguese man-of-war, Physalia (Fig. 70), which floats 
 in tropical seas, shows an extreme case of polymorphism. 
 
170 GENERAL ZOOLOGY 
 
 In this hydroid there is a bladder which bears a colony 
 of hydroids beneath and there are five kinds of individuals. 
 The nematocysts of Physalia are very poisonous; bathers 
 swimming against the trailing submarine batteries experi- 
 ence great pain. 
 
 FIG. 70. The Portuguese man-o'-war. A, general view; B, sectional diagram 
 to show different kinds of individuals in the colony; e, gasterozoid, or feeding 
 individual; /, dactylozoid; g, dactylozoid with tentacle; i, female reproductive 
 individual; n, male reproductive individual. 
 
 CLASS 2. SCYPHOZOA 
 
 In scyphozoans the medusa stage is always much more 
 important than the polyp. The jellyfishes do not have 
 a velum, and many of them are very large; Cyanea capil- 
 lata, which occurs off the New England coast, attaining a 
 diameter of six feet. The polyp of a scyphozoan is al- 
 ways small and inconspicuous. It forms medusae by a 
 peculiar type of budding known as strobilization, i,e., 
 constrictions are formed about the body so that it looks 
 like a pile of saucers; the top " saucer" periodically breaks 
 off and swims away as a young medusa (Fig. 71, D). 
 
 On account of their large size and great abundance in 
 many parts of the ocean, scyphozoans have been much used 
 for experimental work in zoology and physiology. One 
 
PHYLUM CGELENTERATA 
 
 171 
 
 group of experiments dealt with the control of the rhythmical 
 pulsation of the umbrella. There are eight nerve centers 
 around the margin of a jellyfish; these control the pulsa- 
 
 B 
 
 HYDROZOA SCYPHOZOA ANTHOZOA 
 
 FIG. 71. Life histories of typical coelenterates. A, zygote; B, ciliated 
 planula; C, young polyp; D, mature polyp; E, medusa. In the Hydrozoa all 
 stages usually appear in a well developed condition; in the Scyphozoa the polyp 
 is always small and unimportant; the Anthozoa never have a medusa stage. 
 
 tion and all are correlated so that the area dominated by 
 each beats synchronously with other regions. If all but 
 
172 GENERAL ZOOLOGY 
 
 one of the centers are removed the whole umbrella keeps 
 on beating as before ; if all the centers are removed, however, 
 there is no more pulsation. The medusa therefore has a 
 very wide margin of safety in case of injury. 
 
 CLASS 3. ANTHOZOA 
 
 The Anthozoa have no medusa stage, there is, in fact, no 
 indication of metagenesis. The polyps produce eggs and 
 spermatozoa which form zygotes, and these grow into 
 polyps like their parents (Fig. 71, D). Anthozoan polyps 
 are more complicated than those in any other group. 
 There is a gullet leading from the mouth into the digestive 
 cavity, which is divided up into small compartments by 
 thin partitions. 
 
 The anemones are solitary polyps which form no skeleton 
 whereas corals are colonial anthozoans. Corals live in 
 large colonies, often showing great polymorphism, and 
 secrete a skeleton of lime or other substances. Many 
 islands and atolls have been formed entirely from the skele- 
 tons of corals during periods when the bottom of the ocean 
 was slowly sinking. 
 
 GENERAL REMARKS ON OELENTERATES 
 
 When considered in relation to the Protozoa and Porifera, 
 the phylum Ccelenterata is remarkable for several reasons. 
 The following points should be particularly kept in mind: 
 (1) though many Ccelenterata are very complicated in struc- 
 ture, none is much more than a modified gastrula; (2) 
 though tissues are rather poorly developed in some, others 
 have well-defined supporting, nervous and muscular tissues; 
 (3) organs are present; (4) a definite cavity, the enteron, is 
 used for digestion and it presents the simplest condition to 
 be found in any of the Enteroccela (Fig. 18, page 24), having 
 but one opening; (5) colony formation and polymorphism 
 are common ; (6) metagenesis, or alternation of a sexual and 
 
PHYLUM CTENOPHORA 173 
 
 an asexual generation, is here developed as it is nowhere else 
 in the animal kingdom. 
 
 PHYLUM CTENOPHORA 
 
 The ctenophores, comb-jellies, or sea- walnuts (Fig. 72), 
 are extremely transparent, jelly-like animals which swim 
 at the surface of the pcean. They are often shaped like a 
 walnut, as one of their names indicates, and have two re- 
 tractile tentacles which trail behind as they swim. There 
 
 FIG. 72. A ctenophore. 
 
 are eight rows of comb-like paddle plates which propel the 
 body slowly through the water. Ctenophores show a 
 slight superiority over coelenterates in the structure of the 
 digestive system by having an opening, the anus, at the 
 opposite end from the mouth. They are usually protected 
 from enemies by their extreme transparency, which renders 
 them practically invisible during the day. Many species 
 are phosphorescent, and give off light when disturbed. 
 
CHAPTER XVI 
 
 PHYLUM PLATYHELMIA 
 
 Flatworms are bilaterally symmetrical and usually have 
 a broad leaf-like form. Some have a digestive cavity into 
 which there is only one opening, the mouth ; in tapeworms, 
 however, no digestive system is present. There are three 
 classes: 
 
 Class 1. Turbellaria; free living Platyhelmia with cilia on the out- 
 side of the body and a well-developed digestive system. 
 
 Class 2. Trematoda, or flukes; parasitic flatworms, with a tough 
 cuticle on the outside of the body and no cilia. 
 
 Class 3. Cestoidea, or tapeworms, which are parasitic, have a cuticu- 
 lar covering, and lack any digestive system. 
 
 CLASS I. TURBELLARIA 
 
 Turbellarians are the only flatworms that are not para- 
 sitic. They live for the most part in quiet places among 
 vegetation, both in the ocean and in fresh- water, and seek 
 actively for small animals which are swallowed whole. 
 The fresh-water planarians are probably the most familiar 
 representatives, and the activities of one of these worms will 
 be considered. 
 
 Planaria maculata Leidy 
 
 Self -maintenance. This turbellarian (Fig. 73) frequents 
 weedy ponds, springs, and the shores of lakes. It crawls 
 ' about among aquatic plants looking for food. Locomotion 
 is accomplished by the movement of cilia covering the out- 
 side of the body and is also greatly facilitated by the slime, 
 which gives the worm a firm enough hold on solid objects so 
 
 174 
 
PHYLUM PLATYHELMIA 
 
 175 
 
 that the body may be extended and contracted by muscular 
 movements. A planarian crawls about until it encounters 
 some little animal suitable for food. The proboscis is ex- 
 tended through the mouth (Fig. 73), wraps about the food, 
 and is then drawn into the body. Though a planarian has 
 a pair of simple eyes, they are not used to any extent in 
 seeking or capturing food, but it does have the ability to 
 recognize food through chemical substances given off in 
 the water. If an injured animal is encountered which is 
 bleeding slightly, it will be followed about persistently, but 
 
 FIG. 73. Planarians. The individual at the left id in the act of swallowing 
 a small crustacean. On the right is a planarian which has had its head cut off; 
 the body has regenerated a new head, but the small fragment has formed a two- 
 headed monster. 
 
 a sound and whole individual may not be noticed. Pla- 
 naria's mouth is in a rather strange position, at the center 
 of the ventral surface, but that situation gives good oppor- 
 tunity to stretch the proboscis out in any direction. 
 
 After food has been drawn into the body it passes down 
 the proboscis, or pharynx, into the digestive cavity which 
 has three chief branches one leading forward to the ante- 
 rior end of the body and two leading back on either side 
 of the mouth to the opposite end. The three main canals 
 have many small side pouches, and food is thus distributed 
 in the digestive system to all parts of the body. It is, 
 
176 GENERAL ZOOLOGY 
 
 therefore, unnecessary to have a separate circulatory system, 
 and none is present. Planaria does have a very elaborate 
 excretory system, however, which consists of a branching 
 tube on either side of the body. The branches end in 
 "flame cells;" so named because each is hollow and has a 
 bunch of cilia within which resemble the flickering of a 
 candle, when moving. The flame cells force waste prod- 
 ucts down the tubes and ultimately out of the body through 
 two little pores on the dorsal surface. 
 
 Self -protection. Planaria escapes most of its enemies by 
 hiding. Its eyes are of value in enabling it to shun light 
 and seek dark crevices, but the whole surface of the body 
 is also sensitive and a planarian can respond nearly as well 
 after the eyes have been removed. 
 
 If Planaria is captured and half eaten by some predaceous 
 enemy, it is not a very serious matter, for either half of the 
 body is able to regenerate what it may lack and after a 
 time becomes a complete individual again. During such 
 reconstruction no food is taken as a rule, but the body is 
 reorganized and forms a complete new worm from the sub- 
 stance already within it; the size, of course, being smaller. 
 Some species of planarians can regenerate a complete body 
 from about one twenty-fifth of the original, but there is a 
 definite limit and most species show polarity in their bodies. 
 Polarity in this sense means that certain regions are special- 
 ized and can regenerate only particular parts. For ex- 
 ample, if the head of Planaria is cut off just behind the eyes, 
 the anterior piece will not grow a new body but grows 
 another head and becomes a two-headed monster (Fig. 73). 
 A small portion from the opposite end will likewise form a 
 double-tailed individual. Such behavior shows that the two 
 ends of the body are so specialized that they can only form 
 parts like those they already possess. 
 
 Race Preservation. Planaria takes advantage of its 
 remarkable powers of regeneration to produce new individ- 
 uals. A pair of eyes, a proboscis, and other necessary 
 organs are often formed in the posterior half of body, which 
 
PHYLUM PLATYHELMIA 177 
 
 then breaks away from the anterior portion. New in- 
 dividuals are thus produced by transverse fission. 
 
 Planaria also carries on reproduction by sexual processes 
 involving the fusion of egg and sperm cells. Each individual 
 is a hermaphroditic and, therefore, has complete sets of both 
 male and female organs (i.e., testes, vasa deferentia, or sperm 
 ducts, penis; and ovaries, oviducts, sperm receptacle, yolk 
 glands, shell glands) . Though a planarian might ' i fertilize ' ' 
 itself by injecting its own spermatozoa into its female repro- 
 ductive system, it does not commonly do so, but obtains its 
 supply of sperm from some other individual, and in turn 
 gives its sperm to fertilize the other party concerned in the 
 mating. The eggs laid by Planaria are of two types, but 
 both have several yolk cells enclosed in the shell with the 
 ovum. During the summer thin-shelled eggs are laid which 
 develop quickly, but in the autumn thick-shelled "winter 
 eggs" are produced which lie dormant until spring. 
 
 The cleavage and gastrula stages in Planaria' s develop- 
 ment are somewhat peculiar but there is general agreement 
 with the stages usual in metazoans (Fig. 64). Though 
 Planaria is like a gastrula in its retention of the single 
 primitive opening into the enteron, it is specialized in many 
 other respects. There is a large mass of mesoderm which 
 consists mostly of muscles and loose parenchyma cells 
 (Fig. 76) ; the systems of organs are much better organized 
 and more elaborate than in ccelenterates, particularly the 
 reproductive, excretory and nervous systems. Specializa- 
 tion toward better things than have been suggested in the 
 structures of the sponges and coelenterates are shown by 
 Planaria (1) in the assumption of the bilateral type of 
 symmetry, and (2) in cephalization. Most of the dominant 
 and highly specialized animals on the earth are bilaterally 
 symmetrical ; in fact, radial symmetry may usually be taken 
 as a sign of racial conservatism, or even of retrogression. 
 Cephalization, or the placing of important organs at the 
 anterior end, naturally follows the acquisition of bilateral 
 symmetry. A jellyfish may move in any direction and 
 
178 GENERAL ZOOLOGY 
 
 what little power it has of directing its attention must be 
 divided between the eight or more marginal sense organs. 
 A planarian has only one chief nerve center in the head; 
 the eyes and other sense organs are localized at the anterior 
 end. The jellyfish can perceive stimuli from different 
 directions; the planarian always moves with a certain part 
 of the body ahead and this part receives most of the stimuli 
 from the outside world. Planaria is better able to devote 
 its very limited powers of attention to one thing at a time 
 because the chief perceiving and modifying organs are 
 gathered together to some extent, and from a psychological 
 point of view this is an important point. The ability to 
 pay attention comes before the power to draw conclusions, 
 or to reason. Though Planaria, on account of its bilateral 
 symmetry and cephalization, is on the road which leads 
 toward the higher types of mental development, it has made 
 little progress. Its mental powers are at best extremely 
 limited, and its activities consist largely of a few rather 
 simple, and more or less stereotyped, reactions. 
 
 There are a number of different kinds of turbellarians. 
 Some are much more complicated structurally than Planaria, 
 and others are far simpler. The rhabdoco3les usually have a 
 straight unbranched digestive cavity; some even have no 
 definite enteron and the food simply enters irregular spaces 
 in the parenchyma after passing down the pharynx. The 
 polyclads have many intestinal branches leading out from 
 the pharynx. Planaria, like most freshwater turbellarians, 
 belongs with the triclads because its enteron has three chief 
 branches. 
 
 CLASS 2. TREMATODA 
 
 The trematodes are all parasitic flatworms with a cutic- 
 ular covering and a rather simple enteron. They may be 
 divided into two groups, according to habits. The ecto- 
 parasitic trematodes are monogenetic, that is, they live on 
 the outside of one host. They have a simple life history, 
 the adults laying eggs which hatch into small worms like 
 
PHYLUM PLATYHELMIA 
 
 181 
 
 found in the intestine of the animal which eats the herbi- 
 vore. There are species which have their cycles between: 
 pig and man, cow and man, fish and man, mealworm and 
 rat, flea and dog, rabbit and wolf, etc. 
 
 The life cycle of the common beef tapeworm is shown in 
 Fig. 75. An adult (A) in a man's intestine is continually 
 giving off ripe proglottids (B) which pass out and shed 
 eggs (C) upon the ground. A single proglottid may 
 produce several thousand eggs, and the contamination of 
 
 HOST 
 
 FIG. 75. Life history of beef tapeworm. A, adult tapeworm in intestine of 
 man; B, proglottid full of eggs on ground; C, eggs on ground; D, six-hooked 
 larva (onchophore) set free and bores through tissues of cow; E, cysticercus, or 
 bladderworm, in cow's flesh; F, young tapeworm in man. 
 
 the food of cattle is therefore easily accomplished. A ripe 
 egg contains a little 6-hooked embryo (D) which is freed 
 from the egg shell in the cow's intestine and at once 
 bores out into the body. In the liver, or the muscles, it 
 comes to rest and undergoes a complete reorganization. 
 After three months a cysticercus, or bladder- worm (E), 
 has developed and the cow 's flesh may then infect man with 
 tapeworms. The cysticercus is a little bladder with an in- . 
 
182 GENERAL ZOOLOGY 
 
 verted scolex on one side. If it gets into a man's intestine, 
 the scolex is pushed out (F), fastens its suckers on the wall 
 of the intestine and soon begins to bud off proglottids. 
 About 1 per cent, of all the cattle slaughtered in the 
 United States have tapeworm larvae in their flesh, and there 
 is another species which frequently infests pork. Any of 
 these larvae will be killed if meat is thoroughly cooked. A 
 piece of meat if boiled should be cooked about fifteen min- 
 utes for each pound. 
 
 PHYLUM NEMERTINA 
 
 The nemerteans resemble the flatworms in their general 
 form but are more specialized in many respects : an anus is 
 present, the mouth is at the anterior end, there is a retrac- 
 tile finger-like proboscis above the mouth, a definite cir- 
 culatory system is present for carrying blood. Some 
 nemerteans are of large size, attaining a length of ninety 
 feet. Most species live along the seashore, but a few are 
 found in fresh water and on land. 
 
CHAPTER XVII 
 PHYLUM NEMATOIDEA 
 
 The thread- worms, or round- worms, are slender and 
 cylindrical in form. They have a tubular digestive system 
 beginning with a mouth and opening at the opposite end 
 through an anus. There is a body-cavity between the 
 digestive tube and the body wall (Fig. 76). Different 
 species of round-worms vary greatly in habits : some are free 
 living, others are scavengers, and many live as parasites in 
 plants or animals. There are three classes: 
 
 m 
 
 -. ' 
 
 fi 
 ft n 
 
 A 
 FIG. 76. Cross-sections of a flatworm (A) and a round-worm (B) compared. 
 
 Class 1. Nematoda, round-worms with a well-developed digestive 
 system and a rather simple life history. 
 
 Class 2. Gordiacea, horsehair worms. Parasitic Nematoidea with 
 the digestive system often partly degenerate; life history very complex 
 and requiring two or three hosts for its completion. 
 
 Class 3. Acanthocephala, hook-headed worms. Parasitic worms 
 without a digestive system and with a proboscis at the anterior end which 
 is armed with spines; in the life cycle there is an alternation between 
 two hosts. 
 
 CLASS 1. NEMATODA 
 
 Most nematodes are. very resistant to variations in the 
 conditions of life, hence it is easy for them to live in all sorts 
 
 183 
 
184 GENERAL ZOOLOGY 
 
 of odd situations. Many may become parasites if there is 
 opportunity to do so, and a large number of species are 
 permanently parasitic. As an example of the group a 
 parasite of man will be considered. 
 
 Ascaris Lumbricoides Linnaeus 
 
 Self -maintenance. Ascaris is a round-worm nearly a foot 
 long which lives in the small intestine of man. It obtains 
 its nourishment from the partly digested food of its host. 
 As long as the man eats, the worm has a plentiful supply. 
 There are three lobes at the anterior end which act as lips 
 and poke food into the mouth. These are assisted by a 
 muscular pharynx which exerts a sucking action and draws 
 particles inside the body. Behind the pharynx the food 
 traverses a straight intestine, where it is digested and 
 absorbed. Undigested fragments pass out through the 
 anus near the posterior end of the body. No special cir- 
 culatory system is present, but the absorbed food passes 
 through the wall of the intestine into the body-cavity 
 (Fig. 76, B), where it is free to move from one end of the 
 body to the other, and all parts are thus supplied. Excre- 
 tions are collected by two tubes which run longitudinally on 
 either side of the body (ex) and open together to the exterior 
 through a small pore on the ventral side near the anterior 
 end. 
 
 Self -protection. Ascaris is in no danger from predatory 
 enemies, for it rests safe inside some man. It is not digested 
 by the strong secretions in the intestine, as the cuticle on 
 the outside of its body is impervious to most chemicals 
 and furnishes adequate protection. If the man containing 
 an ascarid dies, the worm leaves the intestine and is often 
 found, on post-mortem examination, in the stomach, throat 
 or mouth. If the worm succeeds in escaping from the body, 
 it is able to live for some time and may perhaps find en- 
 trance into another host. As a rule, however, Ascaris is in 
 no danger if it once finds a host but is able to pass its life 
 in quiet and seclusion. 
 
PHYLUM NEMATOIDEA 
 
 185 
 
 Race Preservation. Ascaris is dioecious, that is, the 
 sexes occur in separate individuals. Males are easily dis- 
 tinguished from females by their smaller size, by the coiling 
 of the tail, and by the two little setae which project just in 
 front of the anal opening (Fig. 77). A female has a pair of 
 long tubular ovaries which swing freely in the body-cavity 
 and occupy a large part of its space. The ends of the tubes 
 are enlarged to form a pair of uteri in which the eggs develop 
 to some extent before passing from the body. The males 
 have a single tubular testis which opens through the anus. 
 
 The eggs are fertilized far up in the- ovarian tubules and 
 pass down into the uteri where they have the shell added. 
 
 C D E F G H I J 
 
 FIG. 77. Life History of Ascaris. A, male; B, female; ex, excretory pore; 
 u, female genital aperture; C, zygote; D, zygote without covering: E-H, 
 cleavage stages; I, J, embryos; K, young Ascaris emerging from egg shell. 
 
 A female Ascaris is continually laying eggs, producing as 
 many as 15,000 in a day. A large number of reproductive 
 products is commonly associated with parasitic life and is 
 necessary because the chances of finding a favorable place 
 for development are small. This is, however, the only 
 respect in which Ascaris shows adaptation to parasitism. 
 The eggs pass out of the host and develop into larvae in 
 moist earth (Fig. 77, C-K) but do not hatch unless they get 
 into a man (K). They cannot infect until they have been 
 upon the ground thirty to forty days, and then are intro- 
 duced into the human body on polluted vegetables, in water, 
 or from soiled hands. After gaining entrance the worm 
 hatches, becomes mature, and begins to lay eggs in about a 
 
186 GENERAL ZOOLOGY 
 
 month. Recent investigations indicate that Ascaris may 
 perhaps pass through larval stages in rats or mice before 
 entering man. 
 
 Comparing Ascaris with a flat worm, it shows two great 
 structural advances a body-cavity and an anus. The 
 flatworm must ingest and egest through a single opening; 
 the round-worm has a much more convenient arrangement 
 with an entrance for food and an exit for faeces. The body 
 cavity is not a true ccelom, like that in the earthworm (Figs. 
 16, 89) and other animals, but is sufficiently developed to 
 make the general structure of Ascaris' body a tube-within- 
 a-tube, as described in Chapter II. It is simply a space 
 between the muscles of the body wall (Fig. 76, B, ra) and 
 the digestive tube. This space is a great convenience, for 
 it allows fluids to flow readily from one part of the body to 
 another without having to pass from cell to cell, as is the 
 case in sponges, ccelenterates, and also to some extent in 
 flatworms. 
 
 OTHER NEMATODES 
 
 An Ascaris living in a man usually causes no very serious 
 trouble, but sometimes one will crawl up the bile ducts into 
 the liver and give rise to an abscess, or lodge in the appendix 
 and induce appendicitis. Some other round-worms which 
 live in man are much more injurious. The hook-worm, 
 Necator americanus, is prevalent throughout the south- 
 eastern United States, where it was probably introduced 
 by slaves brought from Africa. It gnaws the inner wall of 
 the intestine and secretes toxins which impoverish the 
 blood and thus weaken the body. The hook-worm has a 
 peculiar life history the adults lay eggs in the intestine; 
 the eggs hatch into larvae which live for a time in excre- 
 ment or in the soil, and when grown enter man by boring 
 through the skin; they then get into the blood and finally 
 gain entrance to the alimentary canal by way of the lungs, 
 trachea, and larynx. Persons with a heavy hook-worm in- 
 fection may be abnormal in many ways: stunted and weak; 
 
PHYLUM NEMATOIDEA 187 
 
 beard poorly developed; perverted appetite for clay, dirt, 
 etc.; dropsical condition, and other peculiarities. The 
 Rockefeller Sanitary Commission has agents throughout the 
 southern states who are trying to better conditions among 
 the poorer people by treatment and by instruction as to 
 proper methods of living. In many districts the shiftless- 
 ness of the population is largely due to the prevalence of 
 hook-worm. Ninety per cent, of the people in some 
 counties in Louisiana have this parasite. I 
 
 Another very injurious round-worm is trichina, Trichinella 
 spiralis, which lives in rats, hogs, men, and a few other 
 animals. This parasite may be acquired only by eating 
 flesh containing the encysted larvae, but if meat is 
 thoroughly cooked there is no danger of infection. Other 
 important human parasites are the pin-worm and the whip- 
 worm, which live in the intestine; and the filaria worms 
 which inhabit the blood and lymph vessels. The latter 
 cause a horrible disease known as elephantiasis, in which 
 the legs and other parts of the human body become greatly 
 enlarged. They occur only in tropical countries, and are 
 transmitted by the bites of certain mosquitoes. There are 
 many round-worms which cause diseases in animals and 
 plants. Tylenchus tritid is an important plant parasite 
 which produces gall-like growths on the roots of tomatoes 
 and other plants. 
 
 CLASS 2. GORDIACEA 
 
 These worms (Fig. 78, A) are commonly known as horse- 
 hairs, horsehair worms, and horsehair snakes, on account of 
 their hair-like form, and there is a belief among ignorant 
 persons that horsehairs will turn into worms if kept in 
 water. They are often very numerous along the shores of 
 ponds and streams during their breeding season. At such 
 times they wrap about each other in great bunches, forming 
 veritable "Gordian knots " (whence the name of the class). 
 Sometimes the adult worms get into man and live for a time 
 as parasites. 
 
188 GENERAL ZOOLOGY 
 
 Most Gordiacea spend the greater part of their lives as 
 parasites in insects and often have very complicated life 
 cycles. Parachordodes tolosanus has two larval stages : the 
 first enters the aquatic larva of the orl-fly and lives through 
 the winter; the second develops in a beetle which has 
 eaten an adult orl-fly and passes another winter in the 
 beetle. The adult lives in the water, as a parasite in 
 man, or in some other animal. 
 
 CLASS 3. ACANTHOCEPHALA 
 
 The hook-headed worms (Fig. 78, B) are parasitic in 
 fishes and other vertebrates, sometimes even in man. They 
 
 B 
 
 A 
 
 FIG. 78. A, a horsehair worm, Gordius; B, a hook-headed worm 
 ( Acanthocephala) . 
 
 have no digestive system but live attached to the wall of 
 the intestine like a tapeworm. At the anterior end there is 
 a protrusible proboscis which is armed with hooks and 
 serves as an organ of attachment. 
 
 The development of acanthocephalans requires an alter- 
 nation of -hosts; the larva usually living in some insect and 
 the adult in a vertebrate. The common June-bug, or June- 
 beetle, carries the larva of Gigantorhynchus suis. This 
 parasite lives as an adult worm in the intestines of pigs, 
 which become infected by eating beetles. In South 
 Russia, where beetles are eaten raw, the worm is said to 
 occur in man. 
 
 PHYLUM ROTIFERA 
 
 On account of the ciliated discs at the anterior end, 
 rotifers (Fig. 79), were called " wheel-animalcules' 7 by 
 the zoologists who first studied them. When the cilia are 
 
PHYLUM ROTIPERA 
 
 189 
 
 in motion they make the discs appear like rotating wheels 
 hence the name Rotifera (fero, I carry; rota, a wheel). 
 
 These animals are remarkable for their small size. Though 
 they are well-developed metazoans with more special- 
 ized systems of organs than round-worms, they are usually 
 no larger than single-celled animals, and most species can 
 barely be seen with the naked eye. The mouth is situ- 
 ated between the trochal (ciliated) discs where it receives 
 the food brought in the vortex created by the cilia. The 
 food is ground in a mastax, or chewing stomach, which is 
 
 FIG. 79. Rotifera. 
 
 provided with chitinous teeth. Behind the mastax there 
 is a glandular stomach where digestion takes place, and the 
 residue passes out through an anus on the dorsal side near 
 the posterior end. The excretory system consists of a 
 pair of branched tubules and a contractile bladder which 
 empties through the anus. Rotifers have a body cavity, 
 but it is not a true ccelom. 
 
 Rotifers are of separate sexes. The males are smaller than 
 the females and are often degenerate sometimes the 
 digestive system is entirely lacking. The females produce 
 two kinds of eggs: summer and winter. The summer eggs 
 are parthenogerietic, have thin shells, and are of two sizes 
 the larger producing females and the smaller males. The 
 
190 GENERAL ZOOLOGY 
 
 winter eggs, which are fertilized, have thick shells, are 
 able to pass through a long resting period before hatching, 
 and always develop into females. 
 
 The ordinary life of a rotifer is about six weeks but certain 
 species, if dried slowly, will form gelatinous cysts and are 
 then able to live in a dormant condition for a long time. 
 When encysted, rotifers can stand great extremes of tem- 
 perature without being killed. 
 
 Rotifers in their adult condition resemble a trochophore 
 larva, which occurs in the development of certain molluscs, 
 annelids, and in some other animals. This has led some 
 zoologists to believe that rotifers are closely related to the 
 ancestral forms of molluscs, worms, etc., but such relation- 
 ship cannot be very close because the rotifers are a very 
 specialized group, and would not, therefore, give rise to 
 new lines of descent. 
 
 PHYLUM BRACHIOPODA 
 
 Brachiopods are marine animals having a calcareous 
 bivalve shell and usually a short stalk which fastens them 
 to the ocean bottom. They resemble clams somewhat in 
 appearance but have no close relationship. The valves 
 of a brachiopod's shell are dorsal and ventral, instead of 
 right and left as in clams. The body is very small and the 
 space within the shell is mostly filled by two ciliated arms 
 which create currents for procuring food. Within the body 
 there is a spacious coelom, a stomach, heart, and digestive 
 gland. An anus may be present or absent. All the struc- 
 tures mentioned show some resemblance to those of other 
 worm-like forms, but are modified to suit a sessile life. 
 
 Brachiopods lead a sluggish existence at the bottom of 
 the ocean, and have done so since the earliest geological 
 times. Most of the shells which appear as fossils are the 
 remains of these animals. The abundance of such deposits 
 indicates the past dominance of the Brachiopoda, which 
 at one time shared the earth with trilobites and archaic 
 
PHYLUM PRYOZOA 191 
 
 molluscs when there were no vertebrates to interfere with 
 their simple activities. 
 
 PHYLUM BRYOZOA 
 
 Bryozoans, or "moss-animals" are usually colonial in 
 habit and resemble hydroids in general appearance. Most 
 species are marine but a few occur in fresh water. They 
 appear as encrustations, or as little- plant-like growths, 
 attached to objects in the water. The individuals of a 
 colony are called zooids. Each has a circle of tentacles 
 about the mouth and shows a superficial radial symmetry, 
 as is the case in many attached animals. The digestive 
 tube is U-shaped and the anus therefore opens near the 
 mouth. Despite their superficial resemblance to hydroids, 
 bryozoans are more nearly related to the worms. They, 
 like the brachiopods, are peculiar on account of their seden- 
 tary existence. 
 
CHAPTER XVIII 
 PHYLUM ECHINODERMATA 
 
 The name of this phylum means " spiny skin," and is 
 fitting for most representatives. Echinoderms are radially 
 symmetrical, and usually have five antimeres; an anus is 
 present in some but non-functional or absent in others ; the 
 ccelom is large and well developed. The distinctive feature 
 of echinoderm structure is the ambulacral, or water-vas- 
 cular, system which serves primarily for locomotion. There 
 are five classes: (1) Asteroidea, starfishes; (2) Ophiuroidea, 
 brittle-stars, or snake-stars; (3) Echinoidea, sea-urchins; 
 (4) Holothuroidea, sea-cucumbers; (5) Crinoidea, stone- 
 lilies. 
 
 CLASS 1. ASTEROIDEA 
 
 The starfishes all have the chief axis short, and the dis- 
 tance from the mouth to the opposite pole of the body is 
 accordingly abbreviated. The central portion of the body, 
 or disc, is not sharply set off from the arms, but grades 
 rather gradually into them. Most starfishes have five arms, 
 or rays, but there are several species which have more; the 
 number sometimes reaching as high as forty. The common 
 starfish along the seashore on the Atlantic coast of the 
 United States is Asterias forbesi (Desor) Verrill, and it will 
 be discussed in detail as an example of the class. 
 
 ' THE STARFISH 
 
 Self-maintenance. Asterias (Fig. 80) moves about by 
 means of the tube-feet, of which there are four rows extend- 
 ing from a longitudinal groove along the underside of each 
 arm. A tube-foot has an adhesive disc at its tip and is con- 
 
 192 
 
PHYLUM ECHINODERMATA 
 
 193 
 
 nected with a little reservoir, or ampulla, within the body 
 wall. Muscles in the wall of the tube-foot make it possible 
 to move the organ about, like an elephant's trunk, and the 
 adhesive disc may thus be put down in a particular spot. 
 When the tube-foot is extended the reservoir connected 
 with its base is small, but if it contracts, the reservoir is 
 distended by the water which is forced into the body. 
 Water first enters the starfishes' water-vascular system 
 through the madreporic plate, or " sieve plate," passes 
 through a straight stone canal to a ring canal around the 
 mouth, and from this a radial vessel which supplies the 
 
 B c 
 
 FIG. 80. Starfishes. A, eating an oyster; B, an individual which has lost 
 four rays and is growing them anew; C, crawling by using the tube-feet. 
 
 tube-feet runs down the under side of each arm. The 
 broad spread of the body and the great number of tube- 
 feet enable starfishes to walk over soft mud, and the ad- 
 hesive discs permit them to climb up vertical surfaces with 
 ease. Though able to move but slowly, starfishes are well 
 equipped to seek for food. 
 
 A starfish can tell when substances are present in the 
 water which indicate that food is near. If an individual is 
 
194 GENERAL ZOOLOGY 
 
 resting quietly in a current and clam juice is added to the 
 water, it will become restless and move about. If a screen 
 is placed so that the starfish cannot reach the food, it will 
 move as near as possible and try to pass by the obstruction. 
 
 On account of its taste for molluscs, the starfish is one of 
 the worst enemies of the oyster and clam fishermen. The 
 method by which a starfish opens an oyster was long a matter 
 of speculation, . erroneous observation, and dispute. One 
 man thought an acid was secreted which dissolved the 
 shell; another asserted that the starfish took the oyster by 
 surprise and stuck its arm in the gaping shell before it 
 could be closed; other equally fantastic theories had advo- 
 cates. The starfish opens oysters by tiring them out. 
 The tube-feet are attached on either side of the shell and a 
 steady pull is thus brought to bear on opposite sides (Fig. 
 80, A). The oyster has two strong muscles to keep the 
 shell closed, but these finally become tired and the starfish 
 then gains access to the food inside. 
 
 A starfish has a most direct route from food to stomach. 
 After an oyster has been opened, soft folds of the stomach 
 are protruded through the mouth and wrapped about the 
 parts suitable for food. Digestion and absorption take 
 place inside the oyster's shell and outside the starfish's 
 body! There is one disadvantage connected with this 
 method of feeding large amounts of digestive fluids are 
 wasted. A starfish therefore has enormous digestive glands 
 which fill most of the space within the body. The anus in 
 most starfishes is not functional, and, indeed, it has small 
 chance to be, for waste seldom enters the short intestine. 
 
 About the big baggy stomach there is a spacious ccelom 
 and in this cavity the food may pass readily to all parts of 
 the body. A set of tubes (hsemocoel) also carries fluid to 
 certain obscure regions, but the starfish has little need of 
 a separate circulatory system. Respiration takes place 
 through the tube-feet and branchiae. The former are 
 admirably fitted for respiratory purposes; the reservoirs, or 
 ampullae, being in the coelomic cavity, and the external por- 
 
PHYLUM ECHINODERMATA 195 
 
 tions with their thin walls swinging about freely in the 
 water. The branchiae are minute, finger-like organs which 
 are scattered plentifully over the whole upper (aboral) 
 surface of the starfish. They are hollow and connect 
 directly with the coelom. Cilia keep currents passing in and 
 out of the branchiae, and the coelomic fluid is thus aerated 
 through the thin membranes exposed to the sea water. 
 Asterias has no special means of excretion, except little 
 amoeboid cells* which wander about in the tissues engulfing 
 waste products. These finally migrate outside the body 
 with their loads. 
 
 Self-protection. Asterias is protected from most prer 
 daceous animals by the hard, spiny skeleton. When left 
 stranded at low tide, however, its watery body readily 
 dries up. A starfish crawls boldly over the rocks at high 
 tide, but creeps into crevices and hides when the ocean 
 ebbs. There is a simple eye at the tip of each ray and the 
 whole surface of the body is more or less sensitive to light ; 
 so Asterias can tell when it is in light or shadow, and is 
 able to crawl toward dark spaces. 
 
 A starfish moves very slowly and the body gathers many 
 particles of dirt which fall from above. Such accumula- 
 tions are continually swept to the edge of the body and off 
 by the cilia which cover the soft parts between the spines. 
 There are many very small animals in the ocean which 
 might gnaw the branchiaB and the flesh of the starfish if it 
 were not for the presence of little seizing organs, the 
 pedicellarise, which are to be found around the bases of the 
 spines. Each pedicellaria consists of a short stalk sur- 
 mounted by a pair of calcareous pinchers. If a small 
 animal starts to walk over the starfish, it is seized and held 
 immobile until it dies (Fig. 80, C). The pedicellariae are 
 rather independent in reacting and will often pull against 
 each other. They grasp anything that moves and hold on 
 until they are jerked loose from the body, or until the thing 
 they hold ceases to move. Through their activities the 
 
196 
 
 GENERAL ZOOLOGY 
 
 soft parts of the body are shielded from small predaceous 
 animals. 
 
 If a starfish meets with an accident and has one of its 
 arms hurt, the injured appendage is cast off near its base and, 
 after a time, a new one grows out. The body may be cut 
 in two and each of the parts can regenerate what it lacks. 
 By careful experiments it has been found that Asterias for- 
 besi is able to form a complete body if left with the disc and 
 one arm (Fig. 80, B). Professor Mead kept a separate 
 arm alive for three months, but it did not regenerate any- 
 thing, although it was apparently in good condition and 
 crept about actively. Some years ago starfishes were com- 
 monly dredged up, cut in two, and thrown back into the 
 
 FIG. 81. Development of starfish. A, egg and sperm cells; B, zygote; 
 C F, cleavage stages; F, morula; G, section through gastrula stage; H, ciliated 
 bilaterally symmetrical larva; a, anus; m, mouth; I, young starfish growing out 
 from side of bilaterally symmetrical larva. 
 
 ocean. The oyster fisherman supposed they were killed 
 by such treatment, but it of course only increased the num- 
 ber of the pests. 
 
 Race Preservation. Starfishes are dioecious animals and 
 both the males and females shed their ripe sexual cells into 
 the ocean. The sperm cells swim to the ova, fertilize them, 
 and development passes through the stages characteristic 
 of all metazoans (Fig. 81, A-G). After gastrulation (G) 
 a bilaterally symmetrical larva is formed, which is known 
 as a brachiolaria (H). It has a mouth (m), digestive tube, 
 
PHYLUM ECHINODERMATA 197 
 
 anus (a), and ccelom. After swimming about for a day or 
 two the brachiolaria attaches itself to a bit of seaweed and 
 five little arms grow out on one side, thus beginning the 
 development of the radially symmetrical starfish (I) . Most 
 of the brachiolaria collapses and in part disintegrates; 
 the young starfish soon starts out to hunt clams and oysters. 
 The growth of a starfish depends upon the amount of food 
 it is able to secure. Mead separated two which changed 
 from larvae at the same time and kept them under obser- 
 vation for thirty-nine days. One was fed, the other received 
 no food, and at the end of the period the former was 
 fifty times as large as the latter. A starfish may become 
 mature in less than a year; and, as each female lays several 
 thousands of eggs, the rate of increase is rapid. 
 
 Although Asterias, like most starfishes, uses only sexual 
 methods of reproduction to produce new individuals, there 
 are some species which are able to multiply asexually. 
 Linckia, for example, breaks off the tips of its arms and such 
 fragments grow into complete new individuals. 
 
 GENERAL REMARKS ON STARFISHES 
 
 The starfish is a fairly successful representative of a 
 somewhat unprogressive biological principle. It has greatly 
 elaborated the radial type of symmetry, and represents, 
 with other echinoderms, the highest degree of speciali- 
 zation along such lines. That the radial arrangement of 
 body parts has some things in its favor cannot be disputed. 
 It is certainly advantageous to be able to move with equal 
 facility in any direction. It is desirable to be able to 
 take food or receive notice of danger on any side of the 
 body. 
 
 But usually the disadvantages more than compensate for 
 the favorable features associated with radial symmetry. 
 As was pointed out in connection with the coelenterates, 
 it is hard for a radially symmetrical animal to correlate its 
 abilities for a particular end. A starfish has five arms 
 
198 GENERAL ZOOLOGY 
 
 bearing five equally important sets of sense organs (eyes, 
 tactile tentacles, etc.) and what little power of attention 
 it has must vacillate between these or be continually divided. 
 This is important because the development of higher 
 psychical processes (memory, power of attention, reason, 
 etc., as they appear in animals with well-centralized nervous 
 systems) depends upon seriously attending to one thing 
 at a time. 
 
 The behavior of a starfish is largely made up of more 
 or less automatic " reflexes." If an individual is turned 
 with its mouth uppermost, it slowly pulls itself over again, 
 by using the tube-feet. Observers have noticed that 
 various individuals may have different methods of righting 
 themselves that is, they have different " habits" of doing 
 the same sort of work, and, therefore, show some individ- 
 uality. Professor Jennings induced starfishes to forsake 
 their usual methods of turning over, and taught them new 
 ways. For example, a certain individual always began to 
 turn with a particular arm, and when prevented from using 
 that, learned to begin with another. Such lessons are very 
 simple, but they show the limits of the starfish's "mind." 
 
 A starfish has its nervous system separated into three 
 divisions; all essentially alike in structure. There are 
 five nerves running longitudinally in each of the arms, and 
 these are connected in the disc. Two divisions are near 
 the oral surface, and the third lies in the aboral body wall. 
 If the central connections are cut, the arms no longer 
 work together, but e.ach can still carry on its own activities. 
 If an individual in which the connecting nerves have been 
 severed is turned over, each of its arms tries to get back by 
 itself. So far as its psychology is concerned, a starfish 
 consists of five centers, or individuals, which usually work 
 in harmony through the influence of the central connections. 
 
 The behavior made necessary by radial symmetry has, 
 then, made the starfish unprogressive, and generally diffuse 
 psychologically. And, if we look at the starfishes' ances- 
 tral tree, there is evidence that there has been retro- 
 
PHYLUM ECHINODERMATA 199 
 
 gression. The embryological evidence (Fig. 81) indicates, 
 according to the Law of Biogenesis, that starfishes were 
 once free swimming bilaterally symmetrical animals (Fig. 
 
 81, H), with all the possibilities that go with that type 
 of bodily arrangement, and that they gave up certain 
 possibilities to become radially symmetrical and unprogress- 
 ive. It is believed that this change came about because 
 echinoderms at one time in their racial history took up 
 a sessile mode of life and grew attached to the ocean bottom. 
 It is desirable for an attached animal to be able to receive 
 stimuli. and capture food from as many directions as possible, 
 hence the assumption of radial symmetry. But echino- 
 derms, as a race, made a serious mistake when in the 
 course of evolution they came to an attached mode of 
 existence, for no sessile animal can progress very far. 
 There are now some points of actual degeneracy. For ex- 
 ample, the anus of the larva is functional (and was probably 
 so in the ancestral types) , but in many adult echinoderms is 
 not used or is absent. In modern times there have been 
 evolutionary attempts to correct early racial errors: 
 most echinoderms, like the starfish, have taken up a free- 
 living existence; some sea-urchins and sea-cucumbers have 
 even become bilaterally symmetrical as adults (Fig. 
 
 82, B, F). 
 
 CLASS 2. OPHIUROIDEA 
 
 The brittle-stars, or snake-stars (Fig. 82, C) are very 
 active, being able to bend the solid arms readily and thus 
 swim about. They feed mostly on minute organisms 
 which they scrape from marine plants with their feet. 
 No anus is present. 
 
 On account of their activity, ophiuroids have been much 
 used for experimental work. Attempts to teach them 
 simple lessons have met with little success. In one series 
 of experiments a rubber tube was slipped over one of an 
 ophiuroid's arms, and behavior in attempting to remove it 
 observed. There was " neither a decrease in the amount 
 
200 
 
 GENERAL ZOOLOGY 
 
 of time taken to solve the problem, nor an increase in the 
 frequency of movements best fitted to solve it," hence no 
 learning. 
 
 Another line of experimental work related to the regenera- 
 tion of lost parts. Ophiuroids readily break off pieces of 
 their arms (whence the name, brittle-star) and many 
 species commonly practise autotomy to escape from ene- 
 mies. If an arm is grasped, it is thrown off and left 
 to wriggle in the claws of the aggressor, while the ophiuroid 
 escapes. Missing parts are readily regenerated and this 
 
 A B C D F 
 
 FIG. 82. Echinodermata. A, crinoid; B, heart urchin; C, brittle star; 
 D, regular sea urchin; E, sea cucumber buried in mud; F, cake urchin, or "sand 
 dollar." 
 
 was made the basis for experimental work. One investi- 
 gator maintained that the rate of regeneration was propor- 
 tional to the degree of injury (i.e., the more cut off, the faster 
 the growth to replace the lost part), but further experiments 
 showed that such a ratio would not hold absolutely. 
 
 CLASS 3. ECHINOIDEA 
 
 The sea-urchins differ from all other echinoderms in 
 possessing (1) an immovable calcareous test, which is 
 
PHYLUM ECHINODERMATA 201 
 
 usually more or less obscured by tube feet and spines, and 
 (2) by the presence of a complicated system of calcareous 
 ossicles which make up a chewing apparatus known as 
 " Aristotle's lantern." Sea-urchins are globular, discoidal, 
 or heart-shaped, and at first glance have little in common 
 with the starfish, but, if one could take a starfish and bend 
 the arms upward so that their ends came together and then 
 fuse the sides, he would have made a sea-urchin. The cal- 
 careous tests of sea-urchins make respiration difficult and 
 many species have therefore developed little brush-like 
 gills about the mouth. 
 
 The cake-urchins and heart-urchins have the same general 
 plan of structure as the/ 'regular" sea-urchins, but have sec- 
 ondarily assumed bilateral symmetry. The cake-urchins, 
 or sand dollars (Fig. 82, F), have the anus at one side (in- 
 stead of at the pole opposite the mouth as in regular forms) , 
 and are very flat, so that they may move about on a soft 
 bottom without sinking in. The heart-urchins have the 
 mouth at one side and the anal opening at the other (B). 
 
 CLASS 4. HOLOTHUROIDEA 
 
 In the sea-cucumbers (Fig. 82, E) the calcareous exoskele- 
 ton, so characteristic of other echinoderms, is extremely 
 degenerate, being reduced to little spicules which are em- 
 bedded in the leathery skin. The chief axis is long, so 
 that the body is worm-like. There is a ring of tentacles 
 for feeding about the mouth. Respiration usually takes 
 place through the anus; water being pumped by muscular 
 action into two great respiratory trees, which branch out 
 among the internal organs and fill up a large part of the 
 space within the body. 
 
 Sea-cucumbers show remarkable instances of autotomy, 
 or the casting off of parts of the body. One species, if re- 
 moved from the sand in which it is accustomed to burrow, 
 will break off pieces from its posterior end until only a twen- 
 tieth of the original body remains. Another species when 
 
202 GENERAL ZOOLOGY 
 
 placed in stagnant water, will throw out almost all of its 
 internal organs (entire alimentary canal, most of the nervous 
 and water- vascular systems), in fact, everything except the 
 body wall and the respiratory tree. Certain tropical species 
 when taken in the hands will turn into a slippery mass of 
 slime which cannot be held. Many of these sea-cucumbers 
 can, however, if conditions become favorable, regenerate all 
 their missing parts in a short time. 
 
 CLASS 5. CRINOIDEA 
 
 The crinoids, or stone-lilies (Fig. 82, A), live for the most 
 part at considerable depths in the ocean. They have five 
 branched arms which are covered with little tentacle-like 
 tube-feet. All crinoids are attached by the aboral pole 
 when young, but some species break loose from the stalk 
 when mature and swim about by waving their arms. The 
 permanently stalked species often attain a great length, 
 some measuring thirty or forty feet. Stone-lilies flour- 
 ished in great numbers during past geological ages, and lime- 
 stone deposits are often filled with their remains, the most 
 common objects being little calcareous discs from the stalks. 
 
CHAPTER XIX 
 PHYLUM ANNELIDA 
 
 The annelids are more completely metameric than the 
 animals in any other phylum. The flatworms and nema- 
 todes do not have any indication of segmentation in their 
 bodies, the rotifers are only imperfectly segmented, but the 
 annelids are always made up of a chain of very similar 
 parts. The phyla which rank above the annelids in 
 structural complexity have the metamerism obscured by 
 the outgrowth of the limbs and other appendages (Arthro- 
 poda, Chordata), or lack any trace of segmentation (Mol- 
 lusca). This phylum therefore shows metamerism as it 
 appears nowhere else in the animal kingdom. As a rule 
 the ccelom of annelids is large and well developed; the body 
 is therefore of the tube-within-a-tube type (Fig. 16). The 
 alimentary canal possesses two openings. Annelids live 
 in the ocean, in fresh water, and on land; some are even 
 parasitic upon other animals. There are two important 
 classes and one or two of less general interest, which will 
 not be considered. 
 
 Class 1. Choetopoda. Annelids with setae, or bristles, along the 
 sides of the body. 
 
 Class 2. Hirudinea. Without setae, but with suckers at either end 
 of the body. 
 
 CLASS CILETOPODA 
 
 The chsetopods (chaite, bristle; pous, foot) are abundant 
 in most parts of the earth and are easily recognized by the 
 setae along the sides of the body. Sometimes the setse pro- 
 trude in great brush-like tufts from each metamere, in 
 other instances they are small and almost completely buried 
 
 203 
 
204 
 
 GENERAL ZOOLOGY 
 
 in the body wall. They usually take the form of stiff horny 
 bristles which serve for locomotion. The earthworms are 
 the most familiar representatives of the class, and one of the 
 commoner species will therefore be considered as an example. 
 
 THE EARTHWORM, Lumbricus hercukus Savigny 
 
 This is the large earthworm found burrowing in humus 
 soil throughout the northeastern United States (Fig. 83). 
 It sometimes reaches a length of ten or twelve inches, and 
 is most often seen above ground after rains. 
 
 Self -maintenance. Lumbricus eats its way through the 
 ground and thus obtains considerable food from the organic 
 matter present. Most of the material excavated passes 
 
 A .* 
 
 FIG. 83. Earthworms. A, castings; B, mating; C, egg cocoon in ground; D, 
 crawling in burrow; E, pulling a leaf into burrow. 
 
 through the alimentary canal and is voided into the burrow 
 behind the worm or taken to the surface at night and left 
 as a pile of castings (Fig. 83, A). The method by which an 
 earthworm moves through its burrow is of interest. The 
 setae and two muscle layers are the chief agents in locomo- 
 tion. If an earthworm is at rest and begins to move for- 
 ward the sequence of events will be somewhat as follows: 
 the setae at the posterior end of the body are protruded and 
 take firm hold on the sides of the burrow, then the circular 
 muscles (running around the body) contract to make the 
 body longer, and as the posterior portion is anchored, the 
 
PHYLUM ANNELIDA 205 
 
 front end then moves forward. The setae at the front end 
 are next extended as those of the posterior end re with- 
 drawn and the longitudinal muscles by contracting pull the 
 body forward. Progress is continued by repeating such 
 activities. 
 
 Though the earthworm is an indiscriminate feeder, it is 
 not without a discriminating sense of taste. At night 
 leaves and other parts of plants are pulled down into the 
 burrows (E), to be eaten later. If foods such as cabbage, 
 onion, and meat are placed on the surface of the ground 
 where worms are abundant and left over night, it will be 
 found that marked preferences are shown. Usually the 
 meat will be eaten first, then the cabbage, and finally the 
 onion. Hurwitz has recently shown that the chemical 
 sense in the earth worm's skin is more acute in some respects 
 than the sense of taste in man. He suspended worms over 
 different solutions so that they could be dipped in quickly. 
 By varying the strength of the solutions and recording the 
 time it took worms to withdraw from them a rather accu- 
 rate test was made. 
 
 The alimentary canal of the earthworm is well suited to 
 extract the small amount of nutriment from the rather 
 indigestible soil. The soft, finger-like prostomium above 
 the mouth continually scrapes away particles so that they 
 may be sucked inside and swallowed by the muscular 
 pharynx. After passing the pharynx and esophagus, the 
 food is held in the large thin-walled crop until it can be 
 received for grinding in the tough gizzard. When thor- 
 oughly pulverized and mixed the mass enters the intestine 
 where it is digested with glandular secretions and absorbed. 
 The large amount of sandy grit in the food, though furnish- 
 ing little or no nourishment, helps to grind up organic 
 materials, just as the gravel does in a chicken's gizzard. 
 
 The ccelom of the earthworm is not continuous, but is cut 
 off at the end of each segment by a membrane, or septum 
 (Fig. 16). The watery fluid in the body-cavity therefore 
 cannot pass freely from one end of the body to the other as 
 
206 GENERAL ZOOLOGY 
 
 in Ascaris. This is of small consequence, however, for 
 Lumbricus has a well-developed circulatory system of the 
 closed* type, with arteries, veins, capillaries, and con- 
 tractile vessels. The blood is forced forward by con- 
 tractions in the walls of the large dorsal vessel, flows 
 through the arteries to capillaries among the internal or- 
 gans and in the skin, then returns to the dorsal vessel 
 through veins. The blood acquires nourishing substances 
 from the digestive system and is aerated in the skin capillar- 
 ies. Like that of vertebrates, it carries both nourishment 
 and gases. Respiration is carried on wholly through the 
 skin, waste gases being eliminated from the blood and re- 
 placed by oxygen. The earthworm's blood contains 
 haemoglobin, and is hence an admirable carrier of carbon 
 dioxide or oxygen. 
 
 An earthworm is well provided with excretory organs, for 
 nearly every one of the hundred or more metameres con- 
 tains a pair of nephridia. These are slender tubes which 
 coil about in the ccelom, and are therefore well situated to 
 absorb and eliminate liquid waste products. A nephridium 
 begins with a closed funnel-like expansion in one metamere, 
 passes back through a septum into the next, coils back and 
 forth three times, and opens to the exterior through a little 
 pore in the body wall. 
 
 Self -protection. Lumbricus is much sought for food by 
 robins, shrews, and other animals. It is perfectly helpless 
 when captured, but escapes from many of its enemies by 
 remaining underground except at night. When some ani- 
 mal tries to pull it from the burrow, all the setae are pro- 
 truded and cling with great tenacity. If the body breaks 
 in two under the strain no particular harm results. To be 
 sure, the portion remaining with the enemy is speedily de- 
 voured and digested, but that in the burrow soon grows the 
 parts needed to make a complete body and is "as good as 
 
 * In a closed circulatory system the blood is always within the vessels 
 and passes through a regular course; in an open system it flows in part 
 through vessels, but also enters large irregular spaces, or sinuses. 
 
PHYLUM ANNELIDA 207 
 
 new. " There is a limit, however, to the ability of fragments 
 to grow into complete worms. If a piece containing only a 
 few metameres is cut from the anterior or posterior end, a 
 double-headed or double-tailed monster, which is incapable 
 of sustaining itself, is produced. Professor T. H. Morgan 
 has made many curious monstrosities by grafting parts of 
 earthworms together individuals with the body single in 
 front and double behind, long composite worms made by 
 fusing parts of two or three individuals, etc. 
 
 Earthworms escape dry seasons and the cold of winter by 
 burrowing deep in the ground and collecting together in 
 clumps for mutual protection. The slime which is secreted 
 from the surface of the body forms a tough coating on the 
 inside of the burrow so that there is little -danger of ob- 
 struction by the caving in of dirt. The tunnel is thus 
 always open for a quick retreat if the necessity arises. 
 
 Race Preservation. Earthworms are hemaphroditic but 
 not self-fertilizing. During the breeding season two in 
 dividuals frequently come together so that their anterio 
 ends overlap, and a slime tube is secreted about th 
 bodies of both (Fig. 83, B). The openings which discharge 
 spermatozoa in each worm are thus brought opposite the 
 little cavities for receiving sperm in the other individual. 
 When the pair separate each has been supplied with sperm 
 by the other (that is, four little pockets in the skin have 
 been filled with spermatozoa), and each is prepared to lay 
 fertile eggs. 
 
 Every mature earthworm has a smooth swollen region, 
 the clitellum, near the anterior end of the body. This gives 
 off a glandular secretion which forms cocoons for receiving 
 eggs and protecting them during development. When a 
 worm is about to deposit a cocoon the clitellum forms an 
 elastic, membranous band which is slipped off over the an- 
 terior end of the body, just as a ring slips from a finger. 
 As the band passes over the openings of the female repro- 
 ductive ducts, eggs are discharged inside it; and as it goes 
 across the pockets where the spermatozoa are stored some 
 
208 GENERAL ZOOLOGY 
 
 of the male reproductive cells are also set free within the 
 moving band. When it pasess off the body, the ends con- 
 tract and a little hollow cocoon is formed. The eggs are 
 fertilized inside the cocoon and undergo the usual series of 
 developmental stages (Fig. 64). The first young worm to 
 hatch usually devours his younger relatives, so that as a 
 rule only one worm is produced from each cocoon. A " fer- 
 tilized " earthworm does not use all its stored sperm at once, 
 but may produce a dozen or more cocoons after each mating. 
 
 GENERAL REMARKS ON EARTHWORMS AND OTHER 
 CH^ETOPODS 
 
 An earthworm's body is made up of a series of very simi- 
 lar segments and more than half of these may be lost without 
 destroying the ultimate effectiveness of the bodily machine. 
 If the control of the earthworm's functions was strongly 
 ocalized at the anterior end, as in some bilaterally symmet- 
 rical animals, such a loss could not be sustained. In this 
 particular, then, the worm presents a primitive condition of 
 metamerism, which is said to be homonomous, because the 
 body segments are much alike. 
 
 An earthworm's nervous system is not strongly central- 
 ized but is made up of a series of short overlapping units. 
 It consists of a dorsal ganglion near the anterior end, two 
 nerve trunks around the esophagus, and a ventral chain of 
 metameric ganglia throughout the rest of the body. As a 
 worm crawls along the impulse for contraction originates at 
 one end and is passed slowly down the whole length. For 
 example, if the circular muscles at the anterior end contract, 
 the same set of muscles is progressively stimulated, and a 
 wave of contraction passes backward. If the worm is cut 
 in two and then stitched together with a thread so that 
 there is a space of several inches between the cut surfaces, 
 the worm moves about as if it had a continuous body; the 
 pull exerted by the forward piece stimulates the one behind 
 to move in the usual way. Such behavior shows that the 
 
PHYLUM ANNELIDA 209 
 
 crawling movements are brought about by " chain reflexes " 
 which pass down the body, each part setting off the one 
 behind. 
 
 The earthworm possesses a great number of small sense 
 organs which, though slightly more abundant toward the 
 ends, are distributed rather uniformly in the skin through- 
 out the body. It is able to taste or smell with equal facility 
 at any point on the outer surface and therefore shows little 
 or no cephalization of the chemical sense. An earthworm 
 has no eyes, but is sensitive to light over its whole surface, 
 and in this respect also shows a diffuse condition. In each 
 segment of the body, sense organs are less abundant in the 
 middle and more numerous toward the anterior and posterior 
 ends. When the body is contracted, therefore, the ma- 
 jority of these organs are folded into the grooves between 
 the segments, and cannot be stimulated. It is possible that 
 the earthworm may rest by contracting the body to cover 
 most of the sense organs, just as a man closes his eyes when 
 asleep. 
 
 Though the earthworm is structurally and physiologically 
 a chain of short units which work with more or less in- 
 dependence and in a rather mechanical way, it is able to 
 carry on some activities which require the correlation of all 
 parts of the body. Above the pharynx is a ganglion, or 
 " brain," and there are three giant nerve fibers which run 
 through the whole nervous system from end to end. When 
 the entire body wiggles, the activities of the short nervous 
 units are probably controlled and coordinated from the 
 brain through these long fibers. There is also one type of 
 sensation which is apparently localized at the anterior end. 
 If an earthworm is allowed to crawl about on a moist surface 
 of limited area, it withdraws when it comes to dry spots at 
 the edge, but if the prostomium (or upper lip) is removed 
 such reactions cease. The ability to discriminate between 
 wet and dry surfaces is, however, the only sense that is 
 known to be thus localized. 
 
 Though the earthworm and its close relatives are rather 
 
210 
 
 GENERAL ZOOLOGY 
 
 strictly homonomous in their metamerism, there are many 
 other chsetopods which are heteronomous. Many of the 
 marine worms have a distinct head which may bear several 
 eyes, tentacles, and other sense organs (Fig. 84, B), thus 
 showing cephalization. Some have the metameres differ- 
 entiated into several groups, so that there are separate body 
 regions with more or less distinctive functions. 
 
 FIQ. 84. Marine annelids on sea bottom. Some burrow through the mud, 
 others make tubes of leathery secretions, sand grains, bits of seaweed or lime, 
 which are buried in the mud or attached to rocks, piles, and seaweeds. A, 
 Hydroides in calcareous tubes on a rock; B, Nereis; C, Polynce; D, Spirorbis on 
 seaweed; E, Cistenides in sand tube; F, Chcetopterus in leathery tube; G, Arenicola. 
 
 Chsetopods are divided mio two great groups. The 
 Oligochceta (few bristled) are mostly confined to land and 
 fresh water, the earthw r orms being common examples. The 
 small aquatic oligochsetes have rather long setae and are 
 often beautifully transparent. Under the microscope all 
 their internal organs may be seen through the body wall. 
 The Polychceia (many bristled) are mostly marine and show 
 great variation in structure and habits. Many species live 
 
PHYLUM ANNELIDA 
 
 211 
 
 in tubes which they make from secretions and bits of foreign 
 substances, such as sand (Fig. 84, E), lime (A, D), and mucous 
 secretions (F). Many of the tube-building species have 
 gills on the head, and when these are brightly colored they 
 often make tne bottom of the ocean look like a flower garden. 
 Some polychsetes have the dorsal surface covered with 
 spines or protective plates (C). 
 
 CLASS 2. HIRUDINEA 
 
 Leeches (Fig. 85), are found in fresh water, on land, and to 
 some extent in the ocean. They are without setae but have 
 
 A B C o E 
 
 FIG. 85. Leeches. A, attached to a turtle's leg; B, swimming; C, hiding 
 beneath a rock and protruding proboscis to catch an insect larva; D, an adult 
 carrying young attached to its ventral surface; E, egg cocoons. 
 
 a sucker at either end. The rings on the outside of the 
 body are much more numerous (70 to 100) than the body 
 metameres (33) . On account of the thickening and distor- 
 tion of the septa, the metamerism within the body is ob- 
 scured; in fact, is not apparent except in the excretory and 
 nervous systems. The ccelom is small when compared 
 with that of chsetopods. 
 
 The anterior sucker of a leech contains the mouth, but 
 the one at the posterior end is merely an adhesive disc, and 
 the anus opens in front of it on the dorsal side- Leeches 
 
212 GENERAL ZOOLOGY 
 
 vary greatly in their food habits. Some live largely on 
 insect larvae (Fig. 85, C), others are scavengers, bub the 
 most important are blood suckers. Some of the blood 
 suckers puncture the skin with a muscular proboscis, others 
 bite a hole with three small teeth. A substance is secreted 
 which inhibits the coagulation of the blood, so that there 
 may be no interference with feeding. The alimentary canal 
 is large and has several side pockets. When an opportunity 
 occurs, a leech gorges itself and is then able to go for several 
 months without food. 
 
 The breeding habits of leeches are of particular interest. 
 In some species one individual sticks a packet of sperm on 
 the outside of another, and the spermatozoa bore in through 
 the skin. This unique method of fertilization is known as 
 hypodermic impregnation. Many leeches produce cocoons 
 Jike those of earthworms which are stuck on the underside 
 of rocks (Fig. 85, E); others carry the eggs and young 
 fastened to the ventral side of the body (D). 
 
CHAPTER XX 
 PHYLUM MOLLUSCA 
 
 Molluscs are bilaterally symmetrical, unsegmented ani- 
 mals which usually have a ventral muscular foot and 
 commonly possess a calcareous shell. All have a mantle 
 of some sort. This is a covering which may have various 
 uses, such as protecting the body, forming the shell, and 
 swimming. Between the mantle and the body proper there 
 is a space, the mantle cavity, which is of great importance in 
 the activities of molluscs. There are five molluscan classes, 
 but only three are of general importance. 
 
 Class 1. Gastropoda; snails, slugs, nudibranchs; with a large flat 
 creeping foot, and usually having a spirally coiled shell. 
 
 Class 2. Lamellibranchiata: clams, oysters, ship worms; with an 
 axe-shaped foot and a shell composed of two flat valves. 
 
 Class 3. Cephalopoda: squids, cuttle-fishes, devil-fishes, octopi; 
 with the body elongated and the foot divided into a number of arms, 
 which usually bear suckers; shell usually small and inclosed within the 
 mantle, or absent. 
 
 CLASS 1. GASTROPODA 
 THE POND SNAIL, Physa gyrina Say 
 
 Self -maintenance. Physa (Fig. 86) lives among aquatic 
 plants where there is an abundance of food. It moves 
 about by making muscular movements of its broad ventral 
 foot. If a Physa is observed from beneath while crawling 
 in a glass dish, little waves will be seen to follow each other 
 from one end of the body to the other. These indicate 
 bands across the body where the foot is temporarily pulled 
 away from the glass and moved forward a little. This is a 
 slow method of progress, but sure. As the snail moves 
 along a slimy trail is left behind, and the slime is of consid- 
 
 213 
 
214 
 
 GENERAL ZOOLOGY 
 
 erable importance in locomotion; in fact, movement is not 
 possible without it. 
 
 Physa is able to vary its density to some extent by pulling 
 the body forward or backward to increase or decrease the 
 air space at the apex of the shell and can therefore rise to the 
 surface of the water or fall to the bottom without being in 
 contact with any solid object. This permits it to spin 
 threads of slime in all directions between the aquatic plants 
 (Fig. 86). The slime threads are sticky and act as traps, 
 
 A B C E F 
 
 Fio. 86. Activities of the snail, Physa. A, eggs in jelly attached to a plant 
 stem; B, dropping from surface; C, taking a breath through the surface film; 
 D, resting; E, crawling on slime thread; F, crawling on bottom. 
 
 catching minute plants and animals. After they have been 
 in the water for a time, Physa eats them to secure the ac- 
 cumulated food. They are also made use of as roadways in 
 passing from one part of a pond to another. 
 
 Physa is largely vegetarian in its food habits. Aside 
 from the minute particles captured by the slime threads, it 
 gnaws away parts of plants with its rasp-like tongue, and 
 eats the scum off the surface of the water. The tongue, or 
 radula, of a snail is a very complicated mechanism, with a 
 
PHYLUM MOLLUSCA 215 
 
 great number of little teeth arranged in regular rows and an 
 elaborate system of muscles. 
 
 A snail's digestive system is of the tubular type. Food is 
 mixed with the secretion from salivary glands in the mouth, 
 passes down an esophagus into a crop, where it may rest for 
 a time ; then enters the stomach and is mixed with glandular 
 secretions. A curved intestine leads from the stomach to 
 the anus, which, instead of being at the posterior end, is 
 situated in the edge of the mantle, high on the left side of 
 the body. The peculiar position of the anus has been 
 brought about by the twisting of the snail's body in order 
 to become adjusted to the shell. The spiral coil of the 
 shell makes it very economical of space, but has twisted the 
 soft parts of the body to such a degree that some of these on 
 the left side have become rudimentary or absent. 
 
 Physa has a closed circulatory system, with a heart, 
 arteries, veins, and capillaries which distribute the colorless 
 blood to all parts of the body. Blood-vessels are spread out 
 abundantly in the walls of the mantle cavity, and it is there 
 that respiration takes place. Physa is a pulmonate gas- 
 tropod (a snail which breathes air) and the mantle cavity 
 serves as a lung. It comes to the top of the water at inter- 
 vals, protrudes its breathing tube through the surface film 
 (C), takes a deep breath of air, and is then able to remain 
 submerged for some time. On account of the twisting of the 
 body Physa has only one kidney, the right. It consists of 
 a glandular portion, which is close to the pericardial cham- 
 ber, or the space around the heart, and a long tube which 
 opens to the exterior near the anus. 
 
 Self -protection. Physa is protected by its calcareous 
 shell, which furnishes an ever ready retreat in time of 
 danger, and shields it from the cold of winter. The shell 
 also makes it possible to endure drought, and Physa some- 
 times lives for a time in the mud left after a pond has dried 
 up. It is not a protection from all dangers, however, and 
 many fish (perch, pumpkinseed, rock bass, etc.) commonly 
 eat snails whole. 
 
216 GENERAL ZOOLOGY 
 
 In addition to dangers which threaten from without, Physa 
 is the subject of attacks by insidious enemies which live 
 within its body. Chief among these are the trematodes, 
 which pass their larval stages in snails and then pass out to 
 mature in other animals (page 179) . Against such parasites 
 Physa probably has no means of protection. 
 
 Race Preservation. Snails are hermaphroditic and dur- 
 ing mating both the individuals taking part are usually 
 fertilized. Physa lays its eggs in a little drop of jelly (Fig. 
 86, A). This adheres to solid objects in the water, protects 
 the eggs, and furnishes nourishment for the young snails 
 developing in it. 
 
 The development of fertilized snail eggs is very similar to 
 that of other metazoans .(Fig. 64), except in regard to the 
 cleavage. In very early cleavage stages cells of different 
 sizes are formed. Instead of lying one above the other, 
 they become spirally twisted ( = spiral cleavage). By the 
 direction of the twisting, the form of the shell which is to be 
 formed is indicated. Snails are of two sorts: right- and 
 left-handed, according to -whether the shell coils like a 
 right- or a left-handed screw. Physa is left-handed, but 
 most of the other common snails are not, and this fact may 
 usually serve for identification. It is of particular interest 
 that the direction of coiling of the shell is indicated in the 
 very earliest cleavage stages. 
 
 Young Physas remain within the jelly for a time after 
 they are fully formed; then break out and are at once able 
 to shift for themselves. The parent takes no care of the 
 eggs after they have been laid. 
 
 GENERAL REMARKS ON GASTROPODS 
 
 Taking the animal kingdom as a whole, the most special- 
 ized and most successful animals are those which have be- 
 come able in their racial evolution to live on land. Among 
 the few groups that have become perfectly adjusted to 
 terrestrial life, snails have an interesting place because they 
 
PHYLUM MOLLUSCA 217 
 
 show, at the present time, all stages of transition from ocean, 
 to fresh water, and to land. Most marine snails breathe 
 directly from the water through feather-like gills, and have 
 a free swimming larva (veliger). The fresh- water species 
 do not have free swimming larvae and, though many 
 breathe like their marine relatives, some, like Physa, have 
 rudimentary gills and respire air through the walls of the 
 mantle cavity. Succinea is a common swamp snail which 
 is amphibious. It breathes air, but may live in the water 
 or on land, and is as often found perched on the top of a 
 cat-tail as among the plants under water. Land snails lay 
 their eggs in holes in the ground and in rotten logs; they all 
 breathe air into the mantle-cavity and are without gills. 
 
 Several of the large marine snails are of considerable 
 economic importance because they are pests to oyster and 
 clam fishermen. They bore holes in the shells of other 
 molluscs with their radulas and extract the soft parts. 
 The slugs, which are snails with very rudimentary shells, 
 and other land snails often do damage to gardens by gnaw- 
 ing holes in the vegetables. The large snail in Europe, 
 Helix pomatia, is a source of trouble in vineyards and fields, 
 but is also of value as a food. It is raised in " snail farms"* 
 and gathered in the fields for local consumption and export. 
 Most of the edible snails used in the United States are sold 
 to the restaurants in the large cities. The Department of 
 Agriculture has recently issued a bulletin describing meth- 
 ods of culture and suggesting that snail farming be under- 
 taken in this country. 
 
 CLASS 2. LAMELLIBRANCHIATA 
 
 Here belong the clams, mussels, oysters, scallops, ship 
 worms, and other " bivalves. " These animals obtain their 
 food by siphoning large quantities of water through the 
 mantle cavity and retaining the minute particles. Cur- 
 rents created by the cilia covering the body and mantle 
 bring in small plants and animals which are swept into the 
 
218 
 
 GENERAL ZOOLOGY 
 
 mouth. The whole alimentary canal is ciliated and the 
 muscles accordingly take little part in moving the food 
 through it, which is very unusual. 
 
 Most lamellibranchs move very slowly by protruding the 
 foot (Fig. 87 A',/) and pressing against the bottom, but some 
 
 
 D' 
 
 FIG. 87. Life history of fresh water mussel. A, adult clam giving off glochidia 
 through excurrent siphon; A', anatomy of mature clam; a, adductor muscle; 
 /, foot; p, marsupium, or swollen portion of gill, in which glochidia mature from 
 eggs. B, B', young glochidia on bottom; C, C', glochidia encysted in fishes' 
 fins; D, D', young clam. 
 
 clams are able to leap a short distance; the scallops swim 
 actively by squirting water from the mantle cavity, and the 
 fresh-water Sphceridce climb about on aquatic plants. A 
 number of clams are able to burrow with great facility. 
 The oyster moves about only while young, for it grows 
 fast to the bottom and is then unable to change its position. 
 On this account oysters are often smothered by mud 
 
PHYLUM MOLLUSCA 219 
 
 deposited after storms or floods. The ship worm, Teredo 
 navaliSj bores in piles, wharves, and even in wooden ships; 
 hence is very destructive to timber. It is soft and worm- 
 like, the shell being restricted to one end of the body and 
 serving as an auger for boring. 
 
 The breeding habits of lamellibranchs are quite variable. 
 Most of them are dioecious and some species show sexual 
 dimorphism, the females usually being larger than the males. 
 Marine species usually have a free-swimming larva (veliger) 
 which is trochophore-like (page 190), but those in freshwater 
 generally bring forth living young. The life history of the 
 fresh-water mussel (Fig. 87) shows great specialization 
 and is well-adjusted to conditions in rivers, ponds, and lakes. 
 The eggs are fertilized within the mantle cavity of the 
 female; then pass into the gills, which serve as brood- 
 pouches, or marsupia, and become greatly swollen. The 
 larvae developed within the marsupia are known as glo- 
 chidia, and there are two types, hooked (Fig. 87, B') and 
 hookless. Both are usually set free in the water and lie 
 on the bottom until a fish comes into contact with them. 
 A hooked glochidium becomes attached to the fish's 
 fins (C) and grows for a time as a parasite within a cyst. 
 The hookless glochidia live as parasites on the gills of 
 fishes. Both types mature within cysts, then break out, 
 ready to take up the usual activities of young mussels (D). 
 
 Lamellibranchs are of great economic value in the United 
 States. Along the seacoast oysters, clams, mussels, and 
 scallops are collected for food. The national and state 
 governments maintain laboratories for the propagation 
 and study of these important natural resources. The ship 
 worm, on the other hand, causes great losses and the govern- 
 ment is trying to find better means to control or exter- 
 minate it. Pearls are formed when small foreign bodies or 
 parasites lodge in the mantle of a clam and become covered 
 with layers of lime. In the United States the chief source 
 of these jewels is from the fresh- water mussels which are 
 dredged from the bottoms of rivers and lakes. The shells 
 
220 
 
 GENERAL ZOOLOGY 
 
 of lamellibranchs are also the source of pearl buttons, and 
 along the Mississippi River a large number of persons 
 (3592 in 1915) are engaged in collecting mussels for this 
 industry. A recent publication of the Bureau of Fisheries 
 lists forty species which are of value. 
 
 CLASS 3. CEPHALOPODA 
 
 Most cephalopods (Fig. 88) are far too active to carry a 
 heavy exoskeleton about, and the shell is as a rule absent or 
 
 ABC 
 
 FIG. 88. Cephalopoda. A, an octopus or devil-fish; B, squids swimming with 
 fish and resting on bottom; m, mantle; s, siphon; C, squid eggs. 
 
 small and enclosed within the mantle. Aside from the rare 
 nautaloids, there are two common types of cephalopods: 
 
 (1) squids, are elongated, have ten arms, and swim actively; 
 
 (2) devil-fishes, or octopods, have eight arms, climb more 
 than they swim, and have short bodies. The mantle (ra) in 
 all cephalopods forms a muscular tube which encloses the 
 whole body, except the head and arms. The method of 
 swimming is peculiar. Water is drawn into the mantle 
 
PHYLUM MOLLUSCA 221 
 
 cavity and forced out of the siphon (s) so powerfully that 
 the whole animal is propelled through the water backward. 
 This movement is accelerated and guided by the lateral 
 fins, thus making cephalopods expert swimmers. 
 
 The food of these molluscs consists of small fish and other 
 marine animals. After being captured, the prey is held 
 with the suckers on the arms, torn apart with the parrot- 
 like beak, and ground up somewhat by the radula before 
 passing down to the stomach. On account of the contrac- 
 tions of the mantle while swimming, all the internal organs 
 are subjected to great pressure and food would be regurgi- 
 tated through the mouth if special valves were not present. 
 Valves are also present in the blood-vessels to prevent back- 
 flow, and there are two accessory hearts which pump 
 blood through the plume-like gills. 
 
 All cephalopods have large eyes on the sides of the head. 
 These resemble those of vertebrates in their general plan, 
 but differ markedly in details of structure and in develop- 
 ment. The eyes enable squids and octopi to see with great 
 acuity, and the pursuit of active animals for food is there- 
 fore easy. When a squid or an octopus is set upon by an 
 enemy, a great cloud of ink is squirted into the water through 
 the anus. Its movements are thus obscured, and escape 
 is made possible. 
 
 The breeding habits of cephalopods are very peculiar. 
 A male takes a spermatophore, or packet of sperm, on one 
 of his arms and places it within the mantle cavity of the 
 female. When a female lays her eggs she ruptures the sper- 
 matophore, a great cloud of spermatozoa swarm over them, 
 and fertilization is thus insured. In some species the male 
 breaks off the tip of his arm and leaves it in the female 
 with the spermatophore enclosed. When this arm tip was 
 first discovered by one of the older zoologists, it was de- 
 scribed as a parasitic worm which was believed to live 
 within the female. A female squid or octopus deposits her 
 eggs on the bottom of the ocean or attaches them to a 
 
222 GENERAL ZOOLOGY 
 
 sea- weed (C). She then leaves them to develop without 
 further care. 
 
 Squids are commonly eaten for food in Italy and are com- 
 ing to be used in other countries. They are also much sought 
 for bait by fishermen. Though some squids attain a total 
 length of forty feet, they are as a rule very shy, and their 
 reputed attacks on men are probably fictitious. 
 
 GENERAL REMARKS ON MOLLUSCS 
 
 The Mollusca are remarkable among other phyla of 
 bilaterally symmetrical animals because they are complex 
 in structure but strictly non-met americ. Though the tro- 
 chophore-like larva (page 190) indicates possible affinities 
 with the Annelida, Echinodermata, Rotifera, etc., the 
 structure of adult molluscs is wholly unlike that of other 
 animals. The Mollusca therefore have no close relation- 
 ship with any other phylum, and have developed along 
 original lines. 
 
 The most striking characteristics of the phylum as a whole 
 are the foot, the shell, the mantle, and the simple nervous 
 system, but all these structures differ greatly in different 
 classes. The foot is flat in gastropods, hatchet-shaped in 
 lamellibranchs, and divided into eight, ten, or more arms in 
 cephalopods. The shell is univalve in gastropods, bivalve 
 in lamellibranchs, and usually rudimentary or absent in 
 cephalopods.' Squids and devil-fishes have a sac-like 
 mantle which encloses most of the body ; clams usually have 
 the mantle divided into two great lobes, completely cover- 
 ing the soft body; but in snails the greater part of the body 
 may be extended outside the mantle cavity. 
 
 Except in cephalopods the nervous system is as a rule 
 poorly developed and its structure is rather similar in all the 
 classes. It consists of three or four pairs of ganglia and a 
 few connective nerves. Molluscs show in a striking way 
 that the degree of specialization in nervous structures is 
 associated with activity. Most of the slow, sluggish clams 
 
PHYLUM MOLLUSCA 223 
 
 are practically without special sense organs (except those 
 for balancing the body and for testing the surrounding 
 water), and depend largely on reflexes in their daily activi- 
 ties, but the scallops (Pecten) which swim actively have 
 a double row of eyes around the edges of the mantle. The 
 snails are somewhat more active than clams, and have 
 simple eyes, organs of taste, and other sensory structures, 
 but are at best rather slow. The most active molluscs are 
 the cephalopods, and they show the most elaborate sen- 
 sory organs: well-developed eyes, balancing organs, taste 
 organs, etc. 
 
 The mental powers of all molluscs are very limited. As 
 would be expected from their superior sensory equipment, 
 the cephalopods excel other members of the group. They 
 are able to see and pursue their prey, elude their enemies, 
 and carry on other activities which are impossible for the 
 sluggish clams and snails. One investigator taught a squid 
 to take food from a hole in a glass tube, and the food was 
 extracted more quickly after some experience had been 
 gained. 
 
CHAPTER XXI 
 
 i 
 
 PHYLUM CHORDATA 
 
 The phylum Chordata includes all the animals commonly 
 known as vertebrates and a number of others that are 
 somewhat less familiar. All chordates have a thick rod 
 running through the body which is known variously as the 
 chorda dorsalis, chorda, or notochord. This is much like 
 a straight Bologna sausage in form and consistency, having 
 a membrane over the outside and a filling of pulp within. 
 
 integument- , 
 
 muscles of 
 "body H/ar//^**, 
 
 chorda- ,,. 
 
 nervous ^ 
 system 
 -enferon-^ 
 . blood . 
 
 -kidney 
 
 orary 
 coelom 
 
 Fio. 89. Cross sections of an earthworm and a lamprey-eel compared. Note 
 the difference in the relative position of the central nervous system. 
 
 The chorda is strictly a skeletal structure, and has nothing 
 to do with the spinal cord, which is a part of the nervous 
 system. 
 
 Besides the chorda, the chordates have two other dis- 
 tinctive features: (1) the gill slits and the gill arches; and 
 (2) a tubular nervous system situated dorsal to the ali- 
 mentary canal (Fig. 89) . The gill slits are paired openings 
 from the pharynx to the outside of the body. In most 
 aquatic chordates they serve as exits for the water drawn 
 
 224 
 
PHYLUM CHORDATA 225 
 
 through the mouth and used for respiration, but are present 
 in terrestrial forms only during development. Between 
 the slits run the aortic arches carrying blood, and these 
 great arteries are often supported by cartilaginous or bony 
 structures. The nervous system of chordates is formed 
 during development by the turning in of the ectoderm on 
 the dorsal surface (Fig. 92). At first there is simply a 
 longitudinal depression (A), and the deeper part of this is 
 later pinched off (B, C) to form a longitudinal tube inside 
 the body wall (C). This tube usually becomes thickened 
 and expanded somewhat at the anterior end to form a brain, 
 and nerves grow out from it to all parts of the body, but 
 even in ah adult man it is still tubular in its fundamental 
 structure. 
 
 In some of the most primitive living chordates, such as 
 the lamprey eels and the lancelets, the chorda is functional 
 and constitutes practically the sole skeletal structure within 
 the body, but in the great majority of the vertebrates it is 
 replaced to a greater or lesser extent^ by a series of verte- 
 brae which together make up the spinal column, or " back- 
 bone." There is also a transition in the aortic arches in 
 passing from primitive to more specialized chordates. 
 The aquatic representatives have functional gill slits and 
 well-developed aortic and skeletal arches. The frogs and 
 toads begin active life with a fish-like form and functional 
 gills, but later lose the gills and have the blood diverted 
 to the lungs and skin for aeration. Reptiles, birds, and 
 mammals never breathe through gills, but nevertheless have 
 gill slits and gill arches during early developmental stages. 
 The primitive arterial arches degenerate in the adults or 
 are diverted to supply new parts with blood (Fig. 105). 
 The skeletal parts of the arches which serve to support gills 
 in primitive chordates are for the most part transformed 
 into jaws, throat cartilages, and the bones of the face in 
 more specialized types. 
 
 Thus, though the same fundamental plan of structure is 
 present in all chordates, parts may be so modified as to 
 
226 GENERAL ZOOLOGY 
 
 Completely change their form, and have often during evolu- 
 tion come to serve entirely different functions from those 
 for which they were originally used. Such changes have 
 usually gone with modifications in modes of life. Though 
 all the primitive chordates are largely aquatic, the special- 
 ized forms are nearly all terrestrial and hence fitted to 
 breathe air. One of the most interesting features of the 
 past evolution of the vertebrates has been their migration 
 from water to land. 
 
 The phylum Chordata is divided into four subphyla. 
 
 Subphylum 1. Enteropneusta. This group includes cer- 
 tain worm-like animals. Dolichoglossus lives buried in the 
 mud along the seashore (Fig. 90, /). Its chorda is small 
 and contained within a swollen proboscis which projects 
 above the mouth. The body is divided into three regions 
 proboscis, collar, and trunk. A large number of paired 
 gill slits connect the pharynx with the exterior. Dolicho- 
 tilossus extracts its food from mud which is forced into the 
 mouth at the front of the collar, passes through the straight 
 enteron, and leaves the body through the anus at the poste- 
 rior end. 
 
 The larvae of some of the Enteropneusta are tornarise, 
 which are much like the larvae of certain echinoderms, and 
 resemble a trochophore (page 190) in general structure. 
 This has led some zoologists to believe that chordates arose 
 from a remote ancestor which resembled a trochophore, and 
 that there is relationship between echinoderms, annelids, 
 molluscs'/ and other animals originating from a trochophore- 
 like larva. 
 
 Subphylum 2. Tunicata. The sea-squirts (Fig. 90, g, h) 
 are degenerate animals which grow attached to piles and 
 other objects in the ocean. They are covered by tough 
 " tunics," through which there are two openings for the 
 entrance and exit of water. When taken from the ocean 
 two little streams of water are ejected from the body; 
 hence the common name, " sea-squirt." In feeding, water, 
 is strained through a porous pharynx, which catches food 
 
PHYLUM CHORDATA 
 
 227 
 
 particles and passes them on to the stomach and intestine. 
 The anus opens near the exhalent aperture inside the tunic. 
 Many species of sea-squirts are colonial and form masses 
 known to fishermen as "sea pork" (&, ti). 
 
 Some tunicates are not attached but swim at the surface 
 of the ocean. The salpas, for example (Fig. 90, c, d) are 
 
 FIG. 90. Simple Chordates. The sub-phyla represented are as follows: 
 a-d, g, h, Tunicata; e, Cephalochorda ; /, Enteropneusta. The separate figures 
 are: a, tunicate tadpoles; 6, Leptoclinum, or "sea pork," a colonial tunicate; 
 c, a chain Salpa ; d, Salpa, a swimming tunicate ; e, amphioxus ; /, Dolichoglossus ; 
 g, Molgula, a typical "sea squirt;" h, Amarcecium, a colonial tunicate. 
 
 beautifully transparent, barrel-shaped animals ^which swim 
 by drawing water into the front of the tunic^and forcing 
 it out behind. They occur singly or in chain-like colonies. 
 The Larvacea are another group of free-swimming tunicates 
 which resemble minute tadpoles in their general form. 
 
 Most adult tunicates have the chorda greatly reduced 
 or absent, but all of them develop from little tadpoles in 
 
228 GENERAL ZOOLOGY 
 
 which it is well-developed and functional. The Larvacea 
 (a) therefore .represent the most primitive types. Most of 
 the common sea-squirts have, like barnacles (Fig. 32), 
 undergone retrogressive metamorphosis, and as adults are 
 adapted to a sessile existence. 
 
 Subphylurn. 3. Cephalochorda. The lancelets (Fig. 90, e) 
 are small fish-like animals which bury themselves tail first 
 in sandy ocean beaches, below low-tide mark. Amphioxus 
 is a well-known representative which has been much dis- 
 cussed by evolutionists as a possible ancestor of vertebrates. 
 It has a well-developed chorda and numerous paired gill 
 slits opening from the pharynx to the exterior. The body 
 wall is divided into a large number of muscle segments, or 
 myomeres. Along the whole dorsal margin there is a nar- 
 row fin which is continuous posteriorly with a broader 
 caudal fin. On the ventral side of the body, a short medial 
 fold (ventral fin) reaches forward from the caudal fin, and 
 in front of this two lateral (metapleural) folds extend to 
 the region of the mouth. The median fins and the lateral 
 folds are continuous, but have the same arrangement as 
 the separate fins of true fishes. The metapleural folds 
 have, therefore, been considered to be the forerunners of 
 the paired limbs of the vertebrates. The cephalochord- 
 ates excel Enteropneusta and Tunicata in their greater 
 cephalization and show closer affinities with the vertebrates. 
 
 Subphylum 4. Vertebrata. This group includes the eel- 
 like cyclostomes, sharks and skates, salamanders, frogs, 
 reptiles, birds, and mammals. Most vertebrates have two 
 characteristics which usually serve to separate them readily 
 from other chordates: (1) vertebrae replace the chorda, 
 which is usually present as such only during embryonic 
 development; (2) two pairs of lateral appendages, or limbs, 
 are present. Though vertebrates as adults are rather di- 
 verse in structure and habits, they are all built according 
 to the same fundamental plan and pass through similar 
 stages in development. There is, therefore, no doubt that 
 vertebrates are genetically related ana, Decause their evolu- 
 
PHYLUM CHORDATA 229 
 
 tion has been more recent than that of any other group of 
 animals, more facts are available concerning the lines 
 specialization has followed. 
 
 In the most primitive vertebrates the chorda is functional 
 throughout life^nd the vertebrae appear only as little carti- 
 lages between the myomeres, or muscle segments. But as 
 the complexity of the skeleton progresses in the vertebrate 
 series, there is a transition from chorda to cartilage, and 
 again to bone. Some vertebrates are as adults only a little 
 beyond the chorda stage/ others, like the sharks, have 
 developed a cartilaginous skeleton, but go no further. The 
 majority of vertebrates, however, pass through three stages 
 in their development: (1) having a chorda, replaced by 
 (2) cartilaginous vertebrae, which in turn give place to (3) 
 bone. Paired limbs % like wise show a rather definite se- 
 quence in their specialization. They are absent in the 
 most primitive vertebrates and appear first as flaps sup- 
 ported by cartilaginous rays. Fins with bony rays are the 
 next step, and these are later replaced by pentadactyl (five- 
 toed) limbs. The pentadactyl limb represents a great ad- 
 vance in racial development when compared with a fin, 
 bi;t it has now been so greatly modified in many groups as 
 to be scarcely recognizable. A bird's wing or a horse's 
 foot show little indication of the ancestral five-toed condi- 
 tion. Most snakes and the blind worms have lost the 
 paired appendages completely. 
 
 The vertebrate circulatory system also affords a good 
 illustration of progressive specialization and development. 
 The most primitive vertebrates have a tubular heart (Fig. 
 105), bent somewhat to be sure, but still nothing but a 
 tube, which pumps blood through six aortic arches to the 
 gills. Very soon the number of arches is reduced to four, 
 then to three* In birds and mammals the arches appear 
 as such only in early development, and in the adult condi- 
 tion all but a single arch on one side of the body have been 
 lost or diverted to other uses. In fishes the heart presents 
 a simple tubular condition; in amphibians and in most rep- 
 
230 GENERAL ZOOLOGY 
 
 tiles it is three-chambered and pumps " half-pure" blood. 
 Birds and mammals show a four-chambered condition; 
 the blood on one side has been aerated and that on the 
 other is charged with waste products. 
 
 There are three kinds of kidneys present in vertebrate 
 animals. These are believed to represent parts of a con- 
 tinuous series of nephridia, somewhat like those of annelids, 
 but in no vertebrate is more than one of the three functional 
 at a time. The most primitive vertebrates (Cyclostomes) 
 have only one kidney, the pronephros, or head-kidney. 
 Fishes and amphibians have a pronephros during early 
 development, but it soon degenerates and is replaced by 
 another kidney, the mesonephros. Reptiles, birds, and 
 mammals have first a pronephros, then a mesonephros, 
 but their functional kidney during adult life is the meta- 
 nephros, which develops after the other two have 
 degenerated. 
 
 Other lines of vertebrate specialization might be described, 
 but enough has been said to show that it is possible to 
 speak in more positive terms concerning this group of ani- 
 mals than of any of those which have been discussed in 
 the preceding pages. Perhaps the most interesting thing 
 to keep in mind as we pass from primitive to specialized 
 vertebrates, is that in general evolution has been from 
 aquatic modes of existence to life on land. Structures and 
 functions have been accordingly modified. The fin, for 
 example, was good enough as an organ of locomotion in 
 the water, but cannot compare with a jointed leg ending 
 in toes for progress on land. 
 
 There are seven classes of vertebrates which may be 
 briefly outlined as follows: 
 
 Class 1. Cyclostomata. Eel-like vertebrates with a chorda and 
 median fins, but without paired fins or true jaws. Lampreys and 
 hagfishes. 
 
 Class 2. Elasmobranchii. Fish-like vertebrates with cartilaginous 
 skeletons, jaws, placoid scales, and a persistent chorda. Sharks, 
 skates, rays. 
 
PHYLUM CHORDATA 231 
 
 Class 3. Pisces. Fish-like vertebrates with cartilaginous or bony 
 skeletons, true jaws, scales, and fins supported by rays. True fishes. 
 
 Class 4. Amphibia. Slimy-skinned vertebrates without exoskele- 
 tons; with pentadactyl limbs; no claws on toes; heart three-chambered. 
 The young are aquatic and breathe by means of gills; the adults are 
 usually more or less terrestrial and breathe through lungs. Frogs, toads, 
 salamanders. 
 
 Class 5. Reptilia. Scaly-skinned vertebrates, which breathe by 
 means of lungs and have a three- or four-chambered heart; claws on 
 the toes. Lizards, snakes, turtles, crocodiles, etc. 
 
 Class 6. Aves. Feathered, scaly-skinned vertebrates with four- 
 chambered heart and warm blood, but no teeth. Birds. 
 
 Class 7. Mammalia. Warm-blooded vertebrates with hair and 
 mammary, or milk, glands. Whales, seals, bats, dogs, horses, monkeys, 
 etc. 
 
 CLASS 1. CYCLOSTOMATA 
 
 The cyclostomes lack true jaws and are without paired 
 lateral appendages. The nasal pit is single instead of 
 double as in other vertebrates, and there are from seven to 
 fourteen pairs of gill slits. Two kinds of cyclostomes are 
 living at the present time hagfishes and lampreys. 
 
 The hagfishes are found only on muddy bottoms in the 
 ocean. They bore their way into fishes in order to eat out 
 the soft parts, and are a source of great annoyance to fisher- 
 men because they commonly attack shad, cod, and other 
 fish caught in nets or on hooks. They have a single tooth 
 which they use for boring into their victims. 
 
 Lampreys live both in the ocean and in fresh water. 
 They have a cartilaginous funnel around the mouth which 
 is used as a sucking disc for feeding or attachment. There 
 are numerous teeth inside the mouth and on the tongue. 
 Petromyzon marinus (Fig. 89, B; 91, ^4.) is a common species 
 about the mouths of rivers along the Atlantic coast of the 
 United States. It lives on blood which is obtained by 
 attaching the oral disc to fishes and rasping a hole with 
 the tongue. The larva of this species differs markedly 
 from the adult, and shows several characteristics which 
 resemble those of Amphioxus (Fig. 90, e). Another cyclo- 
 
232 
 
 GENERAL ZOOLOGY 
 
 , 
 stome, the brook lamprey of North America, Lampetra 
 
 wilderi, has interesting breeding habits. Stones are car- 
 ried in the mouth^and a space is thus cleared in which 
 mating take splace. This species does not eat as an adult, 
 and is therefore not harmful to fishes. 
 
 FIG. 91. Cyclostomes and clasmobranchs. A, lamprey-eels, resting and at- 
 tacking a shark; B, shark; C, sting-ray. 
 
 CLASS 2. ELASMOBRANCHII 
 
 Elasmobranchs (Fig. 91, B, C) resemble true fishes in 
 general form and in having two pairs of lateral fins, but 
 differ from them in possessing a notochord when adult, 
 placoid scales (Fig. 93), a spiral valve in the intestine, clasp- 
 ing organs in the male, and in lacking an air-bladder and 
 a gill-cover, or operculum. They are therefore placed in 
 a separate class. 
 
 There are two common types of elasmobranchs, sharks 
 and rays. The sharks and dogfishes are cylindrical or 
 
:T/LU: 
 
 PHYLUM CHORD AT A 
 
 233 
 
 compressed and swim actively about seeking for food. 
 They are voracious feeders on fish and other animals; there- 
 fore, are readily caught on a hoojUL Though one species 
 attains a length of over thirty feet and sometimes attacks 
 man, most sharks are harmless, except for their ravages 
 among fishes and other small animals. They are of some 
 
 p 
 
 FIG. 92. FIG. 93. 
 
 FIG. 92. Three cross sections (A, B, C) of an embryonic vertebrate to show 
 formation of nervous system, chorda, and coelom. 6, membrane surrounding 
 ccelom; c, chorda; e, enteron; n, nerve tube. 
 
 FIG. 93. Section through a placoid scale of an elasmobranch. d, dentine; 
 p, "pulp;" x, enamel; y, integument. 
 
 value as fertilizer^ and for the oil which is extracted from 
 their livers. The' flesh is also canned to some extent and 
 sold as a cheap substitute for the true fishes. Sharks cause 
 great losses by destroying lobsters and valuable food fishes. 
 , The skates and rays have broad flat bodies and commonly 
 lurk on the bottom of the ocean awaiting small animals 
 
234 GENERAL ZOOLOGY 
 
 which they capture and devour. The sting ray (Fig, 91, 
 C) often inflicts painful wounds on the bodies of bathers 
 and fishermen. It has a barbed poison spine at the base 
 of its tail which is driven with great force into any animal 
 that molests it. A tropical ray, the torpedo, has electrical 
 organs in its body which are under the control of the nerv- 
 ous system. These are capable of giving powerful shocks 
 and serve as effective weapons of defense or offense. 
 
 The placoid scales (Fig. 93) of elasmobranchs are of par- 
 ticular interest because their structure closely resembles 
 that of the teeth in the mouths of true fishes, amphibians, 
 reptiles, and mammals. Each is covered with a layer of 
 enamel, which surrounds a bony dentine layer, and the 
 interior is filled with "pulp" containing blood vessels, 
 nerves, and connective tissue. It is believed that the teeth 
 of vertebrates have been derived from placoid scales such 
 as the sharks and rays possess. In elasmobranchs these 
 organs occur in the skin over the whole body and in the 
 mouth. Those in the mouth are on a portion of the ecto- 
 derm which has been turned inside; they are particularly 
 large and arranged in regular rows. In other vertebrates 
 the teeth are restricted to the mouth, or in other words/ 
 placoid scales are lacking from the skin. 
 
CHAPTER XXII 
 SUBPHYLUM VERTEBRATA, CLASS 3, PISCES 
 
 In true fishes the chorda is usually replaced by bony 
 vertebrae, the gills are protected by an operculum, or " gill- 
 cover/' and the body is covered with scales which are never 
 of the placoid type, as in sharks, but consist of little bony 
 plates which overlap like shingles. In general form and 
 position^the fins are much like those of sharks, but are often 
 supported by bony rays. There are two subclasses of 
 fishes : 
 
 Subclass 1. Teleostomi. Fishes with a skeleton composed largely 
 of bone; with gills, but usually without lungs. 
 
 Subclass 2. Dipnoi. Fishes with a skeleton made up of cartilage 
 and bone, and with a single or paired lung. 
 
 As an example of the subclass Teleostomi, the yellow 
 perch, which is one of the commonest and most widely 
 distributed fishes in North America, will be considered. 
 
 THE YELLOW PERCH, Perca Flavescens Mitchill. 
 
 Self-maintenance. The perch (Fig. 94) is versatile in 
 every way, and this is particularly true of its food habits. 
 It may feed on microscopic animals in the water such as 
 Daphnia or Cyclops (Fig. 31), or eat snails, insect larvae, 
 aquatic insects, minnows, fish eggs, and even plants. The 
 minute animals are secured by straining the water which 
 is continually entering the mouth for respiratory purposes 
 and passing out behind the operculum. This straining is 
 accomplished by a series of minute finger-like projections, 
 the gill-rakers, attached along the inner margins of the 
 gill-arches and spread across the spaces where water leaves 
 
 235 
 
236 
 
 GENERAL ZOOLOGY 
 
 the body. Small particles of food are caught and held 
 until they can be swallowed. A perch may swim about 
 in a swarm of microscopic animals (Fig. 94, B) and strain 
 out enough of them to completely fill its alimentary canal. 
 For capturing larger prey, such as minnows and insects, 
 the perch's mouth is provided with sharp, backwardly 
 
 C D 
 
 FIG. 94. The perch, Perca flavescens. A, caught by a pickerel; B, catching 
 minute animals for food; C, egg string; D, capturing a dragon-fly nymph. 
 
 directed teeth, which, though not used for chewing, serve 
 to prevent the escape cf wriggling animals. 
 
 A perch is a very good swimmer and is even able to leap 
 out of the water to capture small animals. Its shape is 
 such that it can be reasonably successful in any habitat in 
 fresh water. In this respect it differs from fishes which are 
 restricted to certain situations. The basses and sunfishes, 
 for example, are fitted structurally to feed among aquatic 
 

 PISCES 237 
 
 vegetation; suckers have their mouths turned, downward 
 for feeding from the bottom. Perch are equally at home in 
 the depths of lakes or in the dense growth of plants along- 
 shore; also thrive in ponds and streams. They can feed 
 from the surface, strain microorganisms from the open 
 water, snatch snails and the larvae of insects from their 
 retreats among plants or stones, and grub about in the soft 
 bottom mud for minute worms and insect larvae. 
 
 In its sensory equipment for finding food a perch presents 
 a rather generalized condition when compared with many 
 other fresh-water fishes. A bullhead, for example, pays 
 little attention to moving objects unless they touch it, but 
 seeks food by swimming so that the body swings from side 
 to side. The "whiskers, " or barbels, which bear taste 
 organs, thus encounter a considerable part of the area being 
 explored. If a barbel touches something that is suitable 
 for food, the fish at once turns and snaps up the morsel. A 
 trout, on the contrary, depends little on its senses of taste 
 and smell in seeking food, but is quick to investigate any 
 small moving object. It chases bubbles, pursues floating 
 twigs, and examines everything that moves in its imme- 
 diate vicinity, thus capturing many small insects and other 
 organisms. The bullhead in hunting depends primarily on 
 its senses of taste and smell, the trout uses its sense of sight 
 more than any other. The perch, however, has enough 
 versatility to feed in various ways and therefore has greater 
 opportunities for securing food. Its acute eyes enable it to 
 feed on the small moving things sought by the trout; its 
 sense of taste, though not distributed so widely in the skin as 
 that of the bullhead, is still good enough to be of value in 
 locating and judging the value of food. The presence of 
 food substances may also be known through dilute solutions 
 given off in the water which are perceived by the sense of 
 smell. If anything moves that promises food it is investi- 
 gated and perhaps even taken into the mouth, to be rejected 
 again if not suitable. If there is a flavor of snail, or .worm, 
 or insect in the water, it is followed to its source. Small 
 
238 GENERAL ZOOLOGY 
 
 wonder, then, that perch usually exceed other fishes in num- 
 bers. They are far too versatile to be kept down by 
 scarcity, or other variations in the food supply. 
 
 When food enters a perch's mouth, it is taken charge of 
 by the digestive system, and passes down the esophagus to 
 be digested and absorbed in the stomach and intestine. 
 There are three little pockets, or caeca, which enter the 
 intestine just behind the stomach and help to increase the 
 surface. The liver, pancreas, and intestinal glands pour 
 their secretions into the intestine to be mixed with the food. 
 Undigested parts collect in the rectum and are eliminated 
 through the anus. Absorbed food enters the circulatory 
 system and in the blood is distributed to all parts of the 
 body. The blood is pumped by the heart to the gills, where 
 aeration takes place. It then passes through the four aortic 
 arches (Fig. 105) before passing to the head and to the great 
 dorsal aorta which distributes it to all parts of the body. 
 All vessels leading away from the heart (arteries) end in 
 minute capillaries, and these in turn open into veins which 
 carry the blood back to the heart, to be pumped round the 
 circuit again. In short, the perch has a " closed" blood 
 system which carries food, waste products, and oxygen. 
 
 The important organs of respiration in a perch are the 
 gills and the air-bladder. There are four pairs of gills, 
 each bearing a double row of soft finger-like branchial 
 filaments, through the walls of which aeration takes place. 
 As water passes over the filaments, the blood in their 
 capillaries gives up its waste products and acquires oxygen. 
 The air-bladder serves as a storage reservoir for oxygen 
 and enables a perch to invade stagnated regions without 
 danger of suffocation. It contains a high percentage of 
 oxygen and this reserve is used when the surrounding water 
 does not contain a sufficient supply. 
 
 Waste products accumulating in the blood are carried to 
 the kidneys for elimination from the body. A kidney con- 
 sists of a vast collection of uriniferous tubules, or nephridia, 
 which are individually somewhat like those described in the 
 
PISCES 239 
 
 earthworm (page 206), but are not metamerically arranged. 
 The ureter is a duct which carries urine from the kidney 
 outside the body. 
 
 Self-protection. The chief enemies of perch are large 
 predaceous fishes (the pickerel, bass, dogfish) and man. 
 Piscivorous enemies usually lurk in the shore vegetation, 
 and capture their prey by fierce and sudden rushes. From 
 these perch are protected to some extent by various habits 
 and adaptations, but from man, with his nets, hooks, 
 spears, and traps, there is no escape. 
 
 A perch is well protected from slight injuries by its cover- 
 ing of scales. If caught by a pickerel, or some other enemy 
 (Fig. 94, A), it spreads its fins so as to inflict painful pricks 
 when the bony rays are touched. There are also sharp saws 
 along the edges of the opercula which may cause severe 
 cuts. These are made more effective by spreading the 
 opercula to their greatest extent when danger threatens. 
 The slimy skin and the backwardly directed scales make a 
 perch difficult to hold, for with every wriggle the body 
 tends to slip forward a little. 
 
 Fishes have a row of peculiar sense organs extending 
 along either side of the body. These are known as the lat- 
 eral lines, and are believed to serve for the perception of very 
 slow vibratory movements in the water, such as would 
 result from the splashing of falling bodies. The lateral 
 line organs would therefore be of value in giving notice of 
 disturbances in the water. 
 
 The body of a perch is " countershaded " by being paler 
 below, so that when light strikes it from above, the dark 
 back with its brighter illumination is equal to the lighter 
 belly in the shadow. The dark vertical bands blend readily 
 with the lights and shadows in the shore vegetation, or 
 merge with the ripples on the surface of the water when 
 seen from below. A perch, like other fishes, may change 
 its color somewhat to suit the background on which it 
 customarily rests. Such changes are brought about by the 
 expansion and contraction of certain pigmented cells in the 
 
240 GENERAL ZOOLOGY 
 
 skin. If kept on a black background for a month, a perch 
 will become very dark, and if allowed to remain on a white 
 bottom, will assume a very pale color. 
 
 Perch have considerable power of regenerating parts of 
 the body after loss through accident or injury. A fin may 
 be entirely lost, and will grow again. Many fish die, how- 
 ever, after comparatively slight injuries because fish moulds 
 (Saprolegnia) , bacteria, and protozoans gain entrance to the 
 body through the wounds. Perch are very commonly 
 infested by parasites. Nearly every individual examined 
 will be found to have the white " bladders " of larval tape- 
 worms in the liver or among the visceral organs. These 
 cysts if taken into a bass or pickerel will grow into mature 
 tapeworms. The perch's muscles, bile ducts, caeca, and 
 intestine are also often infested with trematodes. 
 
 Perch adjust themselves readily to conditions in the 
 temperate regions of the earth. In winter most of them 
 retreat into the depths of lakes and rivers, where the tem- 
 perature is low but conditions are uniform and small 
 aquatic animals furnish a plentiful food supply. In migrat- 
 ing from one level to another modifications of the internal 
 pressure in the body are necessary, and such adjustments 
 are brought about largely by slow variations of the gas in 
 the air bladder. Perch remain active throughout the year 
 and may be caught on a hook and line at any season. 
 
 Race Preservation. Perch, and all other teleosts, are of 
 separate sexes. Both the males and females have large 
 gonads (testes or ovaries) and are nearly mature before 
 cold weather begins, During the winter both sexes become 
 fully ripe and are ready to begin spawning in April and 
 May. After the ice goes out the males move into shallow 
 water and are soon followed by the females. When a fe- 
 male comes inshore, she is attended by one or more males. 
 As she deposits her "egg string" (Fig. 94, C), which is a 
 crinkled tube of jelly containing 2000 or 3000 eggs, the male 
 squirts sperm over it and fertilization is thus accomplished. 
 
PISCES 
 
 241 
 
 The egg strings often hang on sticks or stones in shallow- 
 water. 
 
 The development of a perch egg after fertilization follows 
 the usual course for metazoans (Fig. 64) ; but appears to be 
 different because the embryo forms on one end of the egg. 
 This restriction of cell-division and embryo-formation to 
 the germinal disc is due to the large amount of yolk which 
 fills the rest of the egg and which, though contained in the 
 cytoplasm, is too inert to permit cell-division. The "fer- 
 tilized" egg (Fig. 95, B) is a zygote formed by the fusion of 
 an ovum (A) and a sperm cell. The nucleus of the egg cell 
 
 F G 
 
 FIG. 95. Embryonic development of fish. A, unfertilized egg; gd, germinal 
 disc; y, yolk; B, zygote formed by union of ovum and spermatozoon; C, D, 
 cleavage; E, young embryo showing neural groove at left; F, showing yolk nearly 
 nearly overgrown by the vascular membrane (blastoderm) growing out from 
 embryo; G, embryo with "yolk sac;" H, young fish, just hatched, with yolk sac 
 not yet absorbed. 
 
 lies at one end in an area of protoplasm called the germinal 
 disc (gd). The rest of the egg cell is filled with yolk 
 material (y). The sperm cell enters the germinal disc, and 
 when cleavage follows fertilization, it is confined to that 
 area. As the embryo grows the yolk is used to nourish 
 it (E, F, G). When the little perch is ready to hatch 
 (G), the yolk containing part of the egg is completely 
 invested by a vascular membrane and forms an appendage, 
 known as the yolk sac, on the belly of the young fish. After 
 leaving the egg envelopes, the young perch (H) takes no 
 food until the egg sac is completely absorbed. 
 
242 GENERAL ZOOLOGY 
 
 Perch take no care of their eggs, and may even devour 
 them or their young. Other species of fishes often follow 
 perch during the spawning season to feed upon the egg 
 strings. The young are very sluggish until the yolk sac has 
 been absorbed, then quickly learn to shift for themselves. 
 
 GENERAL REMARKS ON FISHES 
 
 The thirteen thousand species of fishes now in existence 
 show a wide range of variation in structure and habits. 
 
 The morays and eels have long snake-like bodies; the 
 puffers and globefishes are more or less spherical in form; 
 the goose-fishes have very large heads, the sunfishes have 
 large bodies and very small heads; most fishes are well 
 covered with scales, but the catfish es have none. The fins 
 vary greatly in size, structure and position. There is end- 
 less variation, yet all fishes are recognizable as such, for they 
 breathe through gills and are without fingers or toes on 
 their paired limbs. 
 
 The feeding habits of fishes are often very interesting and 
 peculiar. The herring eats diatoms and other microscopic 
 organisms which it strains from the ocean. One scientist 
 has said that he could predict the herring catch by the 
 number of sunshiny days so directly is the growth of 
 diatoms dependent upon light from the sun, and so im- 
 t portant are these minute plants as food for the herring. The 
 black swallower takes whole fish into its stomach which 
 greatly exceed its own size. Various fishes feed exclusively 
 on plants, or fish, or other particular foods. Some are thus 
 greatly specialized, others will eat practically anything 
 they encounter. 
 
 The only method of locomotion in most fishes is swimming 
 by using the fins, but there are many exceptions. The 
 beach skippers along the shores of some tropical seas seldom 
 enter the water but hop about by using the pectoral fins and 
 tail. The climbing-perch^ is not only able to come on land, 
 but is said to climb trees and catch insects. Flying fishes 
 
PISCES 
 
 243 
 
 leave the water and sail for several hundred yards before 
 alighting. The great majority of fishes are strictly gill 
 breathers, but there are also some exceptions in this in- 
 stance. The dipncjans are able to breathe either through 
 gills, or respire air directly by means of lungs. Some other 
 fishes gulp in air, and breathe through the lining of the 
 throat or intestine. 
 
 Breeding habits of fishes are highly variable and often 
 show extreme adaptation. A cod may lay over nine million 
 eggs, which float at the surface of the ocean and receive no 
 care. The male gaff-topsa'1-catfish carries the eggs laid by 
 
 A B 
 
 FIG. 96. Fish nests. A, stickleback; B, dogfish. 
 
 the female in his mouth until after they hatch, and takes no 
 food for ninety days. The male pipefish obtains eggs from 
 a female and carries them in a pouch in his belly until after 
 they hatch. In some species of sticklebacks the male con- 
 structs a nest (Fig. 96, A). In this he induces a female to 
 lay eggs, and watches them with great care. Many basses 
 and sunfishes make " nests" by clearing a shallow basin on 
 the bottom in which eggs are deposited. 
 
 Some fishes carry on extensive seasonal migrations. The 
 salmon, shad, sturgeon, alewife, and striped bass spend most 
 of their lives in the ocean, but enter rivers to breed. The 
 
244 GENERAL ZOOLOGY 
 
 king salmon on the Pacific Coast of America spawns in 
 November, at the age of four years and when of an average 
 weight of twenty-two pounds. It spends the whole of the 
 preceding summer in ascending the rivers, taking no food 
 during that time, but living on its stores of fat. The salmon 
 that run up the Columbia River travel a thousand miles, 
 and those in the Yukon journey twice as far. Eggs are 
 deposited on the gravelly bottoms of mountain streams >and 
 the fish ^which have made the long journey die soon after. 
 The young salmon spends more than a year in fresh water 
 before passing down to the ocean to mature. The true 
 eels reverse the condition found in the salmon, passing their 
 adult life in fresh or brackish water and returning to the 
 ocean to breed. 
 
 The behavior of fishes shows great diversity in instincts 
 and in the use of particular sense organs. Many species 
 depend largely on their senses of taste or smell for procuring 
 food, others use their eyes exclusively^ and are quick to 
 take any small moving object into their mouths. Carp, 
 mud minnows, trout and other fishes have been taught to 
 come to a certain spot to be fed, and will respond to signals, 
 such as the ringing of bells or the display of objects of cer- 
 tain colors. Instincts and specialized adaptations are 
 many, modification of behavior is often easy, but there is 
 little or no power of reason. 
 
 Early zoologists assumed that, because fishes live in water 
 where all chemical substances that could be smelled or 
 tasted would be dissolved, there could be no sharp distinc- 
 tion between taste and smell, and some went so far as to 
 affirm that fishes could not smell at all. ' Professor Parker 
 has ^demonstrated,, however, that fishes smell very dilute 
 solutionsan the nasal pits, and taste stronger concentrations 
 with the lips, barbels, or other parts of the body. They 
 therefore show the same differentiation in the chemical 
 senses as that in terrestrial vertebrates, smell being used for 
 perceiving greatly diluted substances and taste for stronger 
 solutions. - 
 
PISCES 245 
 
 The evolution of fishes has taken place during the ages 
 still represented by the fossil records and is therefore better 
 known than that of any phylum of invertebrates. Though 
 the exact ancestors of modern fishes are unknown, the 
 general course of events is pretty well recorded in the rocks. 
 Mariy of the ancient groups of fishes are now completely 
 extinct, and a number had peculiar characteristics which 
 died out with them. Some had the body covered with 
 heavy armor, and were provided with a rather flexible 
 neck joint, so that the head could nod. Many lacked 
 teeth and had no paired fins. Among fish-like animals 
 still in existence, tlie cyclostomes and sharks were the first 
 to appear in the past (the most primitive types came first), 
 and were followed somewhat later by the bony-fishes. 
 
 Fisheries are of great economic importance in all parts 
 of the world. The principal species serving for food are: 
 the cod, mackerel, and salmon. In the year 1914 the total 
 catch of fish brought to Boston and Gloucester amounted 
 to 162,589,220 pounds and had a iAie of $4,395,030. 
 The total value of the fishes captured in the United States 
 and on their shores is enormous. The Bureau of Fisheries 
 is constantly at work trying to increase the productiveness 
 of fishing v and improve the quality of its output. Fifty 
 stations are maintained for propagation and study. Dur- 
 ing 1914, 4j288,757,800 eggs and young fishes were planted 
 in suitable waters. 
 

 CHAPTER XXIII 
 
 SUBPHYLUM VERTEBRATA, CLASS 
 AMPHIBIA 
 
 Amphibians have soft moist skins, generally without any 
 sort of exoskeletal structures. This distinguishes them 
 sharply from the reptiles, which have dry scaly skins. On 
 the other hand, amphibians cannot be confused with fishes 
 because, instead of fins, they usually possess paired limbs 
 with toes at their distal extremities. The more primitive 
 amphibians spend their lives in the water breathing by 
 means of gills, but specialized forms, like the frogs and 
 toads, are only strictly aquatic during larval development 
 and spend more or less time on land as adults. Thus, 
 though amphibians are typically aquatic, the group as a 
 whole :.s migrating to the land, and a few species have 
 attained a truly terrestrial life. This is perhaps the point 
 of greatest interest in connection with amphibians some 
 live much like fishes, others start in life as fish-like tad- 
 poles and later become terrestrial. 
 
 There are three orders in the class Amphibia: 
 
 1. Apoda. Degenerate, limbless amphibians which live only in tropi- 
 cal countries. They look much like earthworms and, like them, bur- 
 row about underground. The eyes are very degenerate and the skin 
 contains skeletal plates. 
 
 2. Caudata. Salamanders; aquatic or terrestrial amphibians which 
 have tails when adult. 
 
 3. Salientia. Frogs and toads, which are without t- in the adult 
 condition. 
 
 In order to emphasize the differences between primitive 
 and specialized amphibians, representatives of two orders 
 will be considered an aquatic salamander (Necturus), 
 and the common garden toad (Bufo). 
 
 246 
 
AMPHIBIA 
 
 THE MUD-PUPPY, Necturus maculosus Rafinesque 
 
 247 
 
 North America is fortunate in having this primitive 
 salamander within its boundaries, mostly through the Great 
 Lake Region. Necturus lives among the rocks along the 
 shores of lakes and streams, and is commonly, though 
 incorrectly, called a " lizard" by fishermen. 
 
 Self-maintenance. Necturus (Fig. 97) hunts at night 
 for crayfishes, insects, worms, fishes and their eggs (C). 
 Its eyes are small, poorly developed, and of little or no 
 
 A B c DC 
 
 FIG. 97. The Mud Puppy, Necturus maculosus. A, young; B, swimming; 
 C, catching a crayfish. D, eggs; E, larva with yolk sac nearly absorbed. 
 
 value for procuring food, but the sense of taste is acute and 
 generally distributed in the skin. Necturus is able to 
 discover small animals concealed in the crevices between 
 rocks, and devours them whole. The jaws are armed with 
 double rows of fine pointed teeth for holding the struggling 
 victims of nocturnal forays. Necturus usually creeps about 
 slowly, using the small legs, but when so inclined may 
 
248 GENERAL ZOOLOGY 
 
 swim skillfully by making undulatory movements with its 
 muscular body and tail (B). 
 
 The digestive system is much like that of other verte- 
 brates, consisting of a mouth, pharynx, oesophagus, stom- 
 ach, intestine, liver and pancreas. There is a bladder 
 opening from the ventral wall of the intestine which re- 
 ceives the urine brought by ducts (ureters) from the kid- 
 neys. The heart has three chambers, a ventricle and two 
 auricles. One auricle is filled with " impure" blood from 
 the larger veins, the other receives aerated blood from the 
 gills. The structure of the heart is such that, though both 
 kinds of blood enter the ventricle and are pumped out 
 together, the freshest blood goes to the head and visceral 
 organs, while most of that charged with waste products 
 reaches the gills. The gills are tufts of filaments through 
 which waste gases are exchanged for oxygen. Slender 
 lungs are also present and if the water becomes stagnant, 
 Necturus may go to the surface and gulp in air. 
 
 Self-protection. The mud-puppy is protected in many 
 ways from its enemies. It is acceptable food for water 
 snakes and other predaceous animals, but usually escapes 
 through its nocturnal habits and protective coloration. 
 It may, like other cold-blooded vertebrates, change its 
 colors somewhat to suit the background on which it rests. 
 All amphibians have poison glands in the skin, and the 
 bitter secretion from these makes the mud-puppy distaste- 
 ful to many animals which might otherwise feed upon it. 
 If attacked or captured, Necturus will bite viciously, but 
 the wounds inflicted are not poisonous, as many fishermen 
 believe. 
 
 If Necturus loses a leg or a piece of its tail, the lost parts 
 are regenerated after a time. The process of regeneration 
 and growth in salamanders has been the subject of careful 
 experimental study, and some fundamental facts have been 
 discovered. For example, if the portion of an embryonic 
 salamander eye known as the optic cup is removed and 
 grafted almost anywhere under the skin of the head, a 
 
AMPHIBIA 249 
 
 complete crystalline lens is formed in the new situation. 
 The presence of optic tissue stimulates the ectoderm to 
 form a lens. Metabolic processes have much more latitude 
 in fishes and amphibians than in warm-blooded vertebrates 
 where powers of regeneration are limited and growth 
 processes are not open to much modification. 
 
 The mud-puppy is often attacked by parasites and in- 
 fectious diseases. Fish-mould (Saprolegnia) , a fungus which 
 enters the body through slight abrasions and destroys the 
 tissues, is the most important of these. Another common 
 parasite is a minute trematode worm which attaches itself 
 to the gills and sucks blood. 
 
 Necturus remains active beneath the ice throughout the 
 winter, in this respect resembling fishes and some other 
 aquatic animals which dwell in lakes and rivers. The 
 temperature of the water may be low, but never falls 
 below freezing, and conditions are, therefore, stable enough 
 to permit a sluggish, but not wholly inactive, existence. 
 
 Race Preservation. Amphibians are always dioecious, 
 but Necturus males are much like the females in appear- 
 ance. Though animals are usually found in pairs in 
 autumn, egg laying takes place in May and June. Each 
 female deposits a number of eggs enclosed in little cases 
 and attached by stalks to the under side of stones (Fig. 
 96, D), logs, tin cans, and other objects in the water, where 
 they pass through the usual developmental stages (Fig. 
 65). The whole egg separates during cleavage (Fig. 115), 
 but at one pole the accumulation of yolk retards cell-forma- 
 tion somewhat and the cells are, therefore, larger there. 
 Instead of forming on one side of the egg, as in fishes, the 
 embryo develops entirely around the yolk-containing por- 
 tion, but there is a large yolk sac which protrudes from the. 
 ventral side of the developing embryo until absorbed 
 (Fig. 97, E). 
 
 The young (Fig. 97, A) are striped along the back, and 
 thus differ from the adults. They live in the water plants 
 alongshore where they are effectively concealed by their 
 
250 GENERAL ZOOLOGY 
 
 coloration. Necturus is sometimes called a "mature 
 larva" and it may be properly compared with the well- 
 grown tadpoles of many other amphibians. The gills are 
 functional, but lungs have developed and the sexual organs 
 are mature. 
 
 GENERAL REMARKS ON CAUDATA 
 
 The tailed amphibians show an interesting transition 
 from aquatic to terrestrial habits. Necturus is entirely 
 aquatic, possessing well-developed gills and very small 
 lungs. Siren, which lives in the rivers of the southeastern 
 United States, starts in life as a tadpole, partly loses its 
 gills, and then has them enlarge again to functional size. 
 Cryptobranchus is aquatic throughout life and breathes by 
 means of gills which, however, are covered. Water for 
 respiration enters the mouth and passes out through a pore 
 on either side of the neck. Ambystoma is the common 
 " tiger" salamander throughout the eastern and central 
 United States. It is of particular interest because certain 
 species in the genus show psedogenesis; that is, they be- 
 come sexually mature while in the larval state, and an 
 individual with gills may lay fertile eggs. As a rule, how- 
 ever, the adults are found in damp situations on land, 
 and are often encountered under stones and logs, or in 
 cellars. Diemyctylus is the beautiful vermilion-spotted 
 newt of New England. Its life cycle requires several years 
 for completion. During the first three years it is a green 
 aquatic larva; then migrates to the land and becomes 
 bright orange with vermilion spots. After a period of 
 terrestrial life, the newt returns to the water, becomes 
 green in color, and leads an entirely aquatic existence while 
 breeding. Desmognathus is a truly terrestrial salamander 
 which has no gills or lungs, but breathes by gulping air 
 into the throat, which is very vascular. After becoming 
 mature it returns to the water to breed. Plethodon, the 
 red-backed salamander, does not enter the water even to 
 
AMPHIBIA 251 
 
 breed. It lays its eggs in little packets under logs and 
 stones, and guards them until they hatch. 
 
 Considering the ontogeny of salamanders and the habits 
 of various existing species, the law of biogenesis (page 60) 
 indicates that they are as a group migrating from water 
 to land. Nearly all begin life as a fish-like tadpole which 
 breathes by means of gills, but in the adult condition there 
 are all degrees of adjustment to terrestrial life. 
 
 THE COMMON TOAD, Bufo americanus Le Conte 
 
 Salientians in general are highly specialized. They start 
 in life as tadpoles, but are tailless when mature and show 
 many evidences of specialization. The toad is a typical 
 representative. It is specialized in its marked adjustment 
 to terrestrial life, but shows aquatic affinities in its habit 
 of annually returning to the water to breed. 
 
 Self -maintenance. A toad obtains food by means of 
 its flexible tongue. There is a gland in the roof of the 
 mouth which forms a very sticky secretion. When the 
 tongue is suddenly protruded, it adheres to the food and 
 draws the morsel back into the mouth (Fig. 98, B). The 
 tongue is extended by suddenly filling a lymph space at 
 its base. Toads depend chiefly on their sense of sight for 
 procuring food and will snap at almost any moving object. 
 This habit is general among salientians, and is taken ad- 
 vantage of by fishermen who use a bright-colored rag as 
 bait for catching frogs. A toad has no teeth and insects 
 captured are swallowed at once without any preliminary 
 chewing. If some distasteful object is snapped up, it is 
 quickly spit out, even after it has passed down into the 
 stomach. 
 
 The food habits of toads make them of considerable 
 economic importance. A recent estimate by a scientific 
 expert in the Department of Agriculture places the annual 
 value of a single individual in a garden at $19.44. In cities 
 also a toad may do great good by feeding at night on the 
 insects about street lights. 
 
252 
 
 GENERAL ZOOLOGY 
 
 A toad has two methods of locomotion hopping and 
 walking. The whole body is adapted structurally for 
 progression by leaping, and in moving about that method is 
 usually employed, but at close quarters a toad is able to 
 stalk its food as cautiously as a lynx. An active insect, 
 however, is often secured by leaping upon it from a distance 
 of two or three feet. 
 
 
 A Be 
 
 FIG. 98. Salientia. A, Wood frog and green frog; B, the garden toad, 
 Bufo americanus male singing, catching fly, egg string, tadpoles; C, tree frog 
 and cricket frog. 
 
 The metabolic processes of a toad are much like those of 
 other cold-blooded vertebrates. The digestive and excre- 
 tory organs resemble those of Necturus. The circulatory 
 and respiratory systems are, however, adjusted to life on 
 land. The blood-vessels which supply the gills during 
 larval life are diverted to the lungs and skin when terres- 
 
AMPHIBIA 253 
 
 trial habits are assumed (Fig. 105, B). Most salientians 
 breathe as much through their soft moist skins as 'through 
 the lungs. On this account they are able to endure pro- 
 longed submergence; the blood being aerated through the 
 skin from the surrounding water. 
 
 Self-protection. Though toads have bitter, poisonous 
 secretions in the skin, they are eagerly eaten by snakes, 
 owls, and other animals. They escape from many enemies, 
 however, by doing most of their hunting in the early morn- 
 ing or at twilight, and hiding in their holes during the day. 
 Hopping is a somewhat erratic method of locomotion and 
 toads are hence often able to elude a pursuer. Their colors 
 also render them relatively inconspicuous, and may change 
 somewhat to match different backgrounds. 
 
 Salientians have less power of regeneration when adult 
 than salamanders. If a foot is lost, it does not grow again, 
 but the leg remains a deformed stump. The tadpoles of 
 frogs and toads are rather remarkable for their ability to 
 replace lost parts, and continue to grow after serious in- 
 juries. Eight- and ten-legged frogs, two-tailed tadpoles, 
 and other monstrosities have been produced by splitting 
 embryonic structures. It is also possible to form composite 
 tadpoles by grafting parts of different individuals together. 
 
 On account of their terrestrial life toads cannot remain 
 active during the winter like the strictly aquatic am- 
 phibians. At the approach of cold weather, they burrow 
 into the ground below frost and remain in a torpid condi- 
 tion. During hibernation metabolism is very slow and 
 many activities are dormant; a toad may be cut in pieces 
 without giving any sign of life. 
 
 Race Preservation. Toads show one great advance in 
 complexity of mating instincts when compared with a 
 salamander they make use of sounds. In order to insure 
 the continued existence of any race of animals, the eggs 
 must be fertilized and any adaptation which tends to make 
 this more certain is therefore desirable. In spring when 
 the male toads enter the ponds and utter their shrill notes 
 
254 GENERAL ZOOLOGY 
 
 (Fig. 97) they are giving notice to every mature female 
 within hearing of their presence and ability to fertilize 
 eggs. 
 
 During mating the male clasps the female with his front 
 legs and squeezes the eggs from her body. As eggs emerge, 
 sperm is discharged over them and fertilization takes place 
 in the water. The clasping reflex of the male is controlled 
 in the spinal cord and is very strong during the breeding 
 season. A male while clasping may have his head removed 
 and the whole of the body behind the front legs cut away 
 without letting go his grip; the front legs continue to hold 
 on until they die. 
 
 The eggs of toads are laid in long strings of jelly which 
 protect them. They are usually deposited at night and a 
 single female produces from four to ten thousand each 
 spring. Tadpoles hatch from the jelly about four days 
 after the laying of the eggs. At first they have short tails 
 and a protruding yolk sac on the belly. They have no 
 mouth, but cling to aquatic vegetation by means of a 
 sucker beneath the head until the yolk is absorbed. On 
 the tenth day the mouth is well formed and the tadpoles 
 begin to feed on water plants. The gills, which are feathery 
 tufts on the sides of the head at the time of hatching, are 
 gradually covered by the backward growth of a membrane 
 (the operculum) from the head, and a well-grown tadpole 
 has only one exit for the respiratory water through a pore 
 on the left side. 
 
 The legs grow out as little buds which later develop toes 
 at their distal ends. The front pair are formed inside the 
 opercular cavity and do not appear externally until the 
 tadpole is nearly ready to leave the water. As a tadpole 
 grows older it shows a greater fondness for animal food, 
 and the alimentary canal accordingly grows shorter. It is 
 a general rule among animals that vegetable food is associ- 
 ated with great length of digestive tract, and carnivorous 
 diet with a short enteron. The teeth in the tadpole also 
 change and the earlier chitinous exoskeletal plates associ- 
 
AMPHIBIA 255 
 
 ated with plant food are lost (in frogs they are replaced by 
 bony teeth). 
 
 Tadpoles hatched from eggs laid about the first of June 
 will transform into young toads about the middle of July. 
 As a tadpole nears the end of its sojourn in the water, the 
 gills grow smaller and lungs are developed. Frequent trips 
 are made to the surface of the water and, after the front 
 legs become functional, much time is spent on land, just 
 out of the water. The tail becomes soft and flabby, grows 
 gradually shorter, and finally, some fine day, shrivels up 
 completely. Then the tadpole is no more, but a little toad 
 hops away to hide in the grass and hunt for small insects. 
 Often minute toads migrate from ponds in great armies 
 after a rain, and ignorant people therefore believe that they 
 rained down. The delicate creatures are not yet fully 
 adjusted to life on land and are unusually active when the 
 air is moist. 
 
 If we apply the Law of Biogenesis to the development of 
 a toad, it leads to the same conclusion as in the case of a 
 salamander. The ancestors of amphibians were aquatic 
 fish-like vertebrates which later developed lungs and left 
 the water. Most salientians are more markedly terrestrial 
 than caudate amphibians, and show more striking adapta- 
 tions for life on land. Some species even live in deserts 
 where they pass the dry season in a condition of aestivation, 
 but most amphibians are limited to rather moist situations 
 on account of their soft skins. They are for the most part 
 confined to shady nooks in the shelter of vegetation and 
 seldom hunt in the glare of a noonday sun. They are not 
 as well adapted to the land as reptiles, birds, and mammals. 
 
 GENERAL REMARKS ON SALIENTIANS 
 
 The commonest frogs throughout the United States are 
 the leopard frogs, Rana pipiens, and related species. The 
 green frog, which is often brown or mottled, but may 
 always be recognized by its very large tympanic mem- 
 
256 GENERAL ZOOLOGY 
 
 branes, is also quite common (Fig. 98). The largest and 
 most aquatic species are the bull-frogs; some individuals 
 attain a body length of seven or eight inches. The wood 
 frogs are adapted to terrestrial life, and may spend a large 
 part of the year at considerable distances from water. 
 
 The tree-frogs (C) usually live among vegetation. They 
 have sucking discs at the tips of the toes, which enable 
 them to climb with ease. They also possess remarkable 
 ability to change their colors to match the surroundings. 
 The little brown cricket frog of the United States is a 
 degenerate among tree-frogs which spends most of its 
 life in or near the water and has only rudimentary discs 
 at the ends of its toes. A very specialized tree-frog in 
 Java is able to sail from tree to tree by using the spread 
 webs between the toes. It does not go to ponds to breed, 
 but makes a suspended nest inside a rolled leaf. 
 
CHAPTER XXIV 
 SUBPHYLUM VERTEBRATA, CLASS REPTILIA 
 
 Reptiles are cold-blooded, scaly vertebrates, which 
 breathe by means of lungs. The body is never soft and 
 slimy as in amphibians, but covered with a dry skin which 
 conserves the moisture within and permits existence on 
 land without danger of dessication. The representatives 
 of this class are typically terrestrial ; even the marine turtles 
 and sea snakes, which spend their lives in the open ocean, 
 always return to the land to breed. With the exception 
 of the snakes, limbless lizards, and certain other forms in 
 which the legs are somewhat degenerate, reptiles have 
 claws at the tips of the toes. There are four orders of 
 living reptiles and six times as many which existed in the 
 past but are now extinct. The orders of existing reptiles 
 are as follows: 
 
 Order 1. Squamata. Body elongated and covered with small scales, 
 anus slit-like and extending across the body. 
 
 Suborder 1. Sauria. Limbs, eyelids, and external ear opening 
 usually present. Lizards. 
 
 Suborder 2. Serpentes. Limbs, eyelids, and external ear opening 
 absent. Snakes. 
 
 Order 2. Rhynchocephalia. Represented by a single species, 
 Sphenodon punctatum, a, reptile resembling the lizards, but possessing 
 a large eye-like organ in the center of the forehead. This animal is 
 found only in New Zealand. 
 
 Order 3. Testudinata. Body short, broad, and enclosed between 
 two (dorsal and ventral) shields. Turtles, tortoises. 
 
 Order 4. Crocodilia. Body elongate and adapted to aquatic life; 
 limbs present; anus a longitudinal slit. Alligators, caimans, crocodiles. 
 
 Before discussing the different orders, the activities of a 
 little lizard which inhabits the Eastern United States will 
 be considered in detail. 
 
 257 
 
c= 
 
 258 GENERAL ZOOLOGY 
 
 THE BLUE-TAILED SKINK, Eumeces quinquilineatus 
 Linnaeus 
 
 Self -maintenance. This little lizard hunts among vege- 
 tation for insects, which constitute its principal food. It 
 sometimes eats bird's eggs, young field mice, or other 
 animals of suitable size. It is well fitted to seek and cap- 
 ture food. Its sight is very acute. The sense of taste is 
 also well developed, for ill-flavored substances are often 
 spit out after being taken into the mouth. The claws on 
 the ends of the toes enable the skink to run up and down 
 the trunks of trees with great swiftness. Though the 
 mouth is provided with teeth for holding prey, the food 
 is not chewed, except enough to kill it, before being 
 swallowed. 
 
 The digestive organs of the skink resemble those of other 
 vertebrates, but the respiratory, circulatory and excretory 
 systems are better adjusted to terrestrial life than those 
 of the land amphibians. There is no respiration except 
 through the lungs; the structure of these organs and their 
 associated parts is more specialized than are those of an 
 amphibian. There are little chambers on the inner wall 
 of the lungs which increase the surface exposed and no 
 large branches of the blood-vessels supply the skin. Air is 
 drawn into and expelled from the lungs largely by move- 
 ments of the ribs, whereas frogs and toads have no ribs, 
 but force air in by gulping movements of the throat. The 
 chief excretion product in a reptile is uric acid, which is 
 thrown off as a solid in fact, is insoluble in water and 
 is therefore more desirable for a terrestrial animal (which 
 needs to conserve its water) than urea, which requires 
 liquid for its elimination. 
 
 As the skink grows the "skin" is shed from time to 
 time. The whole epidermis does not come away in one 
 piece, as in many snakes, but sloughs off in patches. 
 Preparation is made in the skin before shedding occurs 
 .and a softening takes place in a particular layer of the 
 

 REPTILIA 
 
 259 
 
 epidermis, so that the outer part may be pushed off with- 
 out injuring what remains. 
 
 Self -protection. The skink is very shy and very swift. 
 When basking in the sun or chasing insects, it is always 
 ready, if danger threatens, to scuttle away to some safe 
 retreat which it knows about. It hunts entirely by day 
 and when inactive usually hides in some crevice with only 
 its head protruding. The watchful eyes give warning of 
 any disturbance, and the skink is hard to catch. If 
 grasped by an enemy, however, it is still not beyond hope 
 
 FIG. 99. The Blue-tailed Skink, Eumeces quinquilineatuA. A, adult male catch- 
 ing insect; B, young; C, female with her eggs in a cavity in a rotten log. 
 
 of escape. The tail may be cast off and left with the 
 enemy while its owner runs away. Most lizards shed the 
 tail very readily and no permanent harm results, for it 
 grows again after a time. 
 
 A skink's colors change with age. In youth the body is 
 jet black with five longitudinal yellow stripes on the back 
 and a bright blue tail. After three or four years the colors 
 become dull, and a mature individual may lose its stripes 
 completely. An adult male has a bright copper-red head 
 and a brown body; a female is usually brown with light 
 
260 GENERAL ZOOLOGY 
 
 longitudinal stripes. In general the colors are suitable for 
 the activities of different kinds of individuals, and skinks 
 are therefore protected from enemies at all ages. 
 
 There is one defect in the skink's means of defense. A 
 reptile of any kind cannot remain active at low tempera- 
 tures. During cool weather, therefore, a dormant skink, 
 if its hiding place is discovered, makes a fine meal for any 
 prowling warm-blooded animal. 
 
 Race Preservation. Skinks, like other reptiles, are of 
 separate sexes. In the species under discussion the 
 mature males are easily recognized by their bright red 
 heads. In the northern states they mate with the females 
 in June, fertilization taking place within the female's body 
 before the shell is formed about the eggs. This is an 
 improvement over the method of fertilization prevalent 
 in amphibians, where the sperm and egg cells are usually 
 discharged into the water. When the eggs are laid, early 
 in July, segmentation has already begun. They are de- 
 posited in little cavities in the rotten wood of old stumps, 
 and the mother remains with them until they hatch (Fig. 
 99, C). In August the young skinks leave the nest~and 
 start out for themselves. 
 
 Though the development of reptiles is similar to that 
 of amphibians, it shows greater specialization for terres- 
 trial life. A skink, or, any other reptile, always lays its 
 eggs on land, whereas amphibians usually deposit theirs 
 in water. The gills in an embryo skink are absorbed long 
 before the egg hatches and are never functional as breath- 
 ing organs. A lizard also shows greater ability to adjust 
 itself to its surroundings; in other words is less stupid 
 than a frog or toad, and even has some glimmerings of 
 intelligence. If a skink encounters a distasteful insect, 
 like a hornet or an ill-flavored caterpillar, it tries to eat 
 it the first time, but after that avoids all insects of similar 
 appearance. It can learn in its dull way that a certain 
 color or form is associated with disagreeable experiences. 
 
REPTILIA 261 
 
 GENERAL REMARKS ON THE ORDER SQUAMATA 
 
 Lizards are clearly more generalized animals than snakes. 
 Most of them possess four well-developed legs with toes, 
 though there are a few which agree with snakes in lacking 
 any external indication of appendages in the adult condi- 
 tion. Snakes are specialized in the absence of appendages, 
 the structure of their jaws, and in other particulars. Yet, 
 despite the divergences between snakes (Serpentes) and 
 lizards (Sauria) , their fundamental plans of structure are 
 so similar that they are placed by systematic zoologists in 
 one order. , 
 
 It cannot be said, however, that many lizards are not 
 specialized; some, indeed, are highly adapted to peculiar 
 modes of life. The limbless species all burrow, and are 
 seldom seen above the surface of the ground. The flying- 
 dragon, Draco volans, has thin lateral expansions on the 
 body which are supported by the ribs and furnish enough 
 Surface so that the animal can sail from tree to tree. The 
 chameleons have prehensile feet and tail, hence are able to 
 climb with great sureness, and their remarkably extensible 
 tongue enables them to snap up insects among the trees. 
 Their unusual ability to change the colors of their skins 
 is also noteworthy. The ," horned toads" are adapted to 
 life in the desert. They are spiny, burrow to escape ex- 
 treme heat or cold, and have other adaptations which 
 enable them to live in arid localities. 
 
 The largest lizards in America are the iguanas, which 
 range from the southern United States southward. They 
 attain a length of six feet, and are often eaten by man. 
 They eat vegetation, insects, bird's eggs, and animals up 
 to the size of a half-grown rabbit. Through the southern 
 states the little aaoles are very common among plants. 
 They are easily tamed and are often sold by curio dealers. 
 The only poisonous lizards known live in the southern 
 United States and Mexico. These Gila monsters (Helo- 
 derma) are sluggish but powerful creatures with bands of 
 
262 GENERAL ZOOLOGY 
 
 bright red and black around the body. They have a 
 poisonous spittle^nd bite viciously when molested. When 
 their bodies are well nourished the tails become swollen 
 with stored fat, and they can then live for some time with- 
 out food. 
 
 Snakes are not only without limbs but show various 
 other adaptations correlated with their slender forms. 
 One lung is usually rudimentary, the visceral organs are 
 peculiarly arranged, and there is no urinary bladder. The 
 two halves of the lower jaw are connected by an elastic 
 ligament, and are loosely fastened to the skull. This 
 arrangement makes it possible for a snake to swallow 
 animals which greatly exceed its own body in diameter. 
 Snakes move by waving the ventral scales backward and 
 wiggling the body. Muscles lead from the scales to the 
 ribs, which help materially in locomotion. There is great 
 variation in the size of snakes a python may have a 
 length exceeding thirty feet and weigh over three hundred 
 pounds; some of the burrowing snakes are less than six 
 inches in length and no thicker than a goose quill. 
 
 Snakes may be viviparous or oviparous; that is, the 
 young of some species are born and those of others hatch 
 from eggs. The garter snakes bear their young in such 
 condition that they are soon able to shift for themselves. 
 The milk snake "lays eggs in holes in the ground. 
 
 Though there are many venomous snakes, the harmless 
 species greatly outnumber them. Of the one hundred 
 and eleven species in the United States less than twenty 
 are poisonous. The venomous snakes have a poison gland 
 on either side of the head which is connected with a grooved 
 or hollow fang in the mouth; the non-poisonous species 
 have the jaws armed with sharp backwardly directed teeth 
 which hold the food and help in swallowing. There are 
 two types of poison fangs grooved and tubular. The 
 former is more primitive, and the hollow type has ap- 
 parently been derived from it by a closing over of the 
 groove. The grooved fang is usually fastened immovably 
 
REPTILIA 263 
 
 to the jaw; but the hollow fang is folded against the upper 
 jaw when not in use and becomes erect when the mouth is 
 opened to strike. Snakes continually protrude the forked 
 tongue when they are moving about and this habit has 
 often led to the belief that the tongue is the fang or "sting," 
 an idea which is of course erroneous. 
 
 In the United States there are only five kinds of poisonous 
 snakes: coral snakes, water moccasin, copperhead, rattle 
 snakes, and opisthoglyphs. Two species of coral snakes 
 (Flaps) are found in the south. They are banded with 
 red, black, and yellow, and have grooved fangs. Four 
 species of opisthoglyphs are found along the southern 
 border of the United States. They are small snakes with 
 grooved fangs which are set far back in the mouth. They 
 cannot readily inject poison by striking quickly at an 
 animal, but must take the prey into the mouth to kill it. 
 The other three types range farther north, have hollow 
 fangs, and belong to the group of venomous snakes known 
 as pit vipers. In these there is a well-defined depression 
 on either side of the head between the nostril and the eye. 
 The water moccasin or cotton-mouth (Ancistrodon pisci- 
 vorus) is a semi-aquatic serpent which frequents the 
 lagoons and sluggish waterways in the southeastern states. 
 It averages about four feet in length but may reach six. 
 The body is very stout and heavy, with an abruptly taper- 
 ing tail and a chunky, ugly head. The copperhead (Ancis- 
 trodon contorlrix) is found east of the Mississippi River from 
 Massachusetts to Florida. It frequents forests and planta- 
 tions, hiding among fallen leaves which it closely resembles 
 in color. There are nineteen species of rattle snakes and 
 the majority of them are found in the United States and 
 Mexico. The diamond-back (Crotalus adamateus) of the 
 /southeastern states is the largest and most deadly of our 
 native serpents. The rattle is an unique organ among 
 snakes. ' The little bells which compose it are formed each 
 time the skin is shed, and are not closely indicative of the 
 snake's age. 
 
264 
 
 GENERAL ZOOLOGY 
 
 ORDER 3. TESTUDINATA (CHELONIA) 
 
 The turtles and tortoises (Fig. 100) are clearly distin- 
 guishable from all other reptiles by the beak-like, toothless 
 jaws and by the shell covering the body. The turtles are 
 aquatic, some species never going on land except to breed, 
 and the tortoises are terrestrial. All chelonians lay eggs, 
 which are buried in shallow excavations on sandy beaches 
 
 Fio. 100. Testudinata. A, the common painted turtle; B, Soft-shell turtle; 
 C, Snapper; D, turtle eggs; E, terrapin. 
 
 or on hillsides (D). Their food varies greatly: the giant 
 tortoises of the Galapagos Islands subsist on cactus; cer- 
 tain marine turtles eat molluscs, fishes, and seaweeds; the 
 fresh- water species feed upon vegetation, molluscs, cray- 
 fishes, insects, fish, frogs, snakes, birds, mice, rabbits, etc. 
 In the interior of the United States there are a number 
 of species of chelonians. The snappers (C) are rough- 
 backed ferocious animals. The common species in the 
 north (Chelydra serpentina) does not weigh over forty 
 pounds, but the southern alligator snapper (Macrochelys 
 
REPTILIA 265 
 
 lacertina) reaches a hundred and forty. The painted, or 
 "mud," turtles (A) are the commonest chelonians through- 
 out the country and may often be seen sunning themselves 
 on logs along swampy shores. The soft-shelled turtles 
 (PlatypeltiSj B) are remarkable for their leathery covering 
 and the extreme length of their necks. 
 
 Through the middle states Blanding's turtle is common 
 along ponds and streams. It is a "semi-box" turtle; that 
 is, there is a hinge across the ventral part of the shell, so 
 that it may be closed over the retracted head and front 
 legs. It spends more time out of water than most turtles, 
 and is often found wandering in fields or woods. In the 
 southeastern states lives the terrapin, Terrapene Carolina 
 (E), which is a true box tortoise. It has the ventral plate 
 so hinged that it completely closes the shell. The terrapin 
 lives in woods and does not enter the water; feeding on 
 berries, earthworms, and insects. There are a number of 
 species of giant tortoises inhabiting islands of the Pacific 
 and Indian Oceans. They never enter the water, and feed 
 on cacti, grass, leaves, and fruits. 
 
 Along the shores of warm seas the large marine turtles 
 are regularly caught for the market, being harpooned in the 
 open ocean or captured while ashore for breeding. The 
 loggerhead turtle may reach a length of four feet and weigh 
 five hundred pounds. The green turtle is somewhat 
 smaller (150 pounds), but is more highly esteemed for the 
 table. Almost any turtle is suitable as food for man, but 
 the marine turtles and tortoises are most generally used. 
 The Bureau of Fisheries has demonstrated by recent experi- 
 ments that the terrapin may 'be reared successfully for the 
 market on small farms. Most of the fresh-water turtles 
 do considerable damage by destroying fishes and small 
 aquatic animals which might serve as fish food. 
 
 ORDER 4. CROCODILIA 
 
 The crocodiles and their relatives resemble the lizards in 
 form, but attain a much greater size and are adapted to 
 
266 GENERAL ZOOLOGY 
 
 aquatic conditions. The nostrils and ears can be closed, 
 and there is a valve at the back of the mouth so that food 
 may be captured under water without danger of filling the 
 lungs. The teeth are set in bony sockets, and there are 
 bony plates beneath the scales in the skin. The heart has 
 four chambers (two auricles and two ventricles) thus resem- 
 bling the condition found in birds and mammals. 
 
 The only crocodilians in the United States are the alli- 
 gator, Alligator mississippiensis, which lives in the rivers 
 emptying into the Gulf of Mexico, and the American 
 crocodile, Crocodilus americanus, in southern Florida. The 
 alligator may attain a length of twelve or fourteen feet. 
 During the breeding season the males bellow like bulls 
 and give off a penetrating odor from two musk glands in the 
 lower jaw. Nests are constructed by heaping mounds of 
 rubbish in swampy places and eggs are deposited in them. 
 The American alligator is very shy, but some Indian and 
 African crocodiles attack man. 
 
 GENERAL REMARKS ON REPTILIA 
 
 Without much doubt reptiles arose during evolution from 
 aquatic salamander-like ancestors. This seems probable 
 both from the palseontological records and the embryo- 
 logical changes in living forms. Fossil remains in the 
 stratified rocks show that the first vertebrates on earth 
 were fishes, and that amphibians preceded reptiles. A 
 modern reptile during its embryology is at first fish-like, 
 then salamander-like, and in both stages possesses gill 
 clefts which never function -as breathing organs but are 
 lost before hatching takes place. 
 
 Modern reptiles are for the most part truly 'terrestrial 
 animals, but are handicapped to some extent by the fact 
 that they are cold-blooded. They are on this account con- 
 fined to the warmer parts of the- earth, whereas^the birds 
 and mammals may invade the frigid regions. Reptiles 
 have mastered air-breathing and water conservation, so 
 
REPTILIA 
 
 267 
 
 that they may live on land without danger of drying up 
 and even dwell in deserts, but have never attained to tem- 
 perature regulation like the "warm-blooded" animals. 
 
 During past ages, however, the reptiles were a mighty 
 race, and at one time ruled the earth. They were special- 
 ized along diverse lines and during early tertiary times 
 were the dominant animals 'in most of the available habi- 
 tats (Fig. 101). Icthosaurs, plesiosaurs, crocodiles, and 
 mososaurs were admirably fitted to prey upon the smaller 
 
 FIG. 101. Restorations of fossil reptiles. (Adapted from Knipe and Lucas.) 
 A pterodactyl soars through the air, two dinosaurs are walking along the shore, 
 and a mososaur swims in the water. , 
 
 animals in the ocean, before there were whales or dolphins. 
 The empire of the air was ruled by great pterosaurs, which 
 were somewhat like birds in form but possessed teeth and 
 long tails. They had, like birds, many adaptations for 
 aerial life such as wings and hollow bones. One species 
 could spread its wings about twenty feet. The land was 
 inhabited by giant dinosaurs and other reptiles, some 
 exceeding a hundred feetT in length. There was great 
 
268 GENERAL ZOOLOGY 
 
 diversity in habits and some species resembled mammals 
 in structure. There were carnivorous dinosaurs which 
 looked much like great lizards or kangaroos; and herbivor- 
 ous species like rhinoceri and buffaloes. The reptiles of 
 past ages occupied the places now taken by birds and 
 mammals and had many of their adaptations. 
 
 But the great abundance of 'reptile life which once domi- 
 nated the earth has now gone forever. Only a few strag- 
 glers remain from those which at one stage in the earth's 
 evolution were the best animals that had been produced. 
 Nature has since brought forth better living mechanisms. 
 But the reptiles did not live in vain, for evolutionists tell 
 us that some of the extinct forms doubtless gave rise to 
 the birds and mammals. 
 
CHAPTER XXV 
 SUBPHYLUM VERTEBRATA, CLASS AVES 
 
 Birds differ from all other vertebrates in possessing 
 feathers, though the scaly feet suggest relationship to 
 reptiles. Certain extinct species possessed teeth ,and had 
 separate toes ending in claws on the front limbs, but all 
 modern birds have the toothless jaws covered with a horny 
 beak and the digits of their wings are more or less rudi- 
 mentary. Birds, like mammals, are " warm-blooded "- 
 that is, are capable of maintaining a constant body 
 temperature. 
 
 There are two subclasses of Aves: 
 
 Subclass 1. Archseronithes.^ Extinct birds having toothed jaws, a 
 long lizard-like tail composed of separate vertebrae, and three separate, 
 clawed digits on each of the fore limbs. 
 
 Subclass 2. Neornithes. Extinct and modern birds in which the 
 terminal vertebrae of the tail are fused to form a pygostyle, or "plow- 
 share" bone; the jaws are toothless, except in some extinct species; the 
 bones of the fore limb are more or less fused and reduced in size. 
 
 SUBCLASS 1. ARCHvEORNITHES 
 
 This group of birds is known from only two specimens 
 and a separate feather, found in the lithographic slate at 
 Solenhofen, Bavaria. All these fossils are referable to a 
 single species, Archceopteryxlithographica (Fig. 102). This 
 bird was about the size of a crow. It had a long flexible 
 tail with feathers along either side, and the wings bore 
 three free digits, each with a claw, while the hind legs had 
 four toes, like those of modern birds. The jaws were 
 armed with a row of strong teeth set in sockets. 
 
 Though this curious flying animal, preserved as a fossil 
 for our inspection from Jurassic times, has characteristics 
 
 269 
 
270 
 
 GENERAL ZOOLOGY 
 
 of both reptiles and birds, as judged by existing representa- 
 tives of those groups, it is classed witji the latter and is 
 looked upon as the most primitive known bird. Archseop- 
 teryx had feathers, and in this respect differed from all 
 reptiles, living or fossil. It is about as nearly a " missing 
 link" as science may reasonably expect to find when the 
 imperfection of the fossil record is considered, and its 
 
 FIG. 102. Archceopteryx (A) and Hesperornis (B), two extinct toothed birds. 
 (After restorations by Knipe and Lucas.) 
 
 structure is believed to indicate, with other evidence, that 
 birds were evolved from reptilian ancestors. 
 
 SUBCLASS 2. NEORNITHES 
 
 The birds in this group are characterized by having 
 several vertebrae at the tip of the tail fused to form a 
 " plowshare bone" which acts as a support for the large 
 tail feathers. The wing bones are reduced and more or 
 

 AVES 271 
 
 less united; there are never more than two free digits with 
 claws, and usually none. Four orders of fossil birds are 
 placed in this subclass, as are seventeen whose representa- 
 tives live today. Space will not permit the discussion of 
 all these; they will, therefore, as a matter of convenience 
 be divided into three groups: (1) primitive fossil birds^ 
 (2) recent flightless birds; (3) recent flying birds. 2^ ' 
 
 The fossil Neornithes had some very peculiar features: 
 Hesperornis regalis (Fig. 101) was a flightless bird about 
 four feet long, adapted for swimming and diving. It 
 had teeth set in grooves in the jaws; the sternum was with- 
 out a "keel" for the attachment of wing muscles, and the 
 strong hind feet were webbed for swimming. The re- 
 mains of this great diver have been found in the Cretaceous 
 deposits in Kansas. In the same region the remains of 
 other aquatic birds which had keeled sterna and jaws 
 bearing teeth set in sockets have been discovered. 
 
 The elephant-birds (dZpyornis, etc.) and the moas 
 (DiorniSj etc.) probably became extinct within the past 
 five hundred years. The former were great flightless 
 creatures which lived in Madagascar. The eggs of 
 ^Epyornis have occasionally been dug up along the sea- 
 shore; some of them are over thirteen inches long and have 
 a capacity of a couple of gallons. Over twenty species of 
 moas formerly lived in New Zealand. They were unable 
 to fly and possessed enormous hind limbs adapted for 
 running. Some were as small as turkeys, while others 
 stood ten feet in height. Probably the early human in- 
 habitants of New Zealand exterminated these birds, for 
 remains are found in caves and in ancient refuse heaps. 
 
 Xmong recent birds which fly but little or not at all, the 
 kiwis (Apteryx) of New Zealand are the most striking. 
 They are about the size of a hen and have a very long beak^ 
 which they use as a probe in seeking worms underground. 
 The wings are very degenerate, being represented by small 
 bones which do not appear outside of the body; tail feathers 
 are wholly lacking. The penguins (ofder^Jjnpennes) of 
 
272 GENERAL ZOOLOGY 
 
 which about twenty species are known, are confined to the 
 Antarctic regions. They live the greater part of the time 
 in the open ocean where their short wings (inadequate for 
 flight) are used for swimming. When on land, penguins, 
 walk erect or, if in haste, slide along on the belly. They 
 nest on land in great colonies during the Antartic summer 
 at temperatures as low as 78F. below zero. Several birds 
 often cooperate in incubating the eggs and caring for the 
 young. y^The cassowaries, emfcus, ostriches, and rheas' are 
 large-inning birds with small wings and large, powerful 
 legs. The. ostrichesDop camol birds (Struthio -eamvlus) , 
 live in the desert regions e^AMea, and travel about in 
 groups. These are the largest living birds, some indi- 
 viduals attaining a height of eight feet and weighing over 
 three hundred pounds. They are very shy and flee swiftly 
 when approached; but do not hide their heads in the sand, 
 as is commonly believed. Ostrich farming is now a well- 
 established industry in South Africa and certain parts of 
 the United States. A domesticated ostrich will yield 
 fifteen or twenty dollars worth of plumes each year. The 
 utilization of such feathers is desirable, for plucking causes 
 the ostrich little inconvenience and the use of the plumes 
 saves the lives of many wild birds which might otherwise 
 be slaughtered on the altar of fashion. The tinamous are 
 birds inhabiting Mexico, Central and South America. 
 They resemble partridges in appearance, but fly very little. 
 The flying birds are divided intovteHr orders. The loons 
 and grebes (Colymbiformes) are strong flyers but are chiefly 
 remarkable "for their ability to swim and dive. A loon can 
 stay under wat&\several minutes, and travel a quarter of 
 a mile while submerged./ The albatrosses, fulmars, shear- 
 waters, and petrels (P^ocellariformes) are gull-like marine 
 birds with tubular nostrils\and long slender wings. They 
 are expedient flyers and riesVin grWt colonies on Oceanic 
 islands. /The stork-like birds (Ciconiiformes) include the 
 tropic birds, Ciinnoran-ts, anhingas, pelicans/ herons/ bit- 
 terns, spoonbills, storks, ibises, flamingoes/etc. Most of 
 
AVES 273 
 
 these birds have long legs, slender necks, elongated bills, 
 and feet fitted for wading or swimming. I'he geese, river- 
 ducks, sea-ducks, fishr ducks, swans, and screamers (Anseri- 
 formes) have webbed feet and are aquatic in their habits. 
 Hawks, eagles, vultures an4 secretary birds (Falconi- 
 formes) are the true birds of prey. The owls, which in the 
 popular mind are associated with them probably as much 
 because of their food habits as for any other reason belong 
 to a different order. All the birds of prey have very strong 
 talons and hooked beaks, which are used for catching and 
 killing animals for food. The owls, together with the 
 kingfishers, hummingbirds, goatsuckers, woodpeckers, and 
 a few other rare forms are placed together in the order 
 Coradiformes because of certain anatomical similarities. 
 Owls are generally nocturnal, but are able to see more or 
 less during the day. Their great expanse of wing, together 
 with a peculiar frilling of the feathers, enable them to. fly 
 with a quietness remarkable in such large birds. The 
 hummingbirds are the smallest of the class Aves, the largest 
 being less than eight inches long, and the smallest, two and 
 three-eighths inches. Over four hundred species are known 
 in America, but only one .is found throughout United States. 
 They are nearly all of gorgeous metallic colors, changing 
 in different lights. The woodpeckers are remarkable for 
 their peculiar beaks, extensible tongues; the stiff tail and 
 peculiarly arranged toes both function in climbing. ,. The 
 rails, cranes, and coots (Gruiformes) are marsh hkds_wjth 
 long legs for wading or with lobate feet for running over 
 aquatic -vegetation. Snipes, plovers, curlews, and jacanas 
 are related'to the gulls, terns, auks, and pigeons (Charadrii- 
 formes). The snipes and their near relatives are waders; 
 the gulls are strong flyers and live largely on fish ; the auks 
 are sea bi^ds with very heavy beaks, and nest in holes in 
 the ground; the gentle pigeons and doves are widely dis- 
 tributed, three hundred species being known. The cuckoo- 
 like birds (Cuculi/ormes) include the true cuckoos, parrots, 
 and a number of tropical birds. The cuckoos are insect 
 
274 GENERAL ZOOLOGY 
 
 eaters, and are chiefly known from the -habits of the 
 European species, which deposits its eggs in the nests of 
 other birds. Parrots live largely on fruits and seeds. 
 Only one rare species, the Carolina Paroquet, occurs in the 
 United States, and it is altogether probable that this has 
 become extinct during the last few years. 
 
 Most of the familiar small birds belong to the order 
 Passeriformes, which includes almost half {7000) of the 
 known species of birds. There are sixty-feur families in 
 this important order the -finches, flycatchers, vireos, 
 thrushes, wrens, blackbirds, jays, swallows, warblers, and 
 many others being included. Passerine birds are usually 
 of small or medium size, but are the most highly organized 
 of the class Aves. The feet are fitted for perching, and 
 representatives of the otfder are often called " perching 
 birds." A familiar example is the American robin, which 
 will be considered in some' detail. This bird is not closely 
 related to the European " robin red-breast," which belongs 
 in-an entirely different family. It is familiar to everyone 
 iiu North America because of its trusting ways and the 
 readiness with which it adjusts itself to the changes accom- 
 panying the advance of civilization. 
 
 THE AMERICAN ROBIN, Planesticus migrat&rius Linnaeus 
 
 Self^-maintenance. The robin eats caterpillars, insects, 
 earthworms, and other small animals. Everyone has seen 
 it straining and tugging to pull a fat worm from the lawn. 
 There has been some dispute among students of bird foods 
 as to whether the robin is beneficial or injurious to man. 
 Barrows says: " There is no question that the robin some- 
 times does a large amount of good in its consumption of 
 insects, especially by eating cutworms and grasshoppers; 
 it must be remembered, however, that the major part of 
 its insect food is taken from the ground and that hence the 
 robin is a factor of small importance in limiting the activity 
 of the spanworms and other caterpillars which defoliate 
 our fruit and shade trees. It also eats large numbers of 
 
AVES 275 
 
 insects which at best are not harmful, and which possibly 
 may be beneficial." Forbes states that, " while the robin 
 is not so precious that we need make it an act of sacrilege 
 to show him the muzzle of a gun in a cherry orchard on 
 the other hand it would be an enormous blunder to wage 
 ourselves, or to permit others to wage, any general or indis- 
 criminate war against him." 
 
 The robin depends for the most part on its acute vision 
 and agility to secure food. Its sharp eyes are quick to spy 
 out any lurking insect in a garden, and its horny beak is an 
 excellent organ for capturing such morsels, for it is im- 
 pervious to bites or stings. Birds are handicapped some- 
 what in feeding by the lack of claws or other seizing organs 
 on the front limbs ; but this is compensated for by the great 
 quickness which is associated with the use of the wings for 
 flight, and by the great flexibility of the neck. 
 
 The feathers are admirably adapted for flight; being 
 light, yet with broad expanse; flexible, but stiff enough to 
 resist the air. The small barbs on either side of the main 
 shaft are provided with minute hooked barbels which bind 
 them together so as to make a broad flat surface; this may 
 be broken up repeatedly, yet on being smoothed over again 
 will be as firm as before. Within the body there are struc- 
 tural adaptations which make it light. The air sacs open- 
 ing from the lungs are good examples of such structures. 
 They lie beween the large muscles, among the internal 
 organs, and are even connected with cavities within the 
 long bones. In order that flight may be swift and sure, 
 the body must be rigid as well as light; there can be no 
 wavering or the flyer will fall. To secure greater rigidity 
 in the body, certain parts of the skeleton which consist of 
 separate bones in other vertebrates are firmly united in 
 birds. Each rib bears a projection (uncinate process) on 
 one side which overlaps the next rib, thus making the body 
 very firm during flight. Perhaps the most striking skeletal 
 adaptation for flight is the great keel on the sternum, which 
 gives attachment to the wing muscles. 
 
276 GENERAL ZOOLOGY 
 
 The robin is not only well equipped to find and capture 
 food, but is also able to profit by experience to a greater 
 degree than a toad or a lizard. It soon learns to avoid un- 
 desirable objects, and usually hunts in favorable places 
 at the most opportune times. After the food has been 
 captured and swallowed, it is stored in the crop, ground 
 among the pebbles which are present in the muscular 
 gizzard, and then absorbed in the intestine. Digestion, 
 absorption, assimilation, and excretion, are assisted by the 
 digestive glands (liver, pancreas) and by certain ductless 
 glands (pancreas, thyroid, thymus) which act " indirectly" 
 by secreting substances into the blood. 
 
 The circulatory system of birds presents some points of 
 special interest. At one time within the egg a robin has 
 several pairs of aortic arches, but these degenerate or are 
 diverted to various parts of the body as development pro- 
 ceeds, and an adult bird has only one great aortic arch, 
 which curves to the right after leaving the heart and then 
 runs backward as the dorsal aorta (Fig. 105). The heart 
 in all birds has four chambers the left auricle and ven- 
 tricle collect and pump blood to the lungs for aeration; 
 the right side receives the "pure" blood from the lungs 
 and forces it through the body. Because of the active life 
 a bird leads, metabolism is very rapid. The body tem- 
 perature is even higher than that of mammals, and respira- 
 tion is accordingly accentuated. A bird breathes through 
 its lungs into air sacs beyond. 
 
 The excretory products of a robin pass into the cloaca 
 from the kidneys and are eliminated through the anus 
 with the faeces. They are chiefly in the form of uric acid, 
 which is a solid and therefore well adapted to an aerial 
 animal. A robin must be light and could not carry a 
 great quantity of water, which would be necessary to 
 dissolve urea. 
 
 Self-protection. A robin's keen eyes and ears usually 
 give adequate notice of approaching danger; any shadow 
 which indicates a prowling hawk causes a speedy retreat 
 
AVES 277 
 
 to some thicket; any unusual sound causes the sharp 
 eyes to seek its source. Raptorial birds, weasels, snakes 
 and other animals catch robins when they can, and are 
 particularly likely to prey upon young individuals. In a 
 general way the coloration of a robin protects it from such 
 sharp-eyed enemies. The body is countershaded (dark 
 above and light below) so that it is not conspicuous when 
 illuminated from above, because the lighter parts are in 
 the deepest shadow. The pigments in the feathers also 
 blend well with the tree trunks and branches which robins 
 usually frequent. When a robin is alarmed or threatened 
 with danger it gives certain call -notes which warn its 
 fellows of the trouble, so that they may either look out 
 for themselves or come to investigate the cause of the 
 disturbance. 
 
 A robin is much better equipped than a lizard to meet 
 the varying conditions of a terrestrial environment. Both 
 have a dry skin which conserves the water within the 
 body, but feathers are better than scales because they 
 keep a layer of air next to the body which serves as an 
 insulator against loss of moisture by evaporation or of heat 
 by radiation. A bird is also more specialized than a lizard 
 in having a constant body temperature which is main- 
 tained in spite of biting cold or extreme heat. Most robins 
 migrate southward to pass the winter in a warm climate 
 where food is abundant, but a few may linger in certain 
 northern localities until spring. The migration of the 
 robin covers a distance of approximately 3000 miles. In 
 autumn the trip takes about eighty days, and in spring, 
 seventy. The birds move northward as the mean daily 
 temperature reaches 35F. 
 
 Birds are subject to attack by various diseases. A robin 
 may acquire bird malaria from the bites of mosquitoes, 
 and certain bacteria may cause internal disorders. Trema- 
 todes and other worms live within the body; bird lice and 
 other ectoparasites dwell among the feathers. Such dis- 
 eases and parasites the robin avoids as far as possible by 
 
278 GENERAL ZOOLOGY 
 
 scrupulous cleanliness. Frequent baths are taken, both 
 in water and dust (the latter being distasteful to ectopara- 
 sites), the feathers are preened with the beak, and every 
 precaution is taken to keep the body in good condition. 
 
 Race Preservation. Though the female robin shoulders 
 the chief responsibilities for the care of the eggs and the 
 rearing of the young, the male is always at hand to en- 
 courage his mate, sound the alarm when danger threatens, 
 or attack intruders. The pair select a suitable site, usually 
 in an elm, maple, or apple because these trees have broad 
 forks in the branches, and the female builds the nest. She 
 first lays down a rough foundation of coarse stems, then 
 brings load after load of grass and mud in her beak and 
 builds a wall around the edge. Each time new material 
 is added she scratches it into place with her feet, smooths 
 and shapes it with her breast, and rounds off loose strands 
 on the outside with her beak. The moulding, turning and 
 smoothing movements are repeated hundreds of times. 
 Finally a lining of soft grass or roots is placed in the com- 
 pleted cup and the nest is ready for the eggs. The male is 
 constantly at hand and seems to furnish incentive to the 
 female to carry on the work. If she goes for mud he fol- 
 lows a few feet behind, singing and showing great interest 
 in all she does. If some other bird or small animal ap- 
 proaches the nesting place, the male attacks the intruder 
 with great ferocity; if a danger threatens that cannot be 
 combated he gives loud warning notes and stays near his 
 mate to render assistance. 
 
 It requires from two to four days to complete a nest and 
 the first egg is usually deposited from the fourth to the 
 seventh day. After three or four eggs have been laid 
 the female begins to incubate them, and for nearly two 
 weeks leaves the nest only for short intervals to snatch a 
 little food. The male is always near at hand, and helps in 
 feeding the young birds after they hatch, but his mate 
 still does the greater part of the work. The young have 
 ravenous appetites, eating more than their own weight 
 

 AVES 
 
 279 
 
 every day. The attentive parents have to work from 
 "early morn till dewy eve" to keep them satisfied. The 
 nest is kept perfectly clean, all excrement or other dirt 
 
 (Photos by A R. Cahn.) 
 
 FIG. 103. "The wind blows east, the wind blows west, 
 The blue egg in the robin's nest 
 Will soon have beak and wings and breast, 
 And flutter and fly away." 
 
 being carried away by the parents. Even after the young 
 have left the nest and are making short flights or hiding 
 in the bushes, the parents watch over them. 
 
280 GENERAL ZOOLOGY 
 
 The long period of care-free youth which robins enjoy 
 enables them to reach a state of comparative independence 
 under the protection of the parents. They do not en- 
 counter the force of the struggle for existence until they 
 have attained somewhat of strength, vigor, and experience. 
 The egg cell is stored with yolk, fertilized, and has passed 
 through early segmentation stages before it is laid. As it 
 passes down the oviduct more nourishment is added around 
 the egg proper in the form of albumen, or " white," and the 
 protective shell is then formed on the outside. While the 
 young robin develops within the egg, it is constantly 
 warmed and watched by the parents. After hatching, 
 it is long the object of solicitous attention. Small wonder, 
 then, that most robin eggs come to maturity. 
 
 A perch lays two thousand eggs, gives no attention to 
 its young, and most of them are destroyed. A robin lays 
 four eggs and usually brings all of them to maturity. Why 
 is one method better than the other? The net result in 
 either case will perhaps be two or three new animals from 
 each pair annually. The robin, however, has one ad- 
 vantage the young pass through a long adolescent period 
 during which they are cared for, and even trained to some 
 extent by having opportunity to imitate their elders. 
 The bird has more chance to acquire experience before 
 being thrown on its own resources, and this gives opportu- 
 nity for a higher grade of psychic development. 
 
 GENERAL REMARKS ON BIRDS 
 
 Birds all agree in having feathers, beaks, arid .character- 
 istic feet, but all these structures show endless variations 
 which are usually correlated with differences in habits and 
 habitats. For example the feathers may be downy, or 
 stiff and hair-like; the feet may be webbedj feathered, or 
 naked; the beak may be hooked for tearing flesh, short and 
 heavy for cracking seeds, or long and slender for probing 
 in the mud. Each type of bird has the bodily parts highly 
 
AVES 281 
 
 adapted for a particular mode of existence, which shows 
 that birds are racially specialized. 
 
 Birds are unusual when compared with other vertebrates 
 in having the ability to fly. Their exceptional agility has 
 made it easy for them to capture food and escape from 
 enemies, but they are not on the whole as versatile as 
 mammals, partly because they are too specialized. They 
 depend largely upon quickness and the acuteness o? their 
 sense organs. This has led to the extreme development of 
 those parts of the nervous system which correlate accurate 
 muscular movements and control reflexes; but the parts 
 which have to do with thinking and scheming are com- 
 paratively simple. In the brain of a bird the cerebellum 
 (which is concerned largely with the coordination of 
 muscular activities) is very large and the cerebral lobes 
 (where higher mental qualities reside) are small. In a 
 mammal both regions are well developed. Birds, then, 
 as a race, have sacrificed their power to develop great 
 mental ability, probably because they became able to fly 
 early in their evolution, and then were so specialized that 
 they could not branch off on new evolutionary lines. The 
 toes on the front limbs became degenerate to allow the 
 formation of more effective organs for flight; the neck 
 grew long and flexible to compensate for the resulting handi- 
 cap in securing food; the teeth gave place to a horny beak. 
 Specialization along such lines for a time made birds so 
 successful that they became modified structurally to such 
 an extent that they can never be racially youthful and 
 have broad possibilities again. Birds as a race are in their 
 old age. 
 
 As long as birds succeed' with their present adaptations, 
 however, they will dominate the air, and must be given 
 credit for their exceptional flying ability. A fish-hawk 
 can move as fast as an express train and may feed at will 
 from the ocean or an inland lake, on the top of a mountain 
 or in the bottom of a canyon. The best flyers among 
 birds are the long-winged gulls, vultures, hawks, and man- 
 
282 
 
 GENERAL ZOOLOGY 
 
 Breeding 
 
 Wintering 
 
 Principal migration routes 
 
 FIG. 104. Distribution and migration of the Eskimo curlew. (From Cooke; 
 Yearbook, U. S. Department of Agriculture, 1914.) 
 
AVES 283 
 
 V 
 
 o'-war birds. Though most short-winded birds are poor 
 flyers and tire easily, some sturdy forms ; like the ducks 
 (which may attain a speed of a hundred miles per hour) 
 are swift and effective on the wing. 
 
 The seasonal migration of birds has been a subject of 
 much interest and speculation, but many of its aspects are 
 still shrouded in mystery. Most small birds move about 
 twenty-five miles a day on their journeys north and south. 
 They usually travel high in air at night and rest during the 
 day in appropriate localities. The Arctic tern annually 
 travels "from pole to pole," thus living in perennial 
 summer. The Eskimo curlew (Fig. 104) covers a great 
 ellipse south across 2500 miles of Atlantic ocean to its 
 winter home and north through the center of North 
 America to its breeding grounds within the Arctic Circle. 
 On the other hand some birds, like the quail and the English 
 sparrow, migrate little or not at all. Migratory birds 
 usually breed in the coolest part of their range and spend the 
 summer in warmer climates. The abundance of food and 
 the presence of appropriate nesting sites are probably 
 important factors in controlling such flights but they are 
 not the sole causes of migration; in some species, in fact, 
 appear to have little or no effect. 
 
 The songs of birds commonly serve for the attraction 
 of mates, but various other characteristic sounds are useful 
 for warning signals, calling the young, etc. The vocal 
 apparatus of a bird is not in the larynx, as in mammals, 
 but lies in the "syrinx" at the lower end of the trachea and 
 is even sometimes imbedded in the .sternum. It is a rather 
 complicated apparatus, capable in many cases of producing 
 a considerable range of sounds. 
 
 The colors of birds are for the most part protective. 
 The body is countershaded (i.e., is dark above and light be- 
 low, so that the effects of light and shadow are eliminated,) 
 and is therefore inconspicuous. Some birds, like the grouse, 
 also have accurate pictures on the feathers of the back- 
 grounds on which they are most apt to be seen. The fact 
 
284 GENERAL ZOOLOGY 
 
 that a bird is bright colored is not necessarily to be taken 
 as indicating that it is conspicuous in its usual habitat. 
 Parrots, for example, are very difficult to see among the 
 foliage of tropical trees. In some instances, however, 
 striking colors, may serve "for attracting the opposite sex, 
 or as recognition marks for other members of the same 
 species. 
 
 The nesting habits of birds show various degrees of com- 
 plexity and specialization in styles of architecture. A 
 nighthawk builds no nest, but deposits the eggs on the 
 bare ground, or sometimes even on a gravel roof. The 
 burrowing owl of the western prairies digs a hole and 
 strews manure on the bottom to keep the eggs off the 
 ground. The bush-turkey of Australia builds great mounds 
 of sticks and dead leaves in which the eggs are left to incu- 
 bate from the heat generated by the decaying rubbish. 
 Most passerine birds build nests like the robin, on the 
 ground or among the branches of plants. The apex of 
 avian architecture is reached in the beautiful basket nests 
 of the orioles. 
 
 With a few exceptions birds incubate the eggs in nests or 
 cavities which they construct. In general the length of 
 the incubation period is proportional to the size of the 
 egg, the smallest hatching most quickly. The emperor 
 penguin carries its single egg between the hind legs, and 
 male and female birds take turns holding it. Eggs laid 
 in open nests are usually colored to match the surround- 
 ings, and this is particularly striking in those which are 
 laid on the ground without a nest or with but a very crude 
 one, as in the nighthawk or killdeer. Birds which nest 
 in hollow trees or in holes in the ground usually lay white 
 eggs. 
 
 Domesticated birds play a considerable part in modern 
 civilization. The numerous varieties of the common hen 
 probably all come originally from the jungle-fowl, Gallus 
 gallus, of India. Domestic pigeons were derived from the 
 blue-rock pigeon, Columba livia, a native of Europe and 
 
AVES . 285 
 
 Eastern Asia. Most of our tame geese originated from the 
 graylag goose of northern Europe, and our ducks from the 
 mallard. The peacock is a native of India, and the Guinea 
 fowl came originally from West Africa. The turkey has 
 been domesticated since the white man came to America, 
 and, though rapidly becoming scarce, is still found in a 
 wild state. 
 
CHAPTER XXVI 
 
 SUBPHYLUM VERTEBRATA, CLASS 
 MAMMALIA 
 
 A mammal is a warm-blooded vertebrate with hair and 
 mammary glands. There are a few apparent exceptions to 
 this definition. For example, some whales as adults are 
 naked, in one instance having the hair reduced to two bris- 
 tles on the upper lip; but before birth all have an abundant 
 hairy covering. Mammary glands are found in all mam- 
 mals. These are simply specialized skin glands which 
 secrete milk to nourish the young. The ancestors of 
 mammals were probably for the most part land animals, 
 but in recent geological history many have become adapted 
 to live in the water. Even the whales appear to have de- 
 scended from quadrupedal forms which lived along the 
 shores of ancient oceans. 
 
 All mammals have four-chambered hearts, like birds and 
 crocodiles. They also have a single aortic arch carrying 
 blood from the heart to the dorsal aorta, but in this case the 
 one on the left side has persisted instead of that on the 
 right as in birds. In passing from fishes to mammals the 
 aortic arches, which in the simplest vertebrates carry blood 
 to and from the gills, are progressively lost or diverted to 
 other uses; the heart on the contrary becomes more and 
 more complicated (Fig. 105). Fishes (A) usually have 
 four pairs of functional arches which supply the gills, and 
 possess a tubular heart; adult amphibians (B) have three 
 pairs of aortic arches (only one of which connects with the 
 dorsal aorta; the other two being diverted to the head, lungs 
 and skin) ; the reptiles (C) have arches somewhat like am- 
 phibians, but in the crocodiles acquire a four-chambered 
 heart; in the birds (D) and mammals (E) only one arch of a 
 
 286 
 
MAMMALIA 
 
 287 
 
 single pair connects with the dorsal aorta and the heart is 
 always four-chambered. 
 
 The teeth of mammals show great variations which in 
 general are correlated with differences in feeding. The 
 reptilian ancestors of the mammals possessed teeth and all 
 existing mammals have them at some time. The curious 
 duck-bill, certain whales, armadillos and the ant eaters 
 have very rudimentary teeth, however, which do not break 
 through the jaw and are resorbed before birth. Most 
 mammals have two sets of teeth a temporary or "milk" 
 dentition and a permanent dentition. In some (guinea 
 
 FIG. 105. A comparison of the heart (dotted) and chief arteries in: A, fish; 
 B, frog; C, lizard; D, bird; E, mammal. 
 
 pigs, bats) the former is lost before birth. The teeth may 
 be all alike (homodont dentition), as in the dolphins, or 
 show a differentiation into incisors, canines, premolars, and 
 molars (heterodont dentition). The molars appear only 
 in the permanent dentition. Very often animals which 
 have a specialized heterodont dentition may lack certain 
 types of teeth, and a diastema, or bare space, is left in the 
 jaw. A squirrel, for example, has very large incisors but 
 no canines .and there is a gap in front of the premolars; a 
 cow lacks incisors in the upper jaw and has no canines 
 whatever. 
 
288 
 
 GENP:RAL ZOOLOGY 
 
 In the mammals structures for reproduction and the 
 nourishment of the young have attained far greater special- 
 ization than in any other group of vertebrates. The sexes 
 are always separate, the egg is fertilized within the female 
 and early stages of development take place within her body. 
 The most primitive mammals lay soft-shelled eggs some- 
 what like those of birds, but the majority have the egg 
 
 body wall 
 
 FIG. 106. Section through body wall and uterus of a placentate mammal to 
 show how the embryo is attached to the wall of the uterus. 
 
 develop within the body and the young are nourished by 
 the mother for some time in many cases, both before and 
 after birth. The young of the marsupials are at first very 
 helpless. They are placed at birth in the marsupial 
 pouch on the ventral side of the abdomen, where they re- 
 ceive protection and are nourished from the mammary 
 glands. All mammals which do not lay eggs or place the 
 young in a marsupial pouch have a placenta (Fig. 106) of 
 some sort. This is a complicated structure which grows 
 

 MAMMALIA 289 
 
 out from the developing embryo and becomes fastened to 
 the uterus of the mother in such a way that there is an 
 exchange of nourishment between mother and offspring 
 through the thin walls of the blood-vessels. All mamma- 
 lian embryos, like those of reptiles and birds, are covered by 
 a protective envelope, the amnion, before birth. This is 
 filled with a watery fluid and its walls enter into the forma- 
 tion of the placenta. 
 
 Mammals are divided into large groups primarily on the 
 basis of the degree of specialization in the reproductive and 
 developmental processes. The groups are as follows : 
 
 Subclass I. Prototheria; egg-laying (oviparous) mammals. 
 
 Subclass II. Eutheria; viviparous mammals. 
 
 Division 1. Didelphia; mammals which carry the young in a mar- 
 supial pouch and nourish them before birth through a placenta which is 
 usually very primitive. 
 
 Division 2. Monodelphia; mammals which nourish the young 
 through a typical placenta, and never carry them in a pouch after birth. 
 
 Fossil records show that the Prototheria and Didelphia 
 appeared on earth before the Monodelphia. There are 
 also rudimentary structures on various representatives of 
 the latter group which indicate that they came from 
 marsupial ancestors. The fourteen orders of mammals 
 which have living representatives will now be considered. 
 
 Order 1. Monotremata. This order includes two primi- 
 tive egg-laying mammals which live in Australia, New Gui- 
 nea, and Tasmania. The skeleton and some other mor- 
 phological features show affinities with birds and reptiles. 
 The eggs hatch in a few hours and the young are at once 
 placed in a hollow in the abdomen which is lined with mam- 
 mary glands. There is no teat and the young lick the 
 milk from the hairy surface of the skin. 
 
 The spiny ant eater, Echidna aculeata, has a long head 
 and a mouth without teeth. Its tongue is very sticky and 
 extensile, serving to capture ants. The duckbill, Orni- 
 thorhynchus anatinus (Fig. 107) is adapted to an aquatic 
 life. It possesses webbed feet, thick fur, and a duck-like 
 
290 
 
 GENERAL ZOOLOGY 
 
 beak with which it catches worms and insects under water. 
 The male has poisonous spurs on the heels of his hind legs. 
 /Order 2. Marsupialia. The marsupials occur chiefly in 
 the region of Australia, where they show considerable diver- 
 sity. The dasyures and Tasmanian devil are carnivorous; 
 the kangaroos (-Fig. 107) and wallabys are vegetarians; the 
 pouched moles burrow underground for insects; the pha- 
 langers live in trees and have prehensile tails. Yet, despite 
 such variations in habits, with corresponding structural 
 adaptations, all marsupials agree in possessing a pouch, 
 
 Fia. 107. Duckbills, and kangaroo with young. 
 
 or marsupium, supported by a pair of marsupial bones 
 which extend forward from the hips. The young are born 
 in a very immature condition, transferred to the pouch, 
 and attach themselves to a teat by means of a sucking 
 mouth. 
 
 The opossums are confined to America, where they are the 
 most important of the marsupial animals. Only one species, 
 the Virginia opossum, Didelphis virginiana, is commonly 
 found in the United States, chiefly in the south and middle 
 west. This animal usually sleeps during the day and comes 
 
MAMMALIA 291 
 
 out at night to hunt for insects, berries, nuts, small birds, 
 mammals, eggs, etc. Two or three litters of four to six are 
 produced annually. The young remain with the mother 
 for a couple of months; at first in the pouch, then clinging 
 to her back. 
 
 Order 3. Edentata. As the name of this order indi- 
 cates, the animals included are without teeth. The 
 sloths, armadillos, and the American ant eaters are com- 
 mon representatives. The only species entering the United 
 States is the nine-banded armadillo which occurs along the 
 Mexican border. This interesting animal bears four young 
 in each litter which are always of the same sex and result 
 from the fragmentation of a single embryo. Such multipli- 
 cation of the individuals from a single fertilized egg is 
 known as polyembryony. 
 
 Order 4. Insectivora. The insectivores are all small 
 in size and live for the most part ^n or in the ground. The 
 moles and shrews are common representatives in North 
 America. The moles (Family Talpidce) are stout, with 
 powerful forefeet suited for digging, rudimentary eyes, 
 and no external ears. The common mole, Scalops aquations. 
 burrows just below the surface of the soil and, though it 
 sometimes disfigures lawns, does considerable good by 
 destroying insects. The shrews are tiny, shy, mouse- 
 like creatures with pointed heads. Some species are among 
 the smallest of mammals. Their food consists of insects, 
 snails, worms, and other suitable objects. 
 
 Order 5. Chiroptera. The bats belong here. The fly- 
 ing squirrels, and a few other hairy animals are able to sail 
 snort distances, but the bats are the only mammals which 
 really fly. The digits on the fore limbs are spread out to 
 support the thin wing membranes, the breast bone has a 
 keel for the attachment of wing muscles, and there are 
 other adaptations for aerial locomotion. Bats have re- 
 markable ability to avoid obstacles while on the wing. An 
 individual in which the eyes have been destroyed is able 
 to fly about in a room crossed by a number of strings with- 
 
292 GENERAL ZOOLOGY 
 
 out touching anything. Young bats cling to their mother 
 and are carried about until able to take care of themselves. 
 
 Bats usually sleep during the day, suspended in some 
 tree, cave, or belfry, and come out at night to seek food. 
 The common bats in the United States are all insectivorous, 
 but certain large, tropical species live on fruit and, in South 
 America, the vampires bite sleeping animals and feed on 
 their blood. 
 
 Order 6. Carnivora. The carnivores may be terres- 
 trial, arboreal or aquatic in their habits, but all have sharp 
 teeth with prominent canines, and poorly developed 
 clavicles. This order contains many animals of great 
 interest to man cats, dogs, fur-bearers, etc. It is usually 
 subdivided into two suborders: (1) Fissipedia, including 
 chiefly terrestrial carnivores; and (2) Pinnipedia, to which 
 the seals, walruses, and sea-lions belong. There are a 
 number of important families of carnivores in North 
 America. The Canidce walk on their toes (digitigrade) 
 and have non-retractile claws. The dogs, foxes, wolves, 
 and coyotes belong to this family. The raccoons (Pro- 
 cyonidce) and bears (Ursidce) walk on the entire foot 
 (i.e., are plantigrade). The martens (Mustelidce) are 
 widespread in North America; the forty-six species includ- 
 ing the mink, weasel, marten, wolverine, skunk, badger, 
 and otter. Many aTe greatly valued for their fur. The 
 family Felidce contains the carnivores with retractile 
 claws cat, puma, leopard, tiger, jaguar, etc. The aquatic 
 carnivores (Pinnipedia) are greatly modified structurally 
 for life in the water. The eared seals (Otariidce), walruses 
 (Odobcenidce) , and the earless seals (Phocidce) all have 
 representatives on the shores of America. 
 
 Order 7. Rodentia. Rodents possess strong incisor 
 teeth fitted for gnawing and lack canines. They are usually 
 of small or of moderate size, the jack-rabbit and beaver 
 being among the largest known. In number of species 
 this is the largest order of mammals, over fourteen hundred 
 being included. Important representatives in North 
 
MAMMALIA 293 
 
 America are the rabbits, hares (Leporidce) ; squirrels, 
 prairie-dogs, woodchucks, chipmunks, ground-squirrels, 
 flying squirrels (Sciuridce) ', beavers (Castoridce)-, pocket- 
 gophers (Geomidce) ; rats, mice, voles, muskrats (Muridce) ; 
 and porcupines (Ccendidce). The beavers are unique 
 among rodents in possessing a flat scaly tail which they 
 use for swimming. The porcupine's quills are peculiar 
 structures, which are to be looked upon as highly modified 
 hairs. They must be touched to do injury and cannot 
 be shot out as is sometimes believed. Rats have recently 
 been the subject of special interest because of the discovery 
 that their fleas commonly carry bubonic plague. 
 
 Order 8. Pholidota. The scaly ant eaters, or pan- 
 golins, are peculiar mammals inhabiting Africa and parts 
 of Asia. Their bodies are covered with flat scales and they 
 roll themselves up into balls when molested, like armadillos. 
 
 Order 9. Primates. This order includes the lemurs, 
 monkeys, apes, and man. Its representatives are mostly 
 found in the warmer parts of the earth and usually live 
 in trees. They are well adapted to arboreal life, usually 
 possessing thumbs and great toes opposable to the other 
 digits so that the hands and feet are admirably fitted for 
 grasping. Most primates are somewhat social in their 
 habits and usually go about in small bands. One young 
 is usually born at a time and it is attended with great 
 care. 
 
 The lemurs (Lemuridce) are dog-like arboreal animals, 
 mostly confined to the island of Madagascar. They usually 
 have a long non-prehensile tail ^and the toes bear both 
 claws and flattened nails. The marmosets (Halpalidce) 
 are found in Central and South America. Their great toes 
 bear flat nails but the others have claws; the tail and ears 
 are long; the thumb is not opposable to the other digits; 
 the brain is rather large; and the space between the nostrils 
 wide. The South American monkeys (Cebidce) have flat 
 nails on all the digits and can oppose both the thumb and 
 great toe; the tail is usually long and prehensile; and there 
 
294 GENERAL ZOOLOGY 
 
 is a wide space between the nostrils. The old world 
 monkeys (Cercopithecidce) as a rule have long tails which 
 are never prehensile; their buttocks are often covered with 
 thick callosities which are usually bright-colored; the 
 nostrils are close together. Many of these apes have cheek 
 pouches for carrying food. They are found in Africa and 
 Asia. 
 
 The anthropoid, or man-like apes (Simiidce), spend most 
 of their time in trees, and do not walk on the palm of the 
 hand when on the ground like most monkeys, but stand 
 more or less erect on the hind legs or walk in a stooping 
 position resting partly on the backs of the hands. The 
 tail is absent and the arms are longer than the legs. There 
 are four genera of anthropoids which include the gibbons 
 (Hylobates), orang-utan (Pongo), gorilla (Gorilla), and 
 chimpanzee (Anthropopithecus) . The gibbous are about 
 three feet high and, though possessing very long arms, do 
 in? t assist themselves with the hands when walking. They 
 are found in Eastern Asia. The orang-utans live in Borneo 
 and Sumatra. They use the knuckles of the hands in 
 walking, and build platforms of sticks in trees. The gorilla 
 is a native of West Africa, where it lives in trees and feeds 
 chiefly on vegetation. It may reach a height of five and 
 a half feet and weigh five hundred pounds. The chim- 
 panzee lives in the same region but is smaller than the 
 gorilla. It is more like man than any other living mammal. 
 It is easily tamed, and has often been trained to perform 
 various simple tasks. 
 
 The family Hominidce includes only one species, Homo 
 sapiens, or man. This is distinguished from other man- 
 like apes by the erect walk, power of articulate speech, and 
 marked ability to reason. There are three great races of 
 men: (1) the Negroid, with dark skin, curly hair, flat nose, 
 thick lips, prominent eyes, and large teeth; (2) the Mon- 
 golian, with black straight hair, yellowish skin, broad face 
 with prominent cheek bones, a small nose, sunken, narrow 
 eyes, and teeth of moderate size; and (3) the Caucasian, 
 
MAMMALIA 295 
 
 having soft straight hair, vigorous beard, retreating cheek 
 bones, narrow prominent nose, and small teeth. 
 
 Order 10. Artiodactyla. This large order includes the 
 hoofed animals with an even number of toes. There are 
 many that are of economic importance, seFving-^as- fur or 
 food for man and as his domestic animals. The pigs, 
 peccaries, and hippopotami do not chew a cud, but rumi- 
 nants (camel, deer, giraffe, prong-horn antelopes, and cattle) 
 crop herbage hastily without masticating it enough for 
 digestion and regurgitate it into the mouth later for 
 thorough chewing. A typical ruminant has the stomach 
 divided into four parts, the functions of which are corre- 
 lated with the peculiar habits of feeding. The deer have 
 solid horns which are shed annually ^but the cattle have 
 horns with a vascular bony core and a hard outer sheath.. 
 The prong-horn antelope of Western North America is 
 unique, having horns like cattle but shedding them annu- 
 ally. The descendants of the British Bos taurus are the 
 commonest of the domestic cattle, though the water 
 buffalo, yak, and others are important in certain districts. 
 
 Order 11. Perissodactyla. The odd-toed hoofed ani- 
 mals include the horses (Equidce), tapirs, and rhinoceroses. 
 Though none of the Equidse were found in America when 
 Columbus discovered it, most of the past evolution of the 
 family took place in the United States. The horses show 
 extreme reduction of the pentadactyl limb ; only the middle 
 toe is functional and the terminal hoof represents the toe- 
 nail. At the present time there are over sixty varieties of 
 domesticated horses, all belonging to one species, Equus ca- 
 ballus. The asses, zebras, and quaggas still run wild in 
 Africa and Asia, but several species have been tamed. 
 
 Order 12. Proboscidia. The two living spebies of ele- 
 phants inhabit Africa and India* respectively. Both have 
 five toes on all the feet, the characteristic elongated trun^: 
 with the nostrils opening at its tip, and a thick loose skin 
 (whence the name, " pachyderm"). Elephants have no 
 canine teeth; the tusks are elongated incisors. In rather 
 
296 GENERAL ZOOLOGY 
 
 recent geological times certain fossil elephants, like the 
 mastodon and mammoth, lived in the Northern Hemi- 
 sphere. 
 
 Order 13. Sirenia. The manitees and dugongs are 
 large aquatic animals which live in shallow waters along 
 the sea shore and in the mouths of rivers. These great 
 animals live on vegetation and the bones are very heavy so 
 that they may remain on the bottom without effort. Their 
 limbs are modified to form flippers which, with the tail, 
 serve for swimming. 
 
 Order 14. Cetacea. There are two kinds of whales: 
 (1) those with teeth (Odontoceti) ; and (2) those in which 
 the teeth are never functionally developed, being present 
 in the young but replaced by baleen or " whalebone" in the 
 adult (Mystacoceti). Though whales are fish-like in 
 general form and wholly aquatic in their habits, they are 
 true mammals. The body before birth has a thick coating 
 of hair, and the young are nourished with secretions from 
 the mammary glands. A whale's nostrils open through 
 the " bio whole" on the top of the head, and a great cloud 
 of spray and vapor is spouted from this aperture when it. 
 comes to the surface to breathe. The toothed whales in- 
 clude the dolphins, porpoises, grampuses, killer whales, 
 sperm whales, and narwals. The baleen whales are as a 
 rule of large size, including the great rorquals, fin-whales, 
 hump-backed, and right whales. " Whalebone" hangs 
 from the roof of a whale's mouth in long plates which are 
 frayed out at the free ends. In feeding, water is taken into 
 the mouth and squirted out between the plates, thus 
 straining out many small animals. The sulphur-bottom 
 whale, Balcenoptera sulfureus, is the largest living animal. 
 It may attain a length of ninety-five feet and weigh nearly 
 300,000 pounds. 
 
 Probably the greatest interest which any man has in 
 mammals relates to his own relationships to animals in 
 general. The next two chapters will accordingly be devoted 
 to man as an animal and his place in the animal kingdom. 
 
CHAPTER XXVII 
 
 MAN, Homo Sapiens Linnaeus 
 
 * The proper study of mankind is man." Pope. 
 
 Though zoologists were at first loath to admit their re- 
 lationship to other monkey-like animals and for a time 
 placed man in a distinct order of mammals, they have been 
 forced by truth-seeking science to connect him closely with 
 other primates. All types of men living at the present 
 time are believed to belong to a single species, Homo sapiens, 
 which is the only one in the family Hominidce. The 
 various races which are united in this species differ some- 
 what from each other but also have much in common. An 
 individual from any one race is fertile in breeding with a 
 representative from another, and there are no fundamental 
 structural differences. Man must be looked upon as an 
 animal belonging to the primates and showing closer simi- 
 larities to his nearest relatives (chimpanzee, gorilla, gibbon, 
 orang-utan) than they do to other monkeys. 
 
 Like all other animals, man's body is a machine which is 
 controlled by such structures as levers, pulleys, muscles, 
 sense organs, and nerves all of which conform in their 
 operation to the laws of chemistry and physics. His body 
 is made up of cells which are like those of other animals; it 
 originates by growth and cell-division from a single cell 
 the fertilized egg; and its cells show differentiation into 
 tissues, corresponding to a similar division of labor in other 
 vertebrates. There are many who believe that man differs 
 from all other animals in possessing a soul, or spirit, which 
 makes him unique. At the present time, science cannot 
 state definitely that man, or any other animal, possesses or 
 lacks a "soul," though there is often heated argument 
 with eminent scientific men on both sides. But this 
 
 297 
 
298 GENERAL ZOOLOGY 
 
 question does not properly come within the scope of this 
 book and our attention will be directed to the more strictly 
 zoological aspects of man's activities. 
 
 Self-maintenance. Judging by the structure of the 
 teeth and other parts concerned with nutrition, man is 
 fitted to live on a diet consisting of fruits, herbs, and flesh. 
 Savages often eat much of their food raw and devour 
 things which have little appeal to civilized tastes. The 
 American Indian revels in dog feasts; the primitive 
 Australians esteem the luscious caterpillar as a great 
 delicacy. Each race has certain food customs which are 
 adhered to more or less strictly. The Italian loves his 
 spaghetti; the Irishman relishes potatoes; the Australian 
 prays to his gods that caterpillars may be abundant; the 
 Hawaiian subsists largely on crabs and fish. 
 
 Though a man's body may be maintained for a con- 
 siderable time by eating nothing but protein food, it thrives 
 best on a varied diet including proteins, carbohydrates and 
 fats.* Men forced to live without vegetables often have 
 the scurvy or other similar diseases due to improper nutri- 
 tion. Recently it has been discovered that certain foods 
 contain very small quantities of substances (so-called 
 vitamines, etc.) which play a very important role in nutri- 
 tion, though they furnish very little actual building material 
 or energy. For example, people living largely on polished 
 rice may be attacked by the disease known as beri-beri ; 
 whereas those eating the same proportion of rice which has 
 the outer brown covering are without such trouble. In 
 the fat of butter there are minute quantities of another 
 substance which stimulates growth. To have the greatest 
 value, proteins, carbohydrates, and fats should occur in 
 fairly definite proportions. The daily requirements for 
 an average man as estimated by Atwater are as follows: 
 
 Protein 125 grammes (4.41 oz.) 
 
 Fats 125 grammes (4.41 oz.) 
 
 Carbohydrates 400 grammes (14.11 oz.) 
 
 * For definitions of these substances, see page 30. 
 
MAN 299 
 
 The protein is used primarily for building body substance, 
 and nitrogen is its most important chemical element. 
 Carbohydrates and fats furnish energy to carry on the work 
 necessary for the activity of the body, and their important 
 element is carbon. If the daily ration is poor in protein, 
 carbohydrates and fats cannot make up the deficiency; 
 if it is lacking in carbonaceous substances, the proteins 
 must be broken down in excess to release energy. In 
 addition to organic foods, water and particular mineral 
 salts are of course necessary. 
 
 Lean Beef 
 L ean smoked ham 
 Bacon smoked 
 Fresh codfish 
 
 Milk 
 
 Buffer 
 
 Oatmeal 
 
 Rice 
 
 IVhite bread 
 
 Dried beans 
 
 String beans 
 
 Cabbage 
 
 Green corn 
 
 Potatoes 
 
 L e/Tuce 
 
 Tomatoes 
 
 Apples 
 
 Almonds 
 
 Chestnuts 
 
 Peanuts 
 
 \SMHProtein I Varhohydrate 
 
 FIG. 108. Constituents of common foods. (From data in Sherman's Chemistry 
 of Food and Nutrition.) 
 
 Man is well-equipped to seek and capture food. A 
 savage depends mostly upon his sight and hearing to dis- 
 cover suitable plants or animals, though the senses of 
 touch, taste, and smell are often employed. The organs 
 of smell are somewhat degenerate and far less effective 
 than in many other mammals, such as the dog or deer. 
 The pos'tion of the eyes compensates for this deficiency, 
 however, by giving man the advantage of binocular vision 
 which permits accurate judgment of distances to objects. 
 The legs are well-suited for pursuing prey, and in some 
 primitive races (Igorrotes) are used also for holding objects. 
 
300 GENERAL ZOOLOGY 
 
 The hands are probably the best organs for seizing and 
 holding that any animal possesses. They are equally 
 effective for strangling a rabbit, holding a spear, or grasp- 
 ing a hoe, and the flat nails make it possible to pick up 
 minute objects. The instinct for survival is strong in 
 man and impels him to keep his body in training so that 
 he may not fail in the chase. His superior mentality gives 
 him a great advantage over other animals in .securing food 
 and he has been able to invent many means to insure a 
 continuous supply. 
 
 Food, once in the hand, is placed in the mouth and goes 
 the usual course being chewed, swallowed, mixed, digested 
 and absorbed. In the mouth food is mixed with saliva 
 which contains the ferment ptyalin, capable of changing 
 starch to sugar. As soon as the chewed food enters the 
 stomach, gastric juice begins to pour out, but since the 
 food remains in the upper part of the stomach for a time, 
 ptyalin continues to act for about half an hour. Later it 
 is neutralized by the free hydrochloric acid in the stomach. 
 In addition to its digestive functions the acid acts as an 
 antiseptic, killing bacteria and rendering certain other 
 injurious substances innocuous. Gastric juice contains a 
 protein-digesting ferment (pepsin) which liquefies the food 
 by dissolving the nitrogenous substances. Most of the 
 mixing and churning is done near the muscular outlet of 
 the stomach and the liquefied food is gradually permitted 
 to pass the sphincter* muscle which guards this opening. 
 In the intestine muscular mixing movements continue and 
 secretions from two great digestive glands (liver, pancreas) 
 are mixed with the liquefied food. Pancreatic juice con- 
 tains three powerful digestive ferments: (1) trypsin, for 
 digesting proteins; (2) amylopsin, converting starches to 
 sugars; and (3) steapsin, which breaks up and emulsifies 
 fats. In the mouth the saliva has a slightly alkaline reac- 
 tion. In the stomach the food becomes acid; is slightly 
 alkaline or neutral after entering the intestine, but later 
 
 * A sphincter is a ring-like mass of muscle which surrounds an opening. 
 
MAN 301 
 
 becomes acid again. The liquefiable nutriment is gradually 
 extracted from the food and the residue of undigested 
 material is finally eliminated through the anus. In a 
 healthy man digestion is usually influenced by numerous 
 bacteria which live in the alimentary canal. Some in- 
 vestigators assert that these unsuspected guests are 
 absolutely indispensable for proper digestion. 
 
 The absorption of food is selective not simply a soaking 
 of liquids through the lining of the alimentary canal. .If 
 fresh blood is placed in the intestine it is not transferred 
 unchanged to the adjacent blood-vessels but goes the usual 
 long round of digestion, absorption, and assimilation. The 
 proteins are mostly changed to amino acids and usually 
 pass through the lining of the intestine into the blood-ves- 
 sels; the fats are emulsified, broken up into fatty acids 
 and glycerin and taken up by the lac teals;* the carbo- 
 hydrates for the most part pass as monosaccharids in 
 the blood to the liver, where they are stored as glycogen 
 or transferred to the tissues to be burned for energy 
 production. 
 
 Most of the activities concerned with digestion are 
 under the control of the nervous system, but usually take 
 place without the knowledge of the person who has eaten. 
 Vomiting is a safeguard for rejecting poisonous or unde- 
 sirable substances; headache may be an indication of an 
 irritated stomach. Yet many of the digestive and assimi- 
 lative processes are controlled by specific chemical sub- 
 stances, which may not even enter the alimentary canal. 
 The pancreas not only manufactures a powerful digestive 
 secretion which it pours into the intestine, but also acts 
 as a ductless gland which is of great importance in con- 
 nection with the utilization of sugar from the blood. When 
 it is diseased the metabolism of carbohydrates is accord- 
 ingly disturbed. Other ductless glands (thyroid, thymus, 
 adrenals, gonads, etc.) all have more or less influence on 
 
 * Lacteals are branches of the lymphatic system which form a network 
 about the digestive organs. 
 
302 GENERAL ZOOLOGY 
 
 metabolic processes, though they take no direct part in 
 the digestion of food. 
 
 The circulatory system of man is a highway for the 
 transfer of food, waste products, gases, and the leucocytes 
 which destroy undesirable materials. The food in the 
 liquid portion of the blood is largely protein. Carbohy- 
 drates and fats are stored in convenient places within the 
 body and enter the blood in small quantities as needed. 
 The red corpuscles carry oxygen from the lungs to the 
 tissues, and the blood returns to the lungs laden with carbon 
 dioxide. The heart pulsates rhythmically for the entire 
 life of a man, without rest, except what it snatches between 
 beats. It forces blood through the arteries to all parts 
 of the body, and is continually filled from the veins. 
 Though the rhythm of pulsation is under the control of the 
 nervous system, the tendency to beat is inherent in certain 
 portions of heart muscle and movements may be induced 
 or modified by the presence of certain salts in the blood or 
 by other stimuli. Through the delicate walls of the capil- 
 laries, which connect the arteries and veins, food and 
 oxygen are supplied to the tissues and waste material 
 enter the blood. Though the amount of blood passing 
 through the heart may be the same at different times, the 
 vasomotor nerves vary the diameter of the small vessels 
 so that different parts of the body have more or less blood 
 as their necessities demand. 
 
 The respiratory system is concerned primarily with the 
 supplying of oxygen to the tissues and the elimination of 
 carbon dioxide. Air is drawn into the lungs w 7 hen the space 
 in the thoracic cavity is increased by the contraction and 
 descent of the diaphragm together with the raising of the 
 ribs by the shortening of the muscles between them. 
 Four hundred and forty liters (888 gallons) of blood pass 
 through the lungs each day. To aerate this an adult man 
 requires about 85,000 liters (3000 cubic feet) of air per hour. 
 The atmosphere may contain 1 to 2 per cent, of carbon 
 
MAN 303 
 
 dioxide and still be fit for breathing, but a greater amount 
 is injurious. 
 
 The excretions formed as a result of metabolism are 
 chiefly eliminated through the kidneys, lungs, and skin. 
 The kidneys discharge the urea, formed as one of the end 
 products when proteins break down. Though urea is 
 excreted through the kidneys, it is formed for the most 
 part in the liver and transferred in the blood. Water passes 
 out through all three of the excretory channels; carbon 
 dioxide is eliminated largely through the lungs, but some 
 goes through the skin. All the excretory organs are con- 
 trolled and coordinated by the nervous system. For 
 example, in cool weather there is less water passing out 
 through the skin and more through the kidneys. 
 
 The activities of the human body are more or less rhyth- 
 mical. The day is usually a time of activity and the body 
 is used up to some extent in doing work; night is a period 
 of rest and losses are made good during sleep. Sleep is a 
 curious phenomenon sense organs which are at other 
 times quick to receive stimuli become inactive and all 
 extracorporal activities cease. There have been a number 
 of theories to explain sleep and the three following may be 
 mentioned: (1) Sleep may be caused by the using up of sub- 
 stances necessary for nervous activity; (2) it may be due 
 to the accumulation of waste products which cause the 
 enlargement of small blood-vessels throughout the body 
 with a resulting scarcity of blood in the brain (though such 
 changes do occur, they probably do not cause sleep); (3) 
 sleep may be due to the contraction of nerve branches so 
 that those of different cells are no longer in contact thus 
 the paths are broken along which nervous impulses travel 
 when the body is awake. Whatever its cause, sleep is 
 necessary for all warm-blooded animals, and the human 
 machine is most efficient when periods of rest and activity 
 alternate with reasonable regularity. 
 
 Self-protection. The instinct for self-protection is as 
 strong in man as in other animals and the body has many 
 
304 GENERAL ZOOLOGY 
 
 natural adaptations for protection. Even its cells and 
 fluids have properties which effectively shield them from 
 dangers likely to be encountered. The skin usually keeps 
 out undesirable materials. If it is broken, however, the 
 clotting of the blood forms an effective plug which checks 
 contamination from the outside. If bacteria or poisons 
 do gain an entrance, they are eaten up by the leucocytes 
 (which swarm through the blood channels to repel invaders), 
 destroyed by antitoxins, or oxidized. Exposed parts of 
 the body are especially protected by heavy growths of hair 
 or callouses. Civilized man has devised clothing, houses, 
 and other protective devices to assist his natural defences. 
 Man is also one of the favored animals which have a con- 
 stant body temperature, maintained by delicate adjust- 
 ments and coordinations of metabolic activities, vaso- 
 motor nerves, and perspiratory glands. He is thus able 
 to survive critical periods of climatic stress, when a cold- 
 blooded animal would die. Man is also endowed with 
 discriminating courage, reasonable fear, and a great degree 
 of resourcefulness, so that he may fight, flee, or escape 
 danger by using his wits. 
 
 Yet, despite all man's versatility in combating dangers, 
 he is continually subject to minor disorders and is often 
 killed outright by accident or disease. There are many 
 chances for the occurrence of defects in the bodily machine. 
 (1) A man may be born with some defect which cannot be 
 corrected hunchback is an example of such an affliction. 
 Among the most pathetic of such cases are those in which 
 the nervous system is defective. There is a malady known 
 as Little's disease in which the body and mind may be 
 perfect, but the nervous connections (pyramidal tracts) 
 never grow down completely from the brain, as they should 
 do about the time of birth. The bodily movements, 
 therefore, are not coordinated because messages cannot be 
 properly carried from the brain. A bright mind may thus 
 wear itself out in a helpless but perfect body over which 
 it has little or no control, and which ultimately dies from 
 
MAN 305 
 
 lack of exercise. This disease is due to nothing but a 
 lack of development and cannot be treated in any way. 
 (2) A defect may be acquired through accident. Man has 
 small power of regeneration when compared with many 
 other vertebrates. A salamander can replace a lost leg 
 but a man cannoc grow a single joint of a finger. Specializa- 
 tion has gone so far that the tissues remaining can at best 
 only close the wound. Accidents which maim or impair 
 the nervous system are most serious; a sudden shock or 
 strain may result in paralysis, insanity, or other perma- 
 nent disorders. (3) The machine may be without seri- 
 ous structural defects but fail to operate properly. The 
 digestive fluids may be too acid; excretions may not be 
 properly eliminated and poison the tissues; the mind may 
 not form sound deductions from observed phenomena. 
 (4) Many occupations are conducive to specific diseases 
 the Indian leads an active life with much exposure and has 
 his old age made unpleasant by rheumatism; the coal miner 
 has his lungs impaired by dust and bad air, hence often 
 acquires tuberculosis; the sedentary business man is dogged 
 by insomnia and indigestion. (5) Clothing may be a 
 source of diseases improper shoes often cause permanently 
 deformed feet; corsets may cause serious disturbances in 
 the digestive, circulatory, or reproductive organs. 
 
 All the disorders mentioned up to this time are due to 
 some defect in man's structural mechanism or in its opera- 
 tion, but there is another class of diseases (6) which are 
 due to plant or animal parasites which actively invade the 
 body. The chief avenues for the entrance for such un- 
 desirable organisms are through the natural openings in 
 the skin, though some parasites enter in other ways (like 
 the tetanus bacillus which gains access through cuts, and 
 malaria which is injected hypodermically by mosquitoes). 
 Such diseases as cholera, typhoid, trichinosis, and tape- 
 worms, enter through the mouth with the food ; diphtheria, 
 infantile paralysis, tuberculosis, and some other diseases 
 are usually spread through nasal discharges; the eggs of 
 
306 GENERAL ZOOLOGY 
 
 certain parasitic worms pass out in the urine and hatch 
 out larvae which bore through the skin; some diseases 
 (syphilis, gonorrhoea) are transmitted through the contact 
 of mucous membranes with those of infected persons and 
 are usually acquired by kissing or through sexual relations. 
 Small wonder, then, that with so many opportunities to 
 acquire diseases, measures are generally taken to combat 
 them. 
 
 Modern medicine attacks disease along three general 
 lines. (1) Preventive medicine educates the public. It 
 spreads knowledge as to the nature of prevalent diseases 
 and tries to lessen chances for their dissemination by im- 
 proving sanitation, conditions of work, and diet. (2) 
 Corrective measures attempt to help the body to work 
 properly. Operative surgery adjusts structural defects; 
 physic retains water in the undigested food and stimulates 
 the flow of water into the intestine; strychnine in small 
 quantities sometimes serves as a tonic to a fagged-out 
 nervous system and allows it to recover its normal efficiency; 
 a change in diet may alleviate some specific irritation ; lenses 
 may correct defective vision. (3) The killing or preven- 
 tion of internal parasites is accomplished in various ways. 
 Quinine is effective in poisoning malarial parasites in the 
 blood corpuscles; thymol given judiciously will kill hook- 
 worms; the salts of mercury are injurious to the organisms 
 causing syphilis. The human body has within itself certain 
 reactions which are taken advantage of in preventing dis- 
 ease. Immunity to diseases may be natural or acquired. 
 Most persons will not grow the virus of infantile paralysis 
 in their bodies, thus showing a natural immunity. A person 
 who has been sick with smallpox will not contract the dis- 
 ease again; he has acquired immunity because his body has 
 formed antitoxins which prevent the growth of smallpox 
 organisms. Modern medicine has made it possible to 
 acquire immunity to certain diseases by contracting them 
 in a mild form through vaccination. Under such circum- 
 stances the body during light attacks of disease develops 
 
MAN 307 
 
 antitoxins which prevent the entrance of more serious 
 infections. 
 
 Frequently the most difficult task of the physician is to 
 tell what is the matter with his patient. The human body 
 is such a complicated mechanism that symptoms may 
 come by very circuitous routes and be difficult of diagnosis. 
 Doctors are often blamed for not giving proper treatment 
 when they have been obliged to choose from two or three 
 possibilities. It is unfortunate that ability to diagnose 
 correctly is usually difficult to acquire and comes only with 
 wide knowledge and long experience, while the mere 
 treatment of most diseases is comparatively easy. 
 
 Race Preservation. Herter* says: "the sex instinct is 
 the only human instinct that can be compared with the 
 instinct of self-preservation in respect to the profundity 
 of its influence on the conduct of man." The activities 
 associated with mating, loving, and the rearing of offspring 
 dominate the life of every normal adult individual. Though 
 sex-determination perhaps depends only upon the presence 
 or absence of an accessory chromosome in the sperm cell 
 which fertilizes the egg and thus initiates a new human 
 being, the two sexes show fundamental differences in their 
 instincts. 
 
 A man is larger and stronger than a woman. His 
 secondary sex characters (beard, deep voice, great strength, 
 and more aggressive disposition) fit him to hunt, fight, or 
 endure other privations necessary for the maintenance 
 of a family. Woman, with her soft voice, mammary glands, 
 and disposition to foster and cherish, is anchored to the 
 central interest of family, forcing her to a relatively sessile 
 life. The structural differences between the sexes are 
 associated with characteristic tastes and habits. "The 
 essential differences between the mental life of woman 
 and that of man apparently depends upon the fact that the 
 cerebral organization represents and reflects the various 
 aspects of the sexual functions, which irradiate as it were, 
 
 * "Biological Aspects of Human Problems," N. Y., 1911. 
 
308 GENERAL ZOOLOGY 
 
 into the brain. The circumstances that the body of the 
 mother nourishes both the embryo and the infant brings 
 her into an organic relationship to family different to. the 
 casual relationship of the male.' 7 * 
 
 In the choosing of a mate the initiative is taken by the 
 man. The primitive native of New Guinea adopts the 
 simple expedient of killing the man who already possesses 
 the woman he wants. In China and Turkey women are 
 sold to men who may desire them. If men are to do the 
 choosing, women must be attractive in order to be selected, 
 and the gentler sex has developed charming qualities, both 
 natural and artificial. The New Guinea woman dresses 
 her hair' by rolling strands in bright-colored clays in order 
 
 / 
 
 FIG. 109. The races of men Mongolian, Caucasian, Negroid. 
 
 that she may be more beautiful. Civilized women show 
 the same instinct in their elaborate dresses, bright hats, 
 ear rings, necklaces, and other ornaments. Forel, the 
 eminent Swiss scientist, believed that many of the basic 
 psychological processes of the human mind had their origin 
 in the desire of the two sexes to appear well in each other's 
 eyes. 
 
 A human being follows the same course of development 
 as other metazoans. In a mature woman an egg cell leaves 
 the ovary and starts down the oviduct about every four 
 weeks. If fertilization does not occur within about two 
 weeks, the egg cell passes out and is later succeeded by 
 another. If a sperm cell unites with an egg cell, the result- 
 ing zygote adheres to the mucous lining of the uterus and 
 development begins. After the usual cleavage stages 
 (Fig. 64, page 152) a little streak (the primitive streak) 
 
 * "Biological Aspects of Human Problems," N. Y., 1911. 
 
MAN 309 
 
 appears on one side of the germ, and later a groove, which 
 is the beginning of the nervous system, forms down its 
 center. The germ layers (ectoderm, entoderm, mesoderm) 
 are soon formed, the enteron and notochord take form, 
 and at the end of the third week the heart of the embryo 
 begins to pulsate (Fig. 115). 
 
 The human embryo has unusual opportunities for de- 
 velopment. It is nourished for nine months in the uterus 
 of its mother from her blood and rests in watery fluids 
 surrounded by protective membranes (Fig. 106, page 288). 
 The fully formed embryo, or fcetus, does not get blood 
 directly from its mother. There is a transfer of nourish- 
 ment and oxygen through thin membranes in the placenta. 
 The blood in the foetal circulation is very poor in oxygen 
 and the fcetus is remarkably resistant to asphyxiation. 
 This is a valuable adaptation for it prevents suffocation at 
 the time of birth when the circulation from placenta to 
 child is often cut off completely for some time. While 
 within the mother the blood of the foetus passes through 
 the foramen ovale, an opening which makes a short cut 
 between the two sides of the heart. There are other short 
 routes so that most of the oxygen in the blood goes to the 
 liver, heart, and head. At birth these channels close and 
 there are fundamental changes in circulation and respira- 
 tion. Sometimes some of them fail to close properly and 
 a "blue baby" or other defective, which usually dies soon, 
 results. 
 
 During the growth of the child the mammary glands of 
 the mother have been changing and soon after birth they 
 begin to secrete milk. This is a true glandular secretion 
 and not merely "filtered blood." A child, then, like other 
 mammals, has unusually advantageous conditions for 
 development in that it may attain an age of nearly two 
 years (after fertilization) without having any nourishment 
 except that supplied by the mother. At about the time 
 of birth important nervous connections are established by 
 the growth of certain tracts of nerves through the brain and 
 
310 GENERAL ZOOLOGY 
 
 spinal cord. A child at birth is rather helpless and does 
 not correlate its movements well, but is nourished, cared 
 for, and protected during this critical period. It also 
 receives education, which even in the most savage races far 
 exceeds that obtained by any other animal. It is taught 
 to walk, talk, get food, care for itself, and to make imple- 
 ments. No wonder that man has been able to dominate 
 the earth. 
 
 The most important structural changes which occur in a 
 growing child after it ceases to be nourished by its mother 
 are in the teeth and reproductive organs. A child usually 
 acquires part of its first set of teeth before it is weaned 
 (six to eighteen months) ; but children are rarely born with 
 all their milk teeth, and others do not acquire any for more 
 than a year. The second set of teeth usually appears 
 between the ages of seven and twelve years, except the 
 " wisdom" teeth which generally break through the gums 
 before the twenty-fifth year but sometimes fail to emerge at 
 all. The reproductive organs become functional in both 
 sexes between thirteen and fifteen ; the voice becomes deeper 
 in the male and there are other accompanying changes. 
 The ovaries cease their activities between the ages of forty 
 and fifty, but the male reproductive organs may remain 
 functional throughout life. 
 
CHAPTER XXVIII 
 
 MAN (Continued) 
 THE MIND OF MAN 
 
 Man excels other animals in his combination of erect 
 attitude, opposable thumb, unusual brain development, 
 and powers of speech. The chief characteristic which has 
 enabled him to outstrip all other competitors in his reason- 
 ing power. This of course has its seat in the nervous sys- 
 tem and is dependent upon growth and cell-division for 
 its full development. The nervous system of man, like 
 that of all vertebrates, is formed by the turning in of a 
 groove which is finally pinched off to form a tube along the 
 dorsal side of the body (Fig. 91). As development pro- 
 ceeds (Fig. 104) tHe wall of the tube thickens and becomes 
 folded in places; long fibers grow out from its nerve cells to 
 all parts of the body and collect in bundles to form nerves. 
 Sense organs develop, usually on or near the outside of the 
 body, and become connected by nerve-cell fibers with the 
 central tube. All the great nervous structure of an adult 
 man has arisen by growth and cell-division from the zygote, 
 formed when an egg cell and spermatozoon fused. 
 
 In its fully formed condition the nervous system consists 
 of three classes of organs: (1) receptors, (2) effectors, and 
 (3) adjusters. According to Herrick there are, instead of 
 "five senses, " about twenty-three kinds of receptor organs* 
 in the human body which receive stimuli from outside or 
 notify the central nervous system of conditions within. 
 
 * Separate receptors are stimulated by: touch and pressure, cold, heat, 
 pain, chemicals, sounds, light, odors, muscle "tonus," tendon "tonus," 
 hunger, thirst, nausea, suffocation, disturbances in circulation, sexual 
 stimuli, distention of cavities (stomach, bladder, etc.), obscure abdominal 
 changes, and tastable substances. 
 
 311 
 
312 
 
 GENERAL ZOOLOGY 
 
 Many of these organs have great range of sensibility the 
 eye can perceive about two hundred pure tints, the average 
 ear is able to discriminate some 11,000 different pitch 
 qualities. Effectors are nerve endings for setting off 
 organs which accomplish some particular work. They 
 activate voluntary muscles, visceral muscles, or glands. 
 The adjusters are groups of nerve cells and their branches, 
 which lie for the most part within the thickened wall of 
 the central nervous tube, or near it. They receive notice 
 of conditions from the receptors, send out stimuli to appro- 
 priate effectors, and make necessary coordinations. A 
 
 neuropore 
 
 cerebr 
 
 um 
 
 cerebrum 
 
 cerede//um< 
 A 0, C 
 
 FIG. 110. Development of the human brain. A, two weeks after fertilization ; 
 B, four weeks; C. during third month of foetal life; D, adult brain. (Largely 
 after Herrick.) 
 
 slight external stimulus may set off a great many effectors 
 and thus cause a violent reaction, or a strong stimulus may 
 be suppressed and cause no response all such matters are 
 arranged by the adjusters. For example, a pioneer may 
 see a human form on the horizon if it is an Indian with a 
 gun, he will run or hide; if it is a member of his family, his 
 nervous system will make no response through effectors to 
 the image received through his eyes. 
 
 Man has many of his important sense organs localized at 
 the anterior end of the body and it is but natural that the 
 great adjusters should be close at hand in the thickenings of 
 the wall of the neural tube which forms the brain. There 
 

 MAN 313 
 
 are two great swellings on the dorsal surface of the brain: 
 the cerebrum, which is the seat of " voluntary" activity, 
 consciousness, memory, and reason; and the cerebellum in 
 which reside many of the centers for controlling the strength 
 and steadiness of muscular activity. There are other ad- 
 justor groups of nerve cells in the brain, spinal cord, and 
 among the internal organs. These for the most part con- 
 trol simple reflexes in organs near them. 
 
 The nervous activities of man are remarkable in two re- 
 spects: (1) a vast number of practically unvarying and 
 more or less automatic reflexes (which greatly exceeds that 
 of other animals, except perhaps birds) is controlled by the 
 nerve centers in certain regions of the brain outside the 
 cerebrum and in the spinal cord; those concerned with the 
 vital functions of the internal organs being dominated by 
 certain parts of the brain (medulla, etc.), and those con- 
 cerned with instinctive reactions of the skeletal muscles 
 being largely controlled in the cord. (2) The higher men- 
 tal faculties concerned with variable activities are developed 
 as in no other animal. The enormous system of branching 
 nerve cells in the outside layer of the cerebrum makes pos- 
 sible, not only the proper adjustment of the body to condi- 
 tions as reported by effectors, but the storing of sensory 
 memories upon which knowledge and the power of reason- 
 ing depend. 
 
 At birth a child has the main tracts in its nervous system 
 formed, but is able during youth to strengthen or con- 
 trol its natural endowment by training. It may be natur- 
 ally " bright" in solving mathematical problems, or "dull" 
 in languages; awkward or graceful; docile or stubborn. 
 Youth is the golden time for improvement, for there comes 
 a day when the fiber tracts are no longer modifiable. As 
 Herrick puts it, "the docile period is past, and though the 
 man may continue to improve in the technic of his per- 
 formance, he can no longer do creative work. Whether 
 this process occurs at the age of twenty or eighty years, it 
 is the beginning of senility. And, alas, this coagulation of 
 
314 GENERAL ZOOLOGY 
 
 mental powers often takes place so early. Many a boy's 
 brains are curdled and squeezed into traditional artificial 
 moulds before he leaves the grades at school. We who seek 
 to enter into the kingdom of knowledge and to continue to 
 advance therein must not only become as little children, 
 but we must learn to continue so." 
 
 ORIGIN OF MAN 
 
 According to Osborn* man probably arose during the 
 Pliocene Period before the Glacial Epoch. At that time 
 there were no men in Europe, but the remains of a very 
 primitive ape-man (Pithecanthropus erectus) have been dis- 
 covered in Java associated with the porcupine, rhinoceros, 
 extinct species of elephants, and other animals which lived 
 in the Pliocene Period. Java was at that time a part of 
 the Asiatic continent and the primitive men were therefore 
 free to migrate north and west, as they apparently did. 
 The absence of hair in man indicates that he probably had 
 his later evolution in a hot country. Osborn says: "It is 
 possible that within the next decade one or more of the 
 Tertiary ancestors of man may be discovered in northern 
 India among the foot-hills known as Siwaliks. Such dis- 
 coveries have been heralded, but none have thus far actu- 
 ally been made. Yet Asia will probably prove to be the 
 center of the human race. We have now discovered in 
 southern Asia primitive representatives or relatives of the 
 four existing types of anthropoid apes, namely, the gibbon, 
 the orang, the chimpanzee, and the gorilla, and since the 
 extinct Indian apes are related to those of Africa and of 
 Europe, it appears probable that southern Asia is near the 
 center of the evolution of the higher primates and that we 
 may look there for the ancestors not only of prehuman 
 stages like the Trinil race but of the higher and truly hu- 
 man types." 
 
 Through recent scientific work the history of man in 
 
 * OSBORN, H. F.: "Men of the Old Stone Age." 1915. 
 
MAN 
 
 315 
 
 Europe is pretty well known. At different times various 
 races came in from the southeast during the interglacial 
 
 EXISTING 
 APES AND 
 MAN. 
 
 GIBBON. 
 Asia. 
 
 GLACIAL OR 
 PLEISTOCENE 
 AGE. 
 
 PLIOCENE 
 AGE. 
 
 MIOCENE 
 AGE. 
 
 OLIGOCENE. 
 
 Primitive Gib- 
 bon of Eu- 
 rope 
 
 (Pliohylobates). 
 
 Earliest 'Gibbons 
 
 of Europe 
 (Pliopithecus). 
 
 MAN 
 
 (Homo sapiens). 
 Asia, Europe. 
 
 Crd-Magnon and 
 other races. 
 
 More primitive spe- 
 cies, human and 
 prehuman. 
 
 Neanderthal race. 
 Piltdown race. 
 Heidelberg race. 
 
 Trinil race 
 (Pithecanthropus) . 
 
 Unknown Pliocene 
 ancestors of man. 
 
 CHIMPANZEE. 
 Africa. 
 
 Ancestral anthro- 
 poids of Asia 
 
 Ancestral anthro- 
 poids of Egypt 
 (Propliopithecus) . 
 
 Primitive anthropoids 
 of Asia and Europe. 
 
 Small monkeys 
 of Egypt. 
 
 Unknown ancestral stock 
 of the Old World pri- 
 mates, including man. 
 
 FIG. 111. Ancestral tree of the anthropoid apes and of man. (From Osborn's 
 Men, of the Old Stone Age. By special permission of the publishers, Charles 
 Scribner's Sons.) 
 
 periods of the Pleistocene. Europe was partly covered 
 with ice four times and between each glaciation there was 
 
316 
 
 GENERAL ZOOLOGY 
 
 a long warm period (Table I) when animals from southern 
 countries invaded the land. In. the deposits left during 
 the first interglacial period eoliths, or unchipped stone 
 fragments, occur which may have been formed by men, but 
 
 TABLE I. 
 
 Showing Conditions in Europe during the Development of Man 
 Adapted from Osborn 's "Men of the Old Stone Age * 
 
 Time 
 
 Climate 
 
 COl-O VTAftM 
 
 diurnal s 
 
 Implements 
 
 Human races 
 
 Postgtoctol 
 25,000 years 
 
 / 
 
 
 Deer, bison, horse, 
 chamois, ibeif 
 
 Iron /OOO BC 
 Bronze /OOO yrs. 
 Pottery 
 PoJishecf stone 
 5OOO yrs. 
 
 Carving, painting 
 
 Clipped flints 
 2 5. OOO yrs. 
 
 Homo sapiens 
 
 Cro-magon Pace 
 Brain capoctfy I8OO c cm 
 
 4 Gloc/al Period 
 25.000 years 
 
 
 
 Reindeer, arctic for, 
 muslfor 
 
 
 3. Interglacial 
 Period 
 
 100. OOO years 
 
 
 } 
 
 Bison, horse, 
 hippopotamus, 
 elephant, lion, 
 rhinoceros, 
 sabre -tooth tiger 
 
 Neanderthal Race 
 Brain capacity I6OO can 
 
 
 
 / 
 
 ' 
 
 Rough flints 
 B5.000 yrs 
 
 Piltdown Pace 
 Brain capacity /4OOcak 
 
 3 Glacial Penod 
 25.000 years 
 
 c 
 
 
 Pemdeer, 
 wooly mammoth 
 
 
 
 
 
 \ 
 
 
 
 
 
 
 
 Hippopotamus, 
 
 
 
 
 
 
 rhinoceros, ' 
 
 
 
 
 
 
 elephant, 
 
 
 Heidelberg Race 
 
 Period 
 
 
 
 stag, bison, 
 
 
 
 200.000 years 
 
 
 ) 
 
 horse 
 
 
 
 2.G/ac,a/ Period 
 25. OOO years 
 
 ( 
 
 
 Pemdeer, 
 wool/ mammoth. 
 
 
 
 ( Interglocia/ 
 Period 
 7S.OOO years 
 
 
 ) 
 
 Hippopotamus, 
 elephant, 
 rhinoceros. / 
 
 (Eoliths?) 
 
 
 1 Glacial Period 
 25.OOO years 
 
 ( 
 
 
 Mush ox in 
 England. 
 
 
 (Trinil race lived in Jaw) 
 Brain copoc/fy 9OOcfm. 
 
 most archaeologists believe that they are not of human 
 origin. In the long second interglacial period the Heidel- 
 berg race lived in central Europe. The men of this race 
 
MAN 317 
 
 (Homo heidelbergensis) did not have a projecting chin like 
 later races and were primitive in other features. 
 
 During the third interglacial period the flint workers 
 entered Europe and attained a considerable degree of simple 
 culture before the last glaciation again drove them south. 
 These men at first frequented open camping sites in 
 sheltered nooks along the hillsides, where the bones of ani- 
 mals slaughtered for food are found mingled with their primi- 
 tive implements. Later when the climate became cooler 
 they often sought the shelter of caves. Their implements 
 were at first rough fragments of stone (eoliths), but later 
 the stones were chipped to form various shapes (palseo- 
 liths). Little is known of the Piltdown race, except that 
 it existed in eolithic times, but the Neanderthals (Homo 
 neanderthalensis) left numerous remains in various parts 
 of Europe and we have considerable knowledge of their 
 customs. The bones of animals hunted for food are mixed 
 with flints and with their own skeletons around the ancient 
 hearths and give many clues to their habits of life. These 
 men hunted the mammoth, rhinoceros, wild horse, bison, 
 cattle, giant deer, and reindeer. Both flesh and pelts were 
 utilized, and the marrow was sought by splitting all the 
 larger bones. The chase was pursued with spears and 
 darts fitted with flint points; also by means of thro wing- 
 stones. The Neanderthals may have used other means 
 to secure large animals, for their small spears were prob- 
 ably not very effective against the mammoth or rhinoceros. 
 At the end of their culture, bone came to be used somewhat 
 for anvils and implements. The Neanderthals as a race 
 possessed many structural peculiarities the brows were 
 heavy and overhanging; the forehead was low; nose, flat; 
 upper lip, long; arms and shins, short. They became 
 extinct during the fourth glacial period and were suc- 
 ceeded by a race which greatly excelled them in physique 
 and intelligence. 
 
 The Cro-Magnon race, with others of less importance, 
 entered Europe at the end of the fourth glacial period 
 
318 
 
 GENERAL ZOOLOGY 
 
 LOWER 
 PALEOLITHIC 
 
 \ Races belonging 
 <? B S 
 
 fjjbsExiifct Stfecies 
 
 i t o// 
 
 % I * 
 
 CL I ^ . 
 
 J *** i I Existing Species of Man 
 
 I / rj J * . J 
 
 .1 / nomo tsapiens 
 
 \fces \ \ ! f ? 
 
 ^"k \l t 
 
 \t J V / 
 
 Cdmtnon Ancesi&f-s 
 
 of 
 Exiinci and Exisiing Species t>J Man 
 
 FIG. 112. Tree showing the main theoretic lines of descent of the chief Pre- 
 Neolithic races discovered in Western Europe. (From Osborn's Men of the Old 
 Stone Age. By special permission of the publishers, Charles Scribner's Sons.) 
 
MAN 319 
 
 about 25,000 years ago. These men are placed in the same 
 species (Homo sapiens) as those of modern times; in fact, 
 some are believed to have existed until the fifteenth century 
 in the Canary Islands. They used bone needles, harpoons 
 with recurved teeth, awls, hammers, borers, polishers, and 
 perhaps bows and arrows. They also had considerable 
 artistic ability, engraving on stone, bone, and ivory; 
 sculpturing in stone, bone, and clay; painting on the walls 
 of caverns; and making conventional ornamental figures. 
 There was probably bartering at this time, for shells and 
 other objects from the Mediterranean and Atlantic are 
 found in Central Europe. 
 
 The Cro-Magnon race declined in later palaeolithic 
 times, though some groups persisted, and their descendants 
 are believed to be now living in the region of Dorgodne 
 and in some other places in France. The Old Stone Age 
 (Palaeolithic) lasted until 10,000 B.C.; then four new races 
 invaded Europe, probably from the south and east. The 
 new inhabitants lived more by fishing than hunting, and 
 stag-horn harpoon points replace the older reindeer-horn 
 spear tips in the deposits from this time. Art also declined 
 and paintings were mostly confined to geometric figures 
 on pebbles, which may have had some religious significance. 
 
 In the Neolithic (New Stone) Age polished stone imple- 
 ments succeeded chipped stone and agriculture began, for 
 instruments for the preparation of the soil and the harvest- 
 ing of crops appear in the deposits from this time. The 
 most distinctive feature of the age, however, was the intro- 
 duction of pottery. Art revived, and some of the draw- 
 ings show the dog accompanying man, indicating that it 
 had been domesticated. In the Old Stone Age the horse 
 was commonly eaten, but that practice died out as it came 
 under domestication. Cattle, sheep, goats, and pigs were 
 also domesticated in the Neolithic age. Before the close 
 of this period all the direct ancestors of the modern races 
 of Europe had not only established themselves, but had 
 begun to separate into colonies. 
 
320 GENERAL ZOOLOGY 
 
 During the Stone Age, "the rudiments of all modern 
 economic powers of man were developed: the guidance of 
 the hand by the mind, manifested in his creative industry; 
 his inventive faculty; the currency or spread of his inven- 
 tions; the adaptations of means to ends in utensils, in 
 weapons, and in clothing. The same is true of the aesthetic 
 powers, of close observation, of the sense of form, of pro- 
 portion, of symmetry, and the appreciation of beauty 
 of animal form and the beauty of line, color, and form in 
 modelling and sculpture. Finally, the schematic repre- 
 sentation and notation of ideas so far as we can perceive 
 was alphabetic rather than pictographic. The religious 
 sense, the appreciation of some power or powers behind the 
 great phenomena of nature, is evidenced in the reverence 
 for the dead, in burials apparently related to the future 
 existence of the dead, and especially in the mysteries of the 
 art in the caverns." 
 
 HUMAN SOCIETY 
 
 All social relations depend upon mutual toleration and 
 cooperation. Though those of man in part resemble the 
 communities of social insects and vertebrates, which de- 
 pend largely on primitive feeding and sheltering instincts, 
 they have more of reasonings, elf interest and altruistic 
 love. The immediate ancestors of man were doubtless 
 social in habits and probably possessed the power of com- 
 munication by speech. The men of the Stone Age must 
 have hunted together for the cave bear or mammoth and 
 cooperated in the division of the spoils. They helped each 
 other in averting dangers to themselves and their families; 
 they dwelt together in the shelter of caves. Such associa- 
 tion of different types of mind led to the exchange of ideas 
 men were stimulated to equal or excel their companions; 
 improved methods originated by one- were imitated by 
 another. Members of the small .society gained mental 
 discipline and self control because they were obliged to 
 sacrifice their own interests for those of the community. 
 
MAN 321 
 
 Thus mind and thought have in part been products of the 
 social medium. 
 
 The daily association of man with man made possible the 
 many inventions and improvements in methods, tools, and 
 other accompaniments of social life. There was progress 
 from the use of rough stones, to the utilization and manu- 
 facture of chipped stones, polished stones, horn, bone, 
 bronze, iron, steel and other metals. Some accidental 
 burning of clay about a hearth perhaps led to the general 
 use of pottery. The difficult and uncertain capturing of 
 food through the chase gave place to the surer method of 
 rearing domestic animals. The keeping of flocks was accom- 
 panied and in part superseded by agriculture. As hands 
 became more skillful and minds more critical the manufac- 
 ture of merely utilitarian implements was not satisfying 
 objects were made more symmetrical^ embellished with 
 engraving, and finally art became an end in itself. By 
 conserving his resources and making his food supply sure 
 man gained some leisure, and this led to the invention of 
 games in order that he might share pleasant experiences and 
 secure training in company with his fellows. 
 
 The collecting of groups of men for common interests and 
 the enjoyment of social relations led to a " consciousness of 
 kind. " Each group drifted into certain methods and habits 
 which became traditions for their children thus clans with 
 characteristic folk ways arose. Primitive races in all parts 
 of the world frequently adopt specific marks to indicate 
 their clan nose rings, ear rings, painting the body, filing 
 the teeth, flattening the head, etc. The fostering of clan 
 spirit led to greater affection among members of a group 
 and also brought about feelings of intolerance for other 
 groups. It was natural, then, that there should be different 
 dialects and customs. Certain laws were adopted by the 
 clan and racial traditions developed so that group loyalty 
 in time gave rise to patriotism. Group loyalty is strongest 
 in primitive people; the most enlightened are tolerant of 
 the idiosyncrasies of others. 
 
322 GENERAL ZOOLOGY 
 
 The first human societies were families. Such primitive 
 family groups still exist among certain tribes in the Philip- 
 pine Islands and elsewhere. Among the American Indians 
 each tribe consists of blood relations who trace their lineage 
 to common ancestors. At the time white men came to 
 America there were about 20,000 Iroquois living in the 
 forests of New York. They were divided into separate 
 "nations/ 5 each consisting of relatives, and to prevent 
 close intermarriage there were laws which required young 
 men to take wives from a different nation. Though these 
 nations were affiliated there was no chief who ruled them all. 
 The feudal chiefs of Bible times and during the Middle 
 Ages were rich in cattle and sheep. In addition to their 
 relatives, they required serfs and vassals to till the soil and 
 tend flocks. These were voluntary retainers who sought 
 the protection of some powerful chief, or captives taken in 
 battle. From affiliations between such overgrown feudal 
 families the nations of the earth arose. 
 
 In the development of nations climate has been an 
 important factor. Where poor soil is coupled with arid 
 conditions, the people are nomads and never form great 
 societies. The struggle with nature for a bare living is too 
 severe. But where there is good soil and abundant rain- 
 fall, agriculture and flocks flourish. Such resources enable 
 men to acquire permanent homes, accumulate wealth, have 
 leisure time for cultural pursuits, and to build up nations. 
 On the other hand adverse climatic changes have made an 
 end of great nations the ruins of ancient Palmyra, which 
 now lie in a desert, were built in the midst of agricultural 
 plenty. 
 
 Through social intercourse man has made progress. 
 There has been a gradual transition from the slaughter and 
 capture of animals or the collecting of natural resources in 
 the way of helpless shell fish, fruits, vegetables and seeds. 
 The chase has been assisted by the invention of traps, 
 arrows, spears, and firearms; experience has led to prepara- 
 tion for times of scarcity by storing reserves, and the inven- 
 
MAN 323 
 
 tion of methods for curing food; the results of agriculture 
 have been multiplied by cultivation and the selection of 
 seeds. Modern economic relations have grown up chiefly 
 through the accumulation and exchange of food products. 
 Even the struggle between capital and labor is the result 
 of primitive instincts to secure, to store up, and to improve 
 the food. In modern civilization food and good living 
 depend upon work or the possession of the results of some- 
 body's lafeor. This has led man to adopt regular habits of 
 life in order to be more effective and thus earn a better 
 living. 
 
 CIVILIZATION 
 
 The Universal Dictionary says: " Civilization consists in 
 what may broadly be called culture in a nation; and a 
 nation may be considered as civilized when a large propor- 
 tion of those belonging to it have their intellectual and 
 moral faculties and all their higher nature in large measure 
 developed and becoming increasingly so with the advance of 
 years. Before this can take place a considerable amount of 
 material prosperity must have been achieved, between 
 which and the culture already described there are continual 
 action and reaction. Regarding progression in material 
 prosperity, certain stages tend to occur: (1) a barbarous 
 one, in which one feeds on roots, fruits, and fishes, when 
 these last can be caught without effort; (2) the state of a 
 hunter; (3) that of the shepherd, in which, to avoid the un- 
 certainty of the result in hunting, wild animals are domes- 
 ticated; (4) the agricultural state; and (5) that of manu- 
 factures and commerce." 
 
 Civilization rests primarily on commerce. In primitive 
 society there is little or no exchange of products; barter 
 depends upon a market for surplus and cannot occur 
 regularly in isolated communities where conditions of life 
 are hard. The Esquimaux tribes, for example, are practi- 
 cally self-sustaining. Civilized nations have gone beyond 
 the exchange of actual products and have money as a 
 
324 GENERAL ZOOLOGY 
 
 medium of exchange. This condition is accompanied by 
 more or less segregation into classes or castes; those with 
 limited abilities or opportunities do the routine labor of the 
 community, those with unusual endowments or good for- 
 tune accumulate stores of desirable assets. There is thus a 
 continual struggle between those who sell labor and those 
 who have wealth. The most pitiable " hangers-on" of 
 modern civilization are those who make no contribution to 
 society, but live parasitically on the fruits of the labor or 
 the wealth of others. Commerce tends to eliminate tradi- 
 tion and the bigoted following of the folk ways of particular 
 communities. As one nation associates with another and 
 finds good workmanship, bright intellects, and shrewd 
 minds, the contact is conducive to mutual respect and 
 esteem. 
 
 The position of women in any nation is a fair standard of 
 its civilization. In primitive societies men fight and hunt, 
 women do all other necessary work. Work, therefore, is 
 often looked down upon as something effeminate and un- 
 worthy of strong manhood. This attitude makes lazy 
 men and overburdened women a strong nation cannot 
 result. Another evil that has often crept into half-civilized 
 nations is slavery. In olden times slaves were one of the 
 recognized rewards of conquerors and women were prac- 
 tically slaves to men. But slavery has decreased because 
 it cannot be successful unless large quantities of new land 
 are available for tillage. Furthermore, slaves are expensive 
 and wasteful. They do not work to their limit of produc- 
 tiveness because they can receive no reward except " self- 
 maintenance." No highly civilized nation countenances 
 slavery today. In modern society, also, women are fast 
 gaining legal, industrial, and intellectual equality with 
 men. This does not mean that one sex will or should be- 
 come like the other, but if civilization progresses each will 
 have equal rights in society. 
 
 In the most cultured nations we can see many evi- 
 dences of the primitive instincts for self-maintenance, self- 
 
MAN 325 
 
 protection, and race preservation. Some dull souls are 
 satisfied with mere self-maintenance and strive only for 
 enough to eat and wear. Many business men are over- 
 cautious, stingy, and conservative, fearing for their own 
 protection; others are greedy, unscrupulous, and aggressive 
 in order to be sure to have enough. Our instincts for race 
 preservation offer many problems shall capital criminals 
 be killed or encouraged to live better; shall hopeless idiots 
 and insane persons be killed, sterilized, or allowed to live 
 and breed other defectives? Class feeling not only dis- 
 plays itself today in patriotism, wars, and great struggles 
 for national industrial supremacy, but is apparent in many 
 harmless idiosyncrasies such as cuffs on trousers, starched 
 collars, the length of coat tails, and the trimming of hats. 
 
 In civilized life there must always be adjustment be- 
 tween individual rights and those of society. Economic 
 conditions are keen, and the cities, which are the greatest 
 industrial centers, show the greatest extremes wealth, 
 leisure, and culture contrasting with long hours, poverty, 
 overcrowding, and ignorance. In civilization there must 
 always be cooperation, sacrifice, and delegated authority. 
 Often it seems that justice is obsolete, if individual cases 
 are considered, but on the whole man's civilization grows 
 higher and better. Certain aspects of modern life are 
 highly desirable the free interchange of ideas, the de- 
 pendence of industrial progress on scientific method and 
 discovery, the intellectual opportunities offered to all. 
 But other sides of this life are not so desirable the growing 
 importance of labor-saving machines leads man more and 
 more indoors where he falls prey to the diseases accom- 
 panying sedentary life; abnormal instincts for self-main- 
 tenance lead to the hoarding of vast fortunes which at times 
 menace society; the wisdom of expending lives and resources 
 in warfare is certainly questionable. 
 
 Religious beliefs of some sort have accompanied man's 
 culture since its beginnings in the Stone Age. Many of 
 the great reforms in society have been brought about by 
 
326 GENERAL ZOOLOGY 
 
 changing religious ideals, and in this field Christianity has 
 done more for improvement than any other religion. 
 Most primitive religious beliefs have arisen from a desire 
 for present help, retribution, or future life, and some races 
 have been particularly " susceptible " to religion. The 
 negroes, for example, in a savage state commonly believe 
 in magic, charms, and the like; and when civilized are often 
 religious enthusiasts. Semicivilized people generally be- 
 lieve in totems, charms, and transmigration. For example, 
 the Alaskan Indian thinks that by making a clay image of 
 his enemy and destroying it, he has injured the enemy; and 
 that by wearing a charm simulating some animal he acquires 
 certain of the qualities of that animal. What the outcome 
 of the " higher criticism" of modern religion will be is un- 
 certain, but one thing is sure no religion which is largely 
 form without high ideals and " service" can survive. 
 
 Probably the greatest burden that civilization and knowl- 
 edge lays on man is responsibility. If man rules the earth, 
 he must do it with wisdom, kindness, and justice. He must 
 conserve natural resources; protect helpless and ignorant 
 nations. Through improvement in educational methods he 
 must find some means to make the great stores of knowledge 
 available as easily as possible. Education must more and 
 more pass from mere training in observing and memorizing 
 to training for the exercising of powers of generalization and 
 ingenuity. Education has as its object the power to con- 
 trol self to do more work and to do it better. Each 
 generation must excel the one before and we do not know 
 what the future of man will be. If he goes ahead as he has, 
 it is possible that finally the universal exchange of ideas 
 may bring about a ripe culture which will mean universal 
 cooperation, sacrifice, and toleration. If, however, a great 
 disaster wipes out the human race, some other animal will 
 have to struggle up through another long evolution and take 
 its place. In either event the manifest duty of man is to 
 do his best to struggle continually to improve. Great 
 nations must have great minds and spirits; these have 
 always come from people who work. 
 
CHAPTER XXIX 
 ANIMALS OF THE PAST 
 
 Chamberlin* maintains that the earth could not have 
 originated from the gradual condensation of a gaseous 
 nebula, as Laplace believed, but was probably ejected from 
 the sun on account of the attraction exerted by a passing 
 star. He believes that it probably solidified very soon and 
 that it grew somewhat by the accumulation of smaller 
 bodies (planetesmals) on its surface. "The juvenile shap- 
 ing of the earth may be said to have begun as soon as the 
 planetesmals began to plunge into the earth-knot of the 
 nebula; and both knot and planetesmals began to gather into 
 a dense body. The drawing of an atmosphere close about 
 the young earth commenced almost simultaneously. The 
 gathering of the primitive waters into the hollows of the 
 earth-surface soon followed. -These three concurrent activi- 
 ties were master-processes in the growth of the infantile 
 earth; they were the geologic triumvirate. They wrought 
 together toward the earth's final shaping into the litho- 
 sphere, the hydrosphere, and the atmosphere. The con- 
 tact surfaces between earth, air and water are the sites of 
 the most distinctive activities of the present day, and as 
 far back as a good record goes these contact zones have been 
 the seats of the most declared denudations and depositions. 
 They have been almost the sole habitats of biological and 
 psychological activity. From the naturalistic point of 
 view these climatic developments embody three great steps : 
 (1) an ascent in the complexity of physico-chemical combi- 
 nation until it attained the organic type; (2) an evolution 
 of physiological processes and of organs subservient to 
 these; and (3) the initiation and the varied development of 
 
 * "The Origin of the Earth," 1916. 
 
 327 
 
328 
 
 GENERAL ZOOLOGY 
 
 Fig, lid. Geological History 
 
 Pertod 
 
 Characteristic Animals 
 
 Firtt 
 
 Occurrence of 
 
 Recent 
 
 P/e/sTocene 
 
 Mammofh. 
 
 Horse. 
 GtcjphodonTs. 
 
 Man 
 
 
 Deer Sloths, 
 Ape . 
 
 Dog . Stag. 
 Came/, 
 
 Ape . Marfl 
 
 Miocene 
 
 Elephant. 
 Sab re. '-tooth 
 
 Car. Bear, 
 
 Monkey, 
 
 Cow , Oeer. 
 
 Mgocene 
 
 hoofed 
 
 ^Lr-^j,^ 
 
 Horse (3 toes) ^ 
 Rhinoceros. 
 Pike 
 
 Eocene 
 
 Mammals. 
 
 Snake. .Lemur 
 BaT. hog. 
 horse. 
 
 Cretaceous 
 
 Mars up ia Is , 
 Salamanders 
 
 Jurrass/c 
 
 Bird. 
 Crocodile. 
 Frog. 
 
 Reptiles . 
 Amphibians, 
 
 Mammals. 
 
 Turtles. 
 Dinosaurs. 
 
 Reptiles 
 
 BryO7.oan$. 
 Echinolds. 
 Ophiurotds . 
 
 Cambrian 
 
 "rustacea ns, 
 Molluscs. 
 Worms . etc. 
 
 Qracniopods, 
 Tr/lotfifeS. 
 
 rocks - no f 053 Us 
 
 FIG. 113. 
 
ANIMALS OF THE PAST 329 
 
 psychological phenomena. These seem to have followed 
 one another in ascensive order." 
 
 Though Chamberlin is willing, on the basis of observed 
 phenomena and mathematical calculations, to postulate 
 that life originated in the surface layers of the soil near 
 water, there is no actual record of such origin. The life of 
 the earliest geological periods has left no trace because the 
 rocks have all been "metamorphosed/' that is, changed by 
 pressure, heat, or other influences so as to destroy all 
 organic remains. Though Walcott has recently discovered 
 a few fossils in Pre-cambrian<rocks, the sedimentary depos- 
 its in the Cambrian are the first which contain abundant 
 animal remains. In the Cambrian most of the chief groups 
 of invertebrate animals appeared and many of them had 
 already attained a high degree of specialization during the 
 preceding formative ages. Fig. 113 gives a condensed 
 summary of the chief events since Pre-cambrian times. 
 The characteristic animals of the four great geological ages 
 will now be considered. 
 
 PALAEOZOIC AGE 
 
 Cambrian Period. The Cambrian fossils, so far as the 
 animals are concerned, are all of marine origin, and it is 
 doubtful if -terrestrial animals existed. In the oceans there 
 were protozoans, sponges, hydrozoans, jelly-fishes, corals, 
 round-worms, annelids, echinoderms (cystoids, starfishes, 
 sea cucumbers), brachiopods, clams, snails, cephalopods, 
 crustaceans and arachnids. Doubtless other types of inver- 
 tebrates were in existence but their remains have not yet 
 been found. Some of the species which existed in Cam- 
 brian oceans did not survive even until the next period, 
 and many whole groups became extinct during the Palae- 
 ozoic Age. Trilobites of various sizes were common. 
 The cephalopods usually had the body enclosed in a straight 
 or slightly curved shell. 
 
 Ordovician Period. In the rocks from this period appear 
 the first remains of several classes of echinoderms (Bias- 
 
330 GENERAL ZOOLOGY 
 
 toidea, Crinoidea, Ophiuroidea, Echinoidea), the Bryozoa, 
 and certain molluscs. The trilobites attained their cli- 
 max here and declined during succeeding periods. The 
 cephalopods began to have coiled shells, which were less 
 cumbersome than the straight types characteristic of the 
 Cambrian. The graptolites, a peculiar group of ccelen- 
 terates, flourished in this time, but had a short racial exist- 
 ence for they were practically extinct at the end of the 
 next period. 
 
 Silurian Period. The most notable event recorded in 
 Silurian rocks is the advent of fish-like animals. The 
 ostracoderms had no paired fins or true jaws and differed 
 from modern fishes in many other respects. Many of 
 them had the anterior end of the body covered with heavy 
 protective plates. There were also primitive sharks in the 
 Silurian seas. In this period also the corals developed suf- 
 ficient colonial life to begin to construct reefs; the brachio- 
 pods reached the height of their racial vigor; crinoids be- 
 came abundant; the trilobites persisted in considerable 
 numbers; the king-crabs had attained some specialization, 
 and primitive sea-scorpions were in existence. 
 
 Devonian Period. Fish-like animals increased greatly 
 and many new types appeared. Though ostracoderms per- 
 sisted and the appearance of new forms indicates that they 
 attained considerable specialization, the whole group died 
 out at the end of the period. Cyclostomes and sharks 
 were present, and some of the latter attained a length of 
 five feet. Several types of true fishes were in evidence- 
 some were apparently ancestral to those living today and 
 some have become extinct. The arthrodirans had the 
 anterior part of the body covered with heavy exoskeletal 
 plates, and were unique in possessing a neck joint so that 
 they could nod the head. The trilobites were now declin- 
 ing and were replaced by crustaceans of more modern type. 
 Most of the cephalopods possessed coiled shells, which in 
 many species had a very complicated structure; clams be- 
 came specialized to such an extent that forms much like 
 
ANIMALS OF THE PAST 331 
 
 modern oysters and razor-clams were in existence. The 
 presence of primitive myriapods indicates that the land had 
 a well-established fauna at this time. 
 
 Carboniferous Period. The rocks of the coal measures 
 are of interest chiefly because they contain the remains of 
 the first known animals with pentadactyl limbs. These 
 were amphibians and some of them at least breathed air 
 into the lungs when in the adult condition. They lived in 
 fresh water and some may have been more or less terres- 
 trial. All the Carboniferous amphibians are called stego- 
 cephalians because they had the head " roof ed-over " with 
 bony plates. Some were several feet in length and a num- 
 ber had teeth of very complicated structure (labyrintho- 
 donts). Sharks were numerous and widespread at this 
 time; true fishes were increasing in numbers. 
 
 Toward the end of the Carboniferous Period insects re- 
 lated to orthopterans, may-flies, and dragon-flies were living 
 in the land vegetation, and there were other types which 
 later became extinct. Various arachnids and a number of 
 species of myriapods were also living on land. The brachi- 
 opods and the older classes of molluscs and echinoderms 
 were declining in numbers. Only a single family of trilo- 
 bites persisted to the end of this period. The abundant 
 land vegetation was largely responsible for the coal deposits 
 that are being mined at the present time. 
 
 Permian Period. The number of species of amphibians 
 increased somewhat during this period, but the most inter- 
 esting feature is the rise of the reptiles, which came into 
 existence in the preceding period and now invaded aquatic 
 and terrestrial habitats. Reptiles soon showed specializa- 
 tion along a number of different lines. Some Anomodontia 
 resembled mammals in many structural features, and in all 
 probability the mammals later arose from this stock. 
 Many of the old types of fishes were dying out, even that 
 which probably gave origin to the amphibians declined, 
 but their places were taken by species more like those of 
 modern times. The trilobites, so characteristic of ancient 
 
332 GENERAL ZOOLOGY 
 
 seas, became extinct and the ancestors of decapod crusta- 
 ceans appeared. The land vegetation consisted largely of 
 coniferous trees. 
 
 At the beginning of the Palaeozoic Age, animals were rep- 
 resented only by invertebrates, but toward its close fish- 
 like chor dates, amphibians and reptiles arose, and gained 
 prominent places. At the beginning also, most of the 
 animals were marine, but, though the ocean continued to 
 be abundantly populated, as life progressed in specialization 
 there was an increasing invasion of fresh-water and land 
 habitats. Many groups that were important in Cambrian 
 times had dwindled (Brachiopoda) or become extinct (Tri- 
 lobita) at the close of the Permian. 
 
 MESOZOIC AGE 
 
 Triassic Period. During Triassic times the land was 
 covered by great forests of coniferous trees, mixed with the 
 older club mosses, ferns, and algae. This luxuriant growth 
 supported numerous insects and such modern types as the 
 beetles had made their appearance. Some of the stego- 
 cephalians at this time were of great size, one species hav- 
 ing a skull measuring more than a yard in length. Reptiles 
 were more diverse and attained greater size than during the 
 preceding period. The giant dinosuars had appeared in 
 various places; heavy-bodied plesiosaurs were beginning 
 to frequent the shores of oceans; whale-like ichthiosaurs 
 competed with fishes in marine habitats; chelonians fre- 
 quented the swamps and rivers. Small mammals also ap- 
 peared in the Triassic period, but did not take an important 
 place among animals until later. 
 
 Jurassic Period. Reptiles rose to the zenith of their 
 development in this period and were distributed in all parts 
 of the earth. Herbivorous and carnivorous dinosaurs 
 (Fig. 101) dominated the land and frequented the borders 
 of marshes; tortoises were numerous along rivers; the ocean 
 

 ANIMALS OF THE PAST 333 
 
 was frequented by large plesiosaurs and ichthiosaurs; the 
 air served as a highway for the ''flying lizards," or ptero- 
 dactyls (Fig. 101). In later Jurassic times primitive birds 
 (Fig. 102) appeared, and for a time shared the ah- with the 
 flying reptiles. Mammals continued to be small in size, 
 but had differentiated into types suggesting moles, opos- 
 sums, and rodents. Fishes began to have more and more 
 the structures associated with those of modern times: the 
 endoskeleton became less cartilaginous and more bony; the 
 heavy armour on the outside of the body decreased; the 
 fins became more effective organs for swimming. 
 
 Cretaceous Period. Dinosaurs and other great reptiles 
 still flourished at the beginning of this period but before 
 the close many whole groups had died out completely. 
 The gigantic size of some of these animals is indicated by 
 Shimer's estimates concerning Brontosaurus, an herbi- 
 vorous dinosaur, which is believed to have weighed twenty 
 tons and to have consumed about 4000 pounds of leaves 
 and twigs per day. The birds of this time were not lizard 
 like. Their tails were short and the wings did not generally 
 possess free digits, but the jaws were armed with teeth. 
 In some types there were signs of extreme specialization. 
 Hesperornis, for example, had lost its power of flight and 
 the wings were degenerate. The mammals were still 
 small and for the most part not highly specialized. 
 
 During the Mesozoic Age the reptiles were the most im- 
 portant animals; though birds and mammals came into 
 existence they did not play a prominent part in the life of 
 the time. Toward the close of the age many orders of 
 reptiles became extinct. Pterodactyls no longer sailed 
 through the air; ichiosaurs and plesiosaurs were absent 
 from the ocean; dinosaurs had disappeared forever. 
 Among invertebrate animals there had also been many 
 changes. In the ocean true shrimps, lobsters, and crabs 
 had come into existence; the archaic cephalopods, so 
 characteristic of ancient seas, were largely replaced by more 
 
334 GENERAL ZOOLOGY 
 
 modern devil-fishes. On land insects were abundant and 
 had probably come to take some part in the pollination of 
 plants. 
 
 TERTIARY AGE 
 
 Eocene Period. There were great changes of scene be- 
 tween the Cretaceous and Eocene. Many old types of 
 reptiles and other animals were extinct; mammals now 
 became the most important land animals. The ancestral 
 stocks of rodents, elephants, bats, ruminants, whales, 
 carnivores, edentates, and insectivores had already reached 
 some degree of differentiation. There were also other 
 primitive mammalian strains in existence which have since 
 become extinct. Birds without teeth were present in 
 considerable numbers and variety. The reptiles, amphi- 
 bians, and fishes all had a more modern aspect. Such types 
 as lizards, snakes, tailless amphibians, and herrings were 
 prevalent. Among the invertebrates also there were hermit 
 crabs, and other specialized modern types. 
 
 Oligocene Period. There were now a number of types 
 of hoofed animals in existence. Among the survivors of 
 the strange herbivores of Eocene times, the rhinoceros- 
 like titanotheres were perhaps the most remarkable, but 
 they were all extinct before the beginning of the next period. 
 The true rhinoceros stock was as yet hornless; the horses 
 had increased to the size of foxes and their feet showed some 
 degree of specialization toward the reduction of toes; pigs 
 were small creatures with four usable toes on each foot; the 
 ruminants were without horns, even the deer. Among the 
 carnivores the sabre-tooth cats were perhaps the most 
 striking though they had not yet attained their greatest size. 
 These animals had the upper canines greatly elongated. 
 Civet cats and carnivores related to modern weasels were 
 in existence. Primate remains have also been found in 
 the deposits from this period. The first pioneers of the 
 Primates were primitive lemur-like animals. Among the 
 birds there were divers like modern grebes, cormorants, 
 
ANIMALS OF THE PAST 335 
 
 gulls, pelicans, rails, ducks, plovers, grouse, parrots and 
 eagles. The insects were represented by such specialized 
 forms as gall-flies, saw-flies, bees, stag beetles, and 
 butterflies. 
 
 Miocene Period. The landscape now had a more modern 
 aspect. Among the plants were irises, pinks, poppies, 
 violets, roses, plums, and pineapples. Various types of 
 primitive monkeys were living in the trees. Porcupines, 
 beavers, martens, skunks, bears, antlered deer, seals and 
 other modern types lived with archaic forms like the horn- 
 less rhinoceri, and three-toed horses. In South America 
 there were great ground sloths, some as much as twenty feet 
 long, armadillos, and strange hoofed animals. A number 
 of new types of birds appeared during Miocene times and 
 other vertebrates were also changing. Snakes were now 
 represented by venomous species. 
 
 Pliocene Period. In this period many of the older 
 mammalian types were still in existence. There were 
 long-jawed elephants; horses in which the outside toes, 
 though off the ground, were still quite large, sabre-tooth 
 tigers, and other archaic creatures. But such old-fashioned 
 animals were outnumbered by the modern types and old 
 strains continued to die out. At the end of the period there 
 were a number of kinds of true apes scrambling about in 
 the trees; the horses were mostly " one-toed ;" and there 
 were many new animals, such as goats, giraffes, and even 
 apes somewhat closely related to man. 
 
 The Tertiary Age is noteworthy for the increase of the 
 warm-blooded animals, particularly the mammals. Though 
 many lines of amphibians and reptiles underwent great 
 specializations, these groups in general decreased, both in 
 numbers and in variety. The birds and mammals, .on 
 the contrary, showed specialization along many and various 
 lines, increased greatly in numbers, and filled many of the 
 habitats occupied during the later Mesozoic times by their 
 cold-blooded predecessors. At the close of the Pliocene, 
 
336 GENERAL ZOOLOGY 
 
 though there were many relics from previous ages, the great 
 majority of animals were much like those of today. 
 
 QUATERNARY AGE 
 
 Pleistocene Period. This was a time of great temperature 
 changes, and the northern hemisphere was invaded four 
 times by great masses of ice which came from the north. 
 Climatic conditions caused great migrations among animals. 
 Europe, for example was at times inhabited by reindeer, 
 arctic foxes, musk-oxen, woolly rhinoceri, mammoths, and 
 other animals characteristic of cool regions. When the ice 
 receded, horses, elephants, sabre-tooth tigers, lions, hippo- 
 potami, and other animals came in. During the warm 
 interglacial periods several races of men dwelt in Europe 
 and attained a considerable degree of primitive culture. 
 In South America giant sloths, peculiar camel-like creatures, 
 and gigantic armadillos (Fig. 113) were still in existence. 
 
 Since the last great glaciation the northern hemisphere 
 has enjoyed a fairly stable climate, vegetation has spread 
 northward, and animals have increased in numbers. The 
 most important feature of the Quaternary Age has been 
 the rise of man and his domination of the earth. 
 
 PAL^ONTOLOGICAL HISTORY OF ELEPHANT AND HORSE 
 
 The discussion of the geological periods necessarily has 
 been very limited and noted only a few of the important 
 events of palseontological history. In order to give a more 
 concrete idea of how modern types of animals have de- 
 veloped from primitive ancestors, two of the best known 
 series will be briefly described. These relate to the ances- 
 tors of the elephant and the horse. 
 
 The earliest elephant remains (Fig. 114, A) were dis- 
 covered in Middle Eocene deposits in Egypt. The crea- 
 tures from which modern types probably descended were 
 small tapir-like animals, about three and a half feet high, 
 which appear to have dwelt in swamps, living upon the 
 succulent, semi-aquatic herbage of that time. Their front 
 
ANIMALS OF THE PAST 
 
 337 
 
 teeth were rather degenerate, indicating a prehensile upper 
 lip, and the second pair of incisors in each jaw were some- 
 what developed as tusks. In the Upper Eocene this 
 
 FIG. 114. Showing fossil ancestors and living representatives of elephant and 
 horse. ( After Lull.) 
 
 primitive elephant (Mceritherium) was asssociated with a 
 more specialized type (Palceomastodon, B) of somewhat 
 larger size. This new elephant had a greatly elongated 
 
338 GENERAL ZOOLOGY 
 
 lower jaw and the structure of its upper jaw indicates that 
 it possessed a short extensile proboscis. No elephant bones 
 are known from Oligocene deposits, but in the Miocene of 
 France a great probocidian, Tetrabelodon, has been dis- 
 covered which is remarkable for the great elongation of the 
 lower jaw and the shortness of the lower tusks (C). In 
 Pliocene times several species of the genus Dibelodon lived 
 in South America. These animals had well-developed 
 tusks in the upper jaw but the short lower jaw was without 
 incisors. The mastodon (D), which lived in North America 
 during glacial times, had the lower jaw still further shortened. 
 Modern elephants (E) apparently took their origin from 
 a straight tusked type, Stegodon, which developed in 
 the Indian region. The genera mentioned are only a few 
 representatives of numerous types of fossil proboscidians. 
 At one time there were elephants in Europe, Africa, Asia, 
 North and South America. From what is known Lull is 
 able to make the following generalizations concerning the 
 racial history of the Proboscidia: 
 
 "The physical changes undergone by the race are also 
 clearly shown, as the palseontological series is very complete. 
 These changes may be thus summarized: Increase in size 
 and in the development of pillar-like limbs to support the 
 enormous weight. Increase in size and complexity of teeth 
 and their consequent diminution in numbers and the de- 
 velopment of the peculiar method of tooth succession. The 
 loss of the canines and of all the incisor teeth except the 
 second pair in the upper and lower jaws and the develop- 
 ment of these as tusks. The gradual elongation of the 
 symphysis or union of the lower jaws to strengthen and 
 support the lower tusks while digging, culminating in 
 Tetrabelodon. The apparently sudden shortening of this 
 symphysis following the loss of the lower tusks and the 
 compensating increase in size and the change in curvature 
 in those of the upper jaw. The increase in bulk and height, 
 together with the shortening of the neck necessitated by the 
 increasing weight of the head with its great battery of 
 

 ANIMALS OF THE PAST 339 
 
 tusks, necessitated the development of a prehensile upper lip 
 which gradually evolved into a proboscis for food gathering. 
 The elongation of the lower jaw implies a similar elongation 
 of this proboscis in order that the latter may reach beyond 
 the tusks. The trunk did not, however, reach maximum 
 utility until the shortening jaw, removing the support 
 from beneath, left it pendant as in the living elephant. 
 The change in the form of the skull developed pari passu 
 with the growth of the tusks and trunk as it is merely a 
 mechanical adaptation to give greater leverage in the 
 wielding of these organs. It may readily be seen that these 
 changes curiously interact upon one another; the result of 
 the evolution of its parts being the development of a most 
 marvelous whole." 
 
 Though elephants passed most of their evolutionary 
 history in Africa, America can claim the distinction of 
 having been the chief field for the evolution of horses. 
 Lull says: " These horse-like animals, to which the name of 
 hyracotheres has been given ranged throughout the entire 
 Eocene period. They were of small size ranging upward 
 from eleven to fourteen inches at the shoulder. The hand 
 bore four digits, while in the foot but three out of the 
 primal five remained. Hyracotherium itself, the oldest of 
 its race, is known from a skull found in the London Clay, 
 while both in England and America is found the widespread 
 Dawn Horse, Eohippus. The scene of the evolutionary 
 drama being shifted to our land, we find Eohippus (Fig. 
 114, F) succeeded by the somewhat larger Orohippus, the 
 mountain horse (G), while Epihippus carries the race close 
 to the close of Eocene time, a duration of at least two million 
 years." During the Oligocene lived Anchiiherium with three 
 functional toes on each of its feet; the speedy Mesohippus 
 (H) was eighteen inches in height at the withers; and its 
 successor, Miohippus (I), stood twenty-four inches high. 
 "The Miocene period was the millenium of mammals, and 
 this was superlatively true of the horses." Though three- 
 toed horses lived in the early part of this period, they were 
 
340 GENERAL ZOOLOGY 
 
 later replaced by one-toed types, and in the Pliocene 
 appeared the genus Equus (J, K) to which all living horses, 
 asses, and zebras belong. 
 
 GENERAL REMARKS ON PAL^OZOOLOGY 
 
 The geological periods and the important features of 
 palseozoology are shown in Fig. 113. There is no doubt 
 that the great landmarks of this history have been correctly 
 interpreted strata of sedimentary deposits have been 
 formed on the earth's crust and the deepest of these are 
 the oldest. From studies of the characteristics of rocks 
 and the animal remains in such deposits the following gen- 
 eralizations may be made : 
 
 1. Conditions on the earth have varied. There have been 
 times of rapid change when whole groups of animals became 
 extinct because they were unsuited to new conditions, and 
 these have alternated with long periods of comparative 
 stability when particular types of animal life multiplied 
 and became specialized along various lines. 
 
 2. The animals of the earliest times were predominantly 
 marine and in later ages the land and fresh water have had 
 an increasing population. Structure is so correlated with 
 habits of life, that, taken with the nature of the sediment 
 in which fossils are found, it enables a palaeontologist to 
 picture the conditions in which an animal lived millions 
 of years ago. 
 
 3. The geographic distribution of animals today is corre- 
 lated with their origin and distribution in the past. A group 
 of limited range is usually of rather recent origin, whereas 
 one which is widely distributed is old. Zittel says, "An 
 understanding of the physical conditions which have gov- 
 erned the perpetuation of recent plants and animals in their 
 respective provinces would be utterly impossible without a 
 knowledge of their distribution in former times." 
 
 4. The simplest, or most primitive, animals came first. 
 This is equally applicable to the sequence in the develop- 
 
ANIMALS OF THE PAST 341 
 
 ment of the different phyla and to specialization within 
 particular groups Protozoa must have preceded Metazoa; 
 crustaceans existed before insects. Fishes, amphibians, 
 reptiles, birds and mammals have arisen since Pre-cambrian 
 times and the order of their appearance agrees roughly 
 with their degree of specialization. What has been said 
 of animals applies equally well to plants. There has been 
 a succession of algae, mosses, ferns, conifers, and finally the 
 seed-plants of today. 
 
 There have been great changes; the types of animals 
 living today are few compared with those which have passed 
 away forever. The animal kindgom past and present may 
 be compared to a cotton-wood tree which is being covered 
 by a moving dune; the living twigs are but a small part of 
 the portions now covered by the shifting sands of time or 
 dune. 
 
CHAPTER XXX 
 EVOLUTION AND HEREDITY 
 
 "It has been a Proteus from the first; changefulness is its most abiding 
 quality; in short, the essence of the creature is its innate creativeness." 
 THOMSON AND GEDDES. 
 
 In the past various theories have been held in regard to 
 the origin and descent of animals. Some scholars have sup- 
 ported the idea of (1) special creation (i.e., that all species 
 of animals were originally created as they now are), but since 
 Lamarck and Darwin there have been an increasing number 
 of those who believe (2) that living types of animals did 
 not originate as they are today but have been transformed 
 while descending from remote ancestors. The evolution 
 theory, then, supports the principle of " descent with modi- 
 fication" as opposed to that of special creation. 
 
 Among scientific men evolution is no longer on trial, but 
 is accepted as a fact. Throughout this book such a point 
 of view has been taken for granted and it may now be well 
 to examine more critically the facts and theories on which 
 present ideas of evolution rest. In this connection there 
 should be no confusion between what is known and what is 
 believed that there has been an evolution scientists do not 
 doubt, but there has been, and is, much difference of 
 opinion as to why and how changes have taken place. After 
 accepting the (1) facts, and evolutionist must still puzzle 
 over the (2) means by which new species have come into 
 being, and why there is (3) adaptation to the environmental 
 conditions in which animals live. 
 
 EVIDENCES OF EVOLUTION 
 
 Palaeontology and Geographic Distribution. As was 
 stated in the last chapter, the earliest stages of the earth's 
 
 342 
 
EVOLUTION AND HEREDITY 343 
 
 fossil records are lacking, but as far back as they can be read, 
 they clearly indicate that the simplest, or most primitive, 
 animals appeared first. Though many primitive types 
 often persisted for long ages, there has been in general a 
 progressive change to new and often improved forms which 
 better met changing conditions of environment. In spite 
 of the imperfection of the fossil record, there are a number 
 of indubitable cases where series of stages which follow 
 each other in order are known (horse, elephant, etc.). 
 Thomson and Geddes say, for example: " What seems clear 
 is this, that in early Eocene times there lived small five- 
 toed hoofed quadrupeds of generalized type that the de- 
 scendants of these were gradually specialized throughout 
 long ages along similar but by and by divergent lines, that 
 they lost toe after toe until only the third remained, that 
 they gained longer necks, more complex teeth and larger 
 brains. So from the short-legged splay-footed plodders of 
 the Eocene marshes there were evolved light-footed horses 
 running on tiptoe on the dry plains. " 
 
 Modern animals for the most part fall readily into a small 
 number of phyla, which are rather sharply set off from each 
 other. Only a few types have come down from the past, 
 but there are some fossils (Archseopteryx, Fig. 102) which fill 
 in certain gaps and indicate what the evolutionary changes 
 have been. We also have living a few "links," like Peripa- 
 tus (Fig. 33) and the duckbill (Fig. 107) which suggest re- 
 lationships that would have remained unsuspected in their 
 absence. Palaeontology tells a story which is fragmentary 
 but may be interpreted only in the light of evolutionary 
 theories, and the story has no events that seriously inter- 
 fere with such an interpretation. 
 
 Groups of modern animals in many instances are known 
 to have had their past evolution in localities where they 
 are now abundant. Each continent has certain types that 
 are characteristic, both as fossils and as animals now living. 
 Furthermore, groups which are known to have originated 
 in rather recent geological ages have limited ranges of 
 
344 GENERAL ZOOLOGY 
 
 distribution when compared with those of ancient lineage. 
 Such facts lead to the conclusion that specific types ori- 
 ginated in certain parts of the earth (centers of origin) and 
 slowly spread to other favorable localities. Furthermore, 
 it is well known that particular animals which have been 
 isolated (on islands, in valleys surrounded by high moun- 
 tain ranges, etc.) develop into types which differ in certain 
 characteristics from those that are free to wander. For 
 example, Australia has long been separated from other 
 continents and has been subjected to comparatively slight 
 climatic changes; its animals are primitive and show few 
 close affinities with those of other continents. The fauna 
 of the Galapagos Islands, while consisting of recent types, 
 is composed largely of unique and peculiar species. 
 
 Generalizing from the facts of palaeontology and the 
 geographic distribution of animals, it may be said that 
 generalized types preceded and gave rise to those of special- 
 ized structure, that particular species arose in certain 
 localities and gradually spread to others, that species 
 isolated and thus prevented from mixing with others are 
 usually peculiar and rather closely adapted to local 
 conditions. 
 
 Ontogeny. The "Law of Biogenesis" asserts that 
 "ontogeny repeats phylogeny" (page 60), and in general 
 the evidence for evolution from the development of indi- 
 vidual animals conforms with the sequence shown by 
 stratified fossil records. All vertebrates, for example, pass 
 through a fish-like stage (Fig. 115); palaeontology shows 
 that fishes were the first vertebrates to appear on the earth. 
 From such facts the older evolutionists reasoned that at 
 one time there were only fish-like vertebrates, and that 
 these later gave rise to other more specialized types. This 
 view has now been modified somewhat and at the present 
 time the same facts are generally interpreted as meaning 
 that all vertebrates are related because they pass through 
 similar stages in development. The "fish-like" stage in 
 the development of a mammalian embryo has gill slits and 
 
EVOLUTION AND HEREDITY 
 
 345 
 
 its limb buds are somewhat like fins, yet such facts do not 
 necessarily mean that mammals were fishes at one stage in 
 their racial development. 
 
 Though the ontogeny of a particular animal usually 
 repeats the general sequence of descent, the record may be 
 warped or distorted by events which did not occur in racial 
 development, or it may fail to agree in other respects. 
 There are cases of " larval adaptation" in which a develop- 
 
 fish 
 
 So/amander 
 
 Frog 
 
 Turtle 
 
 Chicken 
 
 Man 
 
 FIG. 115. Showing stages in the development of vertebrates. The fish and 
 salamander have functional gills in the last stage, but all other animals repre- 
 sented lose them in earlier stages. 
 
 ing animal possesses certain structures (placenta of mam- 
 mals, Fig. 107; anal filament of young crayfish, Fig. 30) 
 which could not have been present in mature free living 
 ancestors when in a comparable stage of evolution. In 
 other instances certain stages which probably existed in 
 ancestral types have been dropped out. In such " accelera- 
 tion in development" the record is not complete. It is 
 
346 GENERAL ZOOLOGY 
 
 also probable that in the ontogeny of any animal there is 
 wide discrepancy as regards the proportion of time between 
 various embryological stages and comparable stages of the 
 ancestors when in similar condition. Despite such imper- 
 fections in the ontogenic narrative, the development of 
 animals, when taken with other evidence, gives valuable 
 support to the theory of evolution. 
 
 To quote Thomson and Geddes: "It must be admitted 
 that the recapitulation doctrine has been often stated in 
 somewhat crude and exaggerated form, so that many saving 
 clauses are necessary. The human embryo is never like a 
 little fish or a little reptile; the resemblance is between 
 embryonic stages. The recapitulation is general, not exact; 
 there is often abbreviation and a masking of the old by 
 the new. . .. . It must also be frankly admitted that 
 we are apt to get into a vicious circle in arguing about 
 recapitulation. We infer the pedigree from the develop- 
 ment, and then say that the development recapitulates the 
 pedigree. . . . But when all is said, there remains good 
 reason for keeping firm hold of this idea that 'ontogeny, 
 or the development of the individual, is a shortened 
 recapitulation of phylogeny, or the evolution of the race." 
 
 Morphology and Classification. If animals are classified 
 on a structural basis without regard to evolutionary theories, 
 they arrange themselves so that those which are genetically 
 related come in the same group and a sequence is established 
 which has evolutionary significance. Studies in classifi- 
 cation and cataloguing first suggested evolution to Lamarck. 
 At the present time systematic zoologists use all the evi- 
 dences of evolution in attempting to establish more 
 " natural" systems. Modern classification is founded on 
 genetic relationship as judged by ontogeny, phylogeny, 
 morphology, and physiology. A correct system of classi- 
 fication is also a genealogical tree. 
 
 Morphology, of itself, furnishes good evidence for 
 evolution. The presence of useless vestigeal organs (man's 
 appendix, whale's teeth) is hard to explain except as a 
 
EVOLUTION AND HEREDITY 347 
 
 survival of structures which at one time in racial history 
 were functional. The principle of "homology" has also 
 been of value hi supporting evolutionary theories. For 
 example, all vertebrates have the same series of bones 
 arranged in the same way in their limbs, but there is great 
 variation in proportions. It is difficult to explain why 
 such diverse organs as a frog's leg, a whale's flipper, a 
 bird's wing, and a horse's foreleg should show homology, 
 unless it is assumed that all vertebrates came from a com- 
 mon ancestral stock. 
 
 Physiology and Ecology. There are many facts in 
 physiology and in the relationships of animals that are 
 readily interpreted by evolutionary theory. The functional 
 activity of the nervous systems shows steady progress 
 from the simpler to more complex animals. Starting in 
 sponges with separate and independent effectors, simple 
 reflexes are established through receptors and effector cells, 
 there is combination and coordination of such reflexes by 
 adjusters, and finally the more or less complete domination 
 of primitive effectors and receptors by a central nervous 
 mechanism. 
 
 It is hard to believe that such intricate relations as exist 
 between parasite and host have been thus from the begin- 
 ning of things. If, however, we assume that a liver fluke 
 (Fig. 74) was first a free-living aquatic worm somewhat like 
 a planarian (Fig. 73), that it gradually took up a parasitic 
 life during succeeding generations and developed the 
 ability to live and multiply during larval stages in the 
 abundant nourishment provided by a snail's body, the 
 case is much more understandable. 
 
 Experimental Evolution. To any thoughtful person the 
 proof of evolution would seem to lie in experiments and 
 many attempts have been made to secure positive evidence 
 in this way, but until the past few years there has been 
 little that is satisfying. Recent investigators have con- 
 tributed some valuable facts as to how animals vary and 
 how particular hereditary characters are transmitted from 
 
348 GENERAL ZOOLOGY 
 
 generation to generation. "In this case and throughout 
 all consideration of ' evidences/ it must be remembered 
 that the evolution idea cannot be logically demonstrated. 
 It is not a simple induction from particulars, thoroughly as 
 particulars support it. It is a way of looking at the becom- 
 ing of things; and it is the only scientific modal interpreta- 
 tion that has been suggested. It is a formula that fits the 
 facts, and all the facts it fits are its ' evidences. " The 
 most recent discoveries resulting from experimental methods 
 will be discussed after the chief evolutionary theories have 
 been stated. 
 
 THEORIES OF EVOLUTION 
 
 Though ancient Greek scholars (Aristotle, Lucretius) 
 and others in later times (Buffon, St. Hilaire) had vague 
 evolutionary ideas, the formulation of the two leading 
 modern theories was due to Lamarck and Darwin. 
 
 Lamarck's Theory. In the author's own words 
 "Changes in environment bring about changes in the 
 habits of animals. Changes in their wants necessarily 
 bring about changes in their habits, the new habits involve 
 the use of new parts, or a different use of old parts, which 
 results finally in the production of new organs and the 
 modification of old ones." The central idea of this theory 
 is the cumulative transmission of functional modifications- 
 use or disuse makes a particular part of an animal increase or 
 degenerate through successive generations. Lamarck fur- 
 ther affirmed, "Nature preserves everything that she has 
 caused the individual to acquire or lose by the influence of 
 the circumstances to which the race has been for a long time 
 exposed, and consequently by the predominant use of a 
 certain organ (or in consequence of its continued disuse). 
 She does this by a generation of new individuals which are 
 produced with the newly acquired organs. This occurs, 
 provided that the acquired changes were common to the 
 two sexes, or to the individuals that produced the new 
 forms." 
 
EVOLUTION AND HEREDITY 349 
 
 There is no doubt that the use of particular parts makes 
 them larger or more effective and that disuse makes them 
 poorly developed and comparatively ineffective. The 
 blacksmith's arm is strong; the banker's sensitive finger 
 tips detect a spurious coin at once; the sedentary business 
 man cannot run like a savage, and the provincial peasant 
 cannot acquire broad knowledge. The chief weakness in 
 Lamarck's theory lies in the fact that such "acquired 
 characters" are not known to be transmitted in heredity. 
 The banker's children are as easily deceived by bad money 
 as was the banker before he began his training; the son of 
 the blacksmith can have powerful arms only by constant 
 exercise. In spite of this difficulty the theory, while not so 
 successful as the next, still has many warm advocates. 
 
 Darwin's Theory. Darwin's conception of evolution 
 was that all animals and plants are subject to (1) small 
 chance variations ; thus certain individuals have a slight ad- 
 vantage over others in the (2) struggle for existence. This 
 leads to (3) natural selection by a (4) survival of the fittest, 
 and in time to the origin of new species. Darwin's theory 
 is commonly known as " Natural Selection." Though it 
 did not wholly discard Lamarck's ideas as to the trans- 
 mission of acquired characters, selection was its important 
 principle. It has been modified and supplemented some- 
 what since Darwin's time, but its chief features have stood 
 criticism remarkably well. Briefly stated, the principal 
 objections urged against natural selection are as follows: 
 (1) that it does not explain how variations that are not use- 
 ful from the beginning may be selected; (2) that variations 
 in some cases at least are most apt to take place when the 
 struggle for existence is not severe; (3) that it will not ac- 
 count for the perfection in detail which many structures 
 have attained (e.g. the wings of insects that are remarkably 
 like leaves, etc.) ; (4) that many apparently useless struc- 
 tures have developed along progressive lines; (5) that the 
 range of variation is not modified by selection; (6) that 
 chance variations cannot be the sole materials on which 
 
350 GENERAL ZOOLOGY 
 
 selection must work to produce new types. These and 
 others that might be cited, however, in no way invalidate 
 the theory as a whole, and it is looked upon by the majority 
 of scientific men as the best working hypothesis we have. 
 
 The following concrete illustration may serve to compare 
 the differences between the two theories. From palseon- 
 tological evidence it is known that the modern horse de- 
 scended from ancestors which had several toes on each foot. 
 According to the Lamarckian hypothesis the one-toed con- 
 dition probably came about through use and disuse. In 
 order to run more swiftly horses stood on the tips of the 
 toes; the middle digit was used most and hence grew larger 
 through successive generations; the toes on either side 
 dwindled away with disuse. A Darwinian explanation of 
 the same facts would suppose that ancestral horses varied 
 among themselves; that those with larger middle toes were 
 able to run faster and hence survived more often in the 
 struggle for existence; that the middle toes were progress- 
 ively larger and the side toes smaller because such condi- 
 tions better fitted the environment. According to the 
 first explanation the habit of using the middle toe and 
 neglecting those on the side led to a one- toed condition; 
 according to the second, the same end was attained by the 
 selection by the environment of variations which made 
 horses better fitted to survive in the struggle for existence. 
 
 VARIATION AND HEREDITY 
 
 Since the scientific world accepted the facts of evolution 
 and had theories as to how the transformations came about, 
 attention has naturally been directed to discovering how ani- 
 mals actually do vary and just how characteristics are trans- 
 mitted from one generation to another. Variations have 
 been carefully studied by statistical methods; thousands 
 of individual animals have been measured and compared. 
 Tests have also been made by breeding experiments to 
 determine how particular characteristics are transmitted 
 
EVOLUTION AND HEREDITY 351 
 
 from parent to offspring. Though all the problems have 
 not been solved, the chief kinds of variations have been 
 determined and some fundamental laws of heredity have 
 been worked out. 
 
 Fluctuating Variations and Mutations. Some character- 
 istics vary according to the laws of chance on either side of 
 an average condition. For example, the height of man is 
 about five feet eight inches and it has been determined by a 
 large number of measurements that the farther men depart 
 in stature from this average, the fewer they are in numbers. 
 There are many men whose height is five feet seven inches or 
 five feet ten, but those that measure seven or five feet are 
 rare. Such departures from the average of a species are 
 called indeterminate, fluctuating, or " chance" variations. 
 Sharply separated from these are those variations which 
 show no gradations, but are clearly differentiated in each 
 individual. The short-legged Ancon sheep appeared sud- 
 denly in stock which had previously possessed legs of normal 
 type. When Ancons are bred with other sheep the off- 
 spring have long or short legs; there are no intermediate 
 conditions. Such new variations which hold their own in 
 heredity are commonly called mutations. 
 
 Orthogenesis. Where variation proceeds in a particular 
 direction through succeding generations it is said to be ortho- 
 genetic. Eimer was the first to point out that many varia- 
 tions do not follow the laws of chance but run in particular 
 grooves. This discovery has been of great value in the 
 understanding of certain points which were not readily 
 explained by natural selection, such as cases of mimicry 
 of one species by another and the very elaborate color 
 patterns developed by some species. It makes the broad 
 differences between groups of animals more easy to under- 
 stand: e.g., why the insects always have three pairs of 
 legs; the spiders, four; and the vertebrates, two. 
 
 Somatic and Germinal Variations. Weismann has made 
 a distinction between variations which occur in the bodies 
 of metazoans and those which influence their germ cells. 
 
352 GENERAL ZOOLOGY 
 
 According to him the former are not inherited but the latter 
 may lead to marked changes in succeeding generations. 
 Weismann's Germ Plasm theory lays the whole responsi- 
 bility for heredity on the germ cells; they are the " vehicle 
 of inheritance" (Fig. 116). The body (soma) of each 
 individual, though carrying the germ cells, is to be looked 
 upon, from this point of view, only as a by-product of them. 
 Each generation of germ cells produces enough soma 
 (which includes all muscle, nerve, gland even the mind 
 and consciousness) to enable it to survive in the struggle 
 for existence. But the germ cells control every detail of 
 the body, and must be modified before there can be heredi- 
 tary changes in the species as a whole. Nothing in the 
 
 -Body 
 
 
 FIG. 116. Diagram illustrating germinal continuity. Through a series of 
 divisions a germ-cell gives rise to a body, or soma, and to new germ cells. The 
 latter, not the body, give rise to the next generation. (From Guyer's Being 
 W ell-Born, Copyright, 1916. By special permission of the publishers, The 
 Bobbs-Merrill Company.) 
 
 body can be inherited. All somatic qualities are caused 
 by some modification of the germ cells, which reproduce 
 all the " hereditary" qualities by which particular species 
 are recognized in each generation. The soma dies at 
 the end of each generation but there is no break in " con- 
 tinuity of the germ plasm." 
 
 Blending Inheritance. Animals produced from a zygote 
 formed by the union of two germ cells partake equally 
 from each parent in many of their characteristics. A large 
 rabbit, for example, when bred with a small one usually 
 produces offspring of intermediate size. In many such 
 cases of supposed blending, however, the succeeding gener- 
 ations do not retain the intermediate or blending condition 
 but are separable into several groups. The increasing 
 number of such " exceptions" has led some investigators to 
 
EVOLUTION AND HEREDITY 
 
 353 
 
 believe that there is no such thing as the perfect blending of 
 hereditary characters. 
 
 Mendel's Law. There are many qualities which do not 
 " blend" but are inherited as "unit characters" that is, 
 they are present or absent, or show one of two alternative 
 conditions without intermediates. Such cases follow Men- 
 del's Law, which supposes (1) unit characters which are 
 (2) usually dominant or recessive and which are (3) in- 
 herited according to the laws of chance. Dominant and re- 
 cessive characters may be present in a single individual 
 but in such instances the former usually masks the latter 
 
 
 
 
 
 FIG. 117. Diagram showing the scheme of inheritance in guinea pigs when 
 black and albino forms are crossed. (From Guyer's Being Weil-Born, Copy- 
 right, 1916. By special permission of the Publishers, The Bobbs-Merrill Co.) 
 
 so that only the dominant character is shown. In other 
 cases the unit characters may produce an apparent " blend" 
 in the first hybrid generation but later separate out and 
 occur in later generations as they were in the original 
 parents. Guinea pigs, for example, may possess a colored 
 coat or be albinos with white hair, and pink eyes. The pig- 
 mented condition is dominant to the albino. Therefore, 
 if a black guinea pig (Fig. 117) is crossed with an albino the 
 offspring in the first generation will all be black. These 
 individuals are hybrids which possess both black and albino 
 qualities, but show only the former because it is dominant. 
 
354 GENERAL ZOOLOGY 
 
 In the next generation (F 2 ) however, the characters separate 
 out according to the laws of chance and three different 
 classes of individuals are produced in the following ratios: 
 
 1 BLACK: 2 BLACK (ALBINO): 1 ALBINO 
 
 The first and third classes will " breed true." If an in- 
 dividual from either of these is bred with another like itself, 
 nothing but black or albino offspring will result. But the 
 "black (albino) 7 ' hybrids if bred together (F 2 , F 3 ) will 
 always separate out into three characteristic classes 1 
 pure black; 2 black (albino); 1 pure albino. 
 
 Many pairs of dominant and recessive characters are 
 known. Among these the following may be mentioned: 
 brown and blue eyes in man; beardlessness and beard in 
 wheat; rough coat and smooth coat in guinea pigs ; normal 
 condition and peculiar waltzing condition in Japanese 
 waltzing mice. In each of the instances just mentioned 
 the dominant precedes the recessive condition. There are 
 other pairs of characters in which one does not dominate the 
 other, but there is always segregation of the characters in 
 the germ cells and chance for recombination after fertili- 
 zation. It is the segregation, or separating out of unit 
 characters, therefore, which is the important thing in 
 Mendelian inheritance. 
 
 Bearers of Heredity. In heredity particular qualities 
 are not passed from parent to offspring but something is 
 transmitted which determines what the character of the 
 offspring shall be. A guinea-pig, for example, develops 
 from a fertilized egg and is nourished by its mother during 
 development. There is no white or black hair in the 
 egg and there is nothing in the nourishment supplied by the 
 mother that makes one or the other develop. Nevertheless 
 there is something in the egg that makes black or white hair 
 develop from the material supplied to the growing embryo. 
 Evolutionists have given up the old idea that the fertilized 
 egg contained a miniature but complete animal which simply 
 enlarged its parts during development. There is not only 
 
EVOLUTION AND HEREDITY 355 
 
 transformation of food material, but changes in structures 
 as well. This has led scientific men to search for deter- 
 miners of one kind or another. Darwin believed that there 
 were particles (gemmules), of as many varieties as there are 
 categories of cells in an organism, which migrated to all 
 the cells during development and determined their specific 
 type. There were serious objections to such a view, but 
 other evolutionists have postulated somewhat similar deter- 
 miners (micellae, pangens, etc.). There is no definite 
 evidence, however, in favor of any such arrangement. 
 
 The only positive evidence that indicates bearers of 
 hereditary qualities points chiefly to the chromosomes. 
 The fact that the chromosomes are divided with such care 
 during mitosis indicates that their accurate separation is 
 important. Biologists have long believed that this was an 
 indication that they were the carriers of hereditary char- 
 acteristics, and during the past few years Professor Morgan 
 believes that he has secured evidence by breeding fruit flies 
 (Drosophila ampelophila) which enables him to assign 
 particular characteristics to each of the chromosomes known 
 to be present in the germ cells. One chromosome, for exam- 
 ple, apparently carries the factors for bent wings and eye- 
 lessness; another, those for banded body, dwarfing, white 
 color of head, and at least twenty others; the two remain- 
 ing chromosomes are believed to carry twenty-eight and 
 forty-seven factors respectively. If chromosomes are the 
 sole bearers of hereditary qualities, those of the fruit fly 
 doubtless carry many more than have as yet been assigned 
 to them. But Professor Morgan speaks with enough assur- 
 ance concerning the behavior of those he has tested by 
 experiment to be able to assign characters to particular 
 positions on the chromosomes. 
 
 CONCLUDING REMARKS ON EVOLUTION 
 
 Morgan* says: "Today the belief that evolution takes 
 place by means of natural processes is generally accepted. 
 
 * "A Critique of the Theory of Evolution," 1916. 
 
356 GENERAL ZOOLOGY 
 
 It does not seem probable that we shall ever again have 
 to renew the old contest between evolution and special 
 creation. 
 
 "But this is not enough. We can never remain satisfied 
 with a negative conclusion of this kind. We must find out 
 what natural causes bring about variations in animals and 
 plants; we must also find out what kinds of variation are 
 inherited, and how they are inherited. If the circumstan- 
 tial evidence for qrganic evolution, furnished by comparative 
 anatomy, embryology and palaeontology is cogent, we 
 should be able to observe evolution going on at the present 
 time, i.e., we should be able to observe the occurrence of 
 variations and their transmission. This has actually been 
 done by the geneticist in the study of mutations and Men- 
 delian heredity." 
 
 Delage and Goldsmith in their "Theories of Evolution" 
 (1913) make the following statements: 
 
 "Two important phenomena mark the evolution of the 
 organic world; one is the appearance of the different species, 
 the differentiation of the various groups recognized in the 
 classification of animals and plants, the increasing com- 
 plexity of organisms, their evolution from the lowest to the 
 highest forms; the other is the adaptation of living things to 
 the conditions and necessities of their environment. The 
 two processes, while simultaneous, differ entirely in their 
 natures and are never superimposed." 
 
 Delage and Goldsmith are supporters of Lamarck's 
 theory and therefore attach special importance to the trans- 
 mission of characters acquired through environmental in- 
 fluences. Morgan may be looked upon as the most recent 
 supporter of the natural selection theory, though his defini- 
 tion of "selection" is different from that given by Darwin. 
 To the Lamarckian, adaptation to environment is of equal 
 importance with evolution; to the selectionist, adaptation is 
 the natural result of evolution. There is no question that 
 animals have followed certain more or less arbitrary lines 
 during their evolution on account of the inflexibility of 
 
EVOLUTION AND HEREDITY 357 
 
 heredity and they have also shown increasing specialization 
 to adapt them to environmental conditions. The Lamarck- 
 ians maintain that animals acquire characters in their 
 reactions to environment, that these are transmitted, and 
 hence that changing environmental relations cause evolution 
 and adaptation. Darwinians claim that there has been 
 evolution because the environment has selected variations, 
 and that there is adaptation because the variations which 
 best fit the environment are selected. For the former view 
 there is little or no evidence; for the latter there is increasing 
 support from experiment and observation. 
 
 Professor Morgan in his recent book proposes a some- 
 what new method of selection as the basis of evolution. He 
 believes that selection does not operate on small fluctuating 
 variations but on numerous small mutations. 
 
 "Evolution of wild species appears to have taken place 
 by modifying and improving bit by bit the structures and 
 habits that the animal or plant already possessed. Evolution 
 from this point of view has consisted largely in introducing 
 new factors that influence characters already present in the 
 animal or plant. ... In other words, the emphasis may be 
 placed less on the competition between the individuals of a 
 species (because the destruction of the less fit does not in 
 itself lead to anything that is new) than on the appearance 
 of new characters and modifications of old characters that 
 become incorporated in the species, for on these depends 
 the evolution of the race. 
 
 " The mechanism of heredity has, I think, been discovered 
 discovered not by a flash of intuition but as the result of 
 patient and careful study of the evidence itself. With the 
 discovery of this mechanism I venture the opinion that the 
 problem of heredity has been solved. We know how the 
 factors carried by the parents are sorted out to the germ 
 cells. The explanation does not pretend to state how fact- 
 ors arise or how they influence the development of the 
 embryo. ... So, I repeat, the mechanism of the chromo- 
 
358 GENERAL ZOOLOGY 
 
 somes offers a satisfactory solution of the traditional problem 
 of heredity." 
 
 There is perhaps some question as to whether hereditary 
 characters may be modified by selection. That is, whether 
 selection in a given direction may cause variation in the 
 same direction. The bulk of the evidence, however, ap- 
 pears to indicate that such is not the case. According to 
 Morgan, then: 
 
 " Evolution has taken place by the incorporation into the 
 race of those mutations that are beneficial to the life and 
 reproduction of the organism. Natural selection as here 
 defined means both the increase in the number of individuals 
 that results after a beneficial mulation has occurred (owing 
 to the ability of living matter to propagate) and also that 
 this preponderance of certain kinds of individuals in a 
 population makes some further results more probable than 
 others. More than this natural selection cannot mean, if 
 factors are fixed and not changed by selection." 
 
 The question of evolution must be left in this state: the 
 fact that there has been an evolution is proved, the methods 
 by which evolution takes place have during the past few 
 years become pretty well known, but the causes of variation 
 and heredity are as yet largely unknown. 
 
INDEX 
 
INDEX 
 
 Acanthocephala, 188 
 
 Acarina, 131 
 
 Acquired characters, 349 
 
 Acridinae, 76, 77 
 
 Adaptation, 36 
 
 Adephaga, 114 
 
 ^Epyornis, 271 
 
 Agassiz, 11 
 
 Albatross, 272 
 
 Alligator, 266 
 
 Alternation of generations, 168, 
 
 183, 188 
 Amitosis, 42 
 Amnion, 288 
 Amoeba, 135 
 Amphibia, 246 
 Amphioxus, 228, 231 
 Amphipoda, 58, 59 
 Ancestor of arthropods, 45 
 Anemone, 172 
 Annelida, 203 
 Ant lion, 101 
 Antelope, 295 
 Anthozoa, 172 
 Antimere, 17 
 Ants, 119 
 Ape, 293, 294 
 Aphids, 96 
 Aphis lion, 101 
 Apoda, 246 
 Aptera, 69 
 Arachnida, 123 
 Archseopteryx, 270 
 Archseornithes, 269 
 Aristotle, 6 
 Armadillo, 291 
 Arthrodires, 330 
 Arthropoda, 45 
 Artiodactyla, 295 
 Ascaris, 184 
 Asteroidea, 192 
 Aves, 269 
 
 Bark lice, 72 
 Barnacle, 58, 61 
 Bats, 291 
 
 Bearers of heredity, 354 
 Bears, 292 
 Beaver, 292 
 Bed-bug, 100 
 Bee, 119 
 Beetle, 113 
 
 Bilateral symmetry, 17, 177 
 180, Binary fission, 137, 144, 145 
 Biogenesis, 60, 344 
 Biramous appendage, 45, 57 
 Bird lice, 72 
 Birds, 269, 280 
 Bittern, 272 
 Bladder worm, 181 
 Blending inheritance, 352 
 Body cavity, 154, 183, 186 
 Book lice, 72 
 Bot-fly, 110 
 Brachiopoda, 190 
 Brachycera, 109 
 Brain, 312 
 Brittle starfish, 199 
 Bryozoa, 191 
 Bufo, 251 
 Bug, 95, 98 
 Butterfly, 103, 104 
 
 Caddis-fly, 102 
 Carabidae, 114 
 Carbohydrate, 30 
 Carnivora, 292 
 Cats, 292 
 Cattle, 295 
 Caudata, 246, 250 
 Cecidomyiidse, 109 
 Cell, 15, 37 
 Cell-division, 40 
 Cell theory, 43 
 Centipede, 65 
 361 
 
362 
 
 INDEX 
 
 Cephalization, 156, 177, 228 
 
 Cephalochorda, 228 
 
 Cephalopoda, 220 
 
 Cercaria, 180 
 
 Cestoidea, 180 
 
 Cetacea, 296 
 
 Chsetopoda, 203, 208 
 
 Chalcid-fly, 118 
 
 Chelonia, 264 
 
 Chilopoda, 65 
 
 Chiroptera, 292 
 
 Choanocyte, 157, 158 
 
 Chordata, 224 
 
 Chromatin, 39 
 
 Chromosomes, 40, 42 
 
 Cicada, 98 
 
 Ciliates, 142 
 
 Circulatory system, 286, 302 
 
 Civilization, 323 
 
 Clam, 217 
 
 Classification, 13, 15, 346 
 
 Clavicornia, 115 
 
 Cleavage, 153 
 
 Closed blood system, 206, 215, 238 
 
 Cockroach, 72 
 
 Ccelenterata, 163, 172 
 
 Coelom, 18, 152, 154, 186 
 
 Coleoptera, 113 
 
 Colloid, 29 
 
 Conjugation, 137, 144, 145 
 
 Coral, 172 
 
 Crab, 58, 59 
 
 Crayfish, 50 
 
 Cricket, 74 
 
 Crinoidea, 202 
 
 Crocodilia, 257, 265 
 
 Crustacea, 50, 57 
 
 Ctenophora, 173 
 
 Cuckoo, 273 
 
 Culicidse, 106 
 
 Cuvier, 10 
 
 Cyclops, 58, 61 
 
 Cyclostomata, 231 
 
 Cypris, 58, 61 
 
 Damsel-fly, 70 
 Darwin, 10, 349 
 Decapoda, 59 
 
 Deer, 295 
 
 Definitions, 5 
 
 Dermestidse, 115 
 
 Devil-fish, 220 
 
 Didelphia, 289 
 
 Digestion, 300 
 
 Diploblastic, 16 
 
 Diplopoda, 64 
 
 Dipnoi, 235 
 
 Diptera, 106 
 
 Diseases, 111, 139, 140, 141, 179, 
 
 181, 186, 304 
 Dobson-fly, 101 
 Dogs, 292 
 
 Domesticated birds, 284 
 Dragon-fly, 70 
 Duck, 273 
 Duck-bill, 289 
 Dugong, 296 
 Dytiscidae, 114 
 
 Earthworm, 18, 204, 224 
 Echinodermata, 192 
 Echinoidea, 200 
 Ecology, 6, 347 
 Edentata, 291 
 Elasmobranchii, 232 
 Elephants, 295; fossil, 336 
 Embryology of man, 307 
 Endoskeleton, 18 
 Enteropneusta, 226 
 Enzymes, 300 
 Ephemerida, 69 
 Eumeces, 258 
 Eutheria, 289 
 
 Evolution, 6, 342, 348, 355, 358 
 Excretion, 35, 303 
 Exoskeleton, 18, 19 
 
 Fsoces, 35 
 Fasciola, 179 
 Ferments, 300 
 Fertilization, 153 
 Fishes, 235, 242 
 Fission, 137, 144 
 Fissipedia, 292 
 Flagellates, 139, 140 
 Flame cell, 176 
 
INDEX 
 
 363 
 
 Flat worms, 174 
 
 Fleas, 112 
 
 Fluctuating variations, 351 
 
 Flukes, 178 
 
 Fly, 106, 109, 111 
 
 Fossil reptiles, 267 
 
 Fossils, 328 
 
 Frogs, 255 
 
 Galen, 7 
 Gall-fly, 117 
 Gall-gnat, 109 
 Gastropoda, 213, 216 
 Gastrula, 152, 153, 165 
 Geographic Distribution, 343 
 Germ layers, 16, 153 
 Germinal variations, 351 
 Gill arches, 224, 225 
 Gill slits, 224, 225 
 Glochidium, 218 
 Goose, 273 
 Gordiacea, 187 
 Grasshopper, 73, 76, 79 
 Grebe, 272 
 Gull, 273 
 Gyrinidae, 114 
 
 Hagfish, 231 
 Harvey, 9 
 Hawk, 273 
 Hemiptera, 95 
 Heredity, 354, 357 
 Hermaphrodite, 150, 177, 207 
 Heterocera, 103 
 Heteromera, 116 
 Heteroptera, 98 
 Hibernation, 253 
 Hirudinea, 211 
 Holothuroidea, 201 
 Homoptera, 96 
 Hook-headed worms, 188 
 Hookworm, 186 
 Horse, 295 
 Horse-fly, 109 
 Horsehair worms, 187 
 Horses, fossil, 339 
 Huxley, 12 
 Hydra, 16, 18 
 
 Hydrophilidae, 115 
 Hydrozoa, 163 
 Hymenoptera, 116 
 
 Infusoria, 142 
 Insecta, 66 
 Insectivora, 291 
 Isopoda, 58, 59 
 Isoptera, 72 
 
 Kangaroo, 291 
 Katydid, 73 
 Kidneys, 230 
 Kiwi, 271 
 
 Lamarck, 348 
 
 Lamellibranchiata, 217 
 
 Lamellicornia, 115 
 
 Lamprey, 231 
 
 Lancelet, 228 
 
 Law of Biogenesis, 60, 251, 255, 344 
 
 Law of Recapitulation, 60, 251, 255, 
 
 344 
 
 Leech, 16, 211 
 Lemurs, 293 
 Lepidoptera, 102 
 Lice, 95 
 Life, 25 
 Life cycle, 35 
 Linnaeus, 9 
 Living substance, 28 
 Living things, 33 
 Lizard, 258, 261 
 Locust, 73, 76, 79 
 Loon, 272 
 Lumbricus, 204 
 
 Malaria, 108, 141 
 Mammalia, 286 
 Man, 293, 294, 297 
 Manitee, 296 
 Mantis, 74 
 Mantle, 213 
 Marsupial pouch, 288 
 Marsupial ia, 290 
 Marsupium, 218 
 Mastigophora, 139 
 Maturation, 151 
 
364 
 
 INDEX 
 
 May-fly, 69 
 
 Mechanism, 30 
 
 Medusa, 17, 163, 165, 168, 170 
 
 Melanopus, 79 
 
 Mendel's Law, 353 
 
 Mesozoic Age, 332 
 
 Metabolism, 34, 35 
 
 Metagenesis, 168, 169, 171, 172 
 
 Metamerism, 16, 156, 203, 208, 222 
 
 Metamorphosis, 68, 104 
 
 Metapleural folds, 228 
 
 Metazoa, 147 
 
 Midge, 108 
 
 Migration, bird, 283 
 
 Migration, fish, 244 
 
 Milliped, 64 
 
 Mind of man, 311 
 
 Mite, 131 
 
 Mitosis, 40 
 
 Mole, 291 
 
 Mollusca, 213, 222 
 
 Monkey, 293 
 
 Monodelphia, 289 
 
 Monotremata, 289 
 
 Morphology, 6, 346 
 
 Mosquito, 106 
 
 Moth, 103, 105 
 
 Moulting, 93 
 
 Mud-puppy, 247 
 
 Muscidse, 111 
 
 Mussel, 218, 220 
 
 Mutations, 351 
 
 Myriapoda, 64 
 
 Natural selection, 11, 349 
 Necturus, 247 
 Nematocyst, 166, 167 
 Nematoda, 4, 183, 186 
 Nematoidea, 183 
 Nemertina, 182 
 Neornithes, 269, 270 
 Nests, bird, 284 
 Nests, fish, 243 . 
 Neuroptera, 100 
 Nutrition, 298 
 
 Obelia, 163 
 Octopus, 220 
 
 Odonata, 70 
 (Edipodinse, 76, 77 
 (Estridse, 110 
 Oligochseta, 210 
 Ontogeny, 344 
 Ontogeny of metazoa, 151 
 Onycophora, 63 
 Open blood system, 81, 206 
 Ophiuroidea, 199 
 Opossum, 290 
 Organic substances, 30 
 Organs, 149, 155, 169 
 Origin of earth, 327 
 Origin of life, 25 
 Origin of man, 314 
 Orthogenesis, 351 
 Orthoptera, 72 
 Ostracoderms, 329 
 Ostrich, 272 
 xOwl, 273 
 Oyster, 194, 218, 219 
 
 Psedogenesis, 250 
 
 Paired appendages, 45, 47, 57, 128, 
 228, 229 
 
 Palseozoology, 5, 340 
 
 Parazoa, 160 
 
 Parthenogenesis, 150 
 
 Passeres, 274 
 
 Pasteur, 27 
 
 Pearls, 219 
 
 Pedicellaria, 195 
 \ Pelican, 272 
 ^Penguin, 271 
 
 Pentadactyl limb, 229 
 
 Perch, 235 
 
 Perching birds, 274 
 
 Peripatus, 63 
 
 Perissodactyla, 295 
 
 Petromyzon, 231 
 
 Phalangida, 130 
 
 Philidota, 293 
 
 Phyla of animals, 19, 21, 24 
 
 Physa, 213 
 
 Physiology, 6, 347 
 
 Phytophaga, 115 
 
 Pigs, 295 
 
 Pinnipedia, 292 
 
INDEX 
 
 365 
 
 Pisces, 235 
 
 Placenta, 288 
 
 Placoid scale, 232, 233 
 
 Planaria, 16, 174 
 
 Planesticus, 274 
 
 Platyhelmia, 174 
 
 Plecoptera, 71 
 
 Pliny, 7 
 
 Poisonous snakes, 263 
 
 Polychaeta, 210 
 
 Polymorphism, 72, 119, 165, 169, 172 
 
 Polyp, 163, 165, 166, 170, 172 
 
 Porcupine, 293 
 
 Porifera, 157 
 
 Primates, 293 
 
 Proboscidia, 295 
 
 Proglottid, 180 
 
 Protective coloration, 84, 283 
 
 Protein, 30 
 
 Protoplasm, 28 
 
 Prototheria, 289 
 
 Protozoa, 134, 145 
 
 Quaternary age, 336 
 
 Rabbit, 292 
 
 Radial symmetry, 17, 19 
 
 Radula, 214, 217, 221 
 
 Hat, 293 
 
 Rays, 232 
 
 Recapitulation Theory, 60 
 
 Recognition colors, 91 
 
 Redi, 26 
 
 Redia, 179 
 
 Regeneration, 53, 86, 159, 176, 196, 
 
 200, 202, 206, 240, 248, 305 
 Rejuvenescence, 146 
 Reproduction, 36, 145, 150 
 Reptilia, 257, 266 
 Respiration, 67, 68, 82, 302 
 Retrogressive metamorphosis, 62, 
 
 228 
 
 Rhinocerus, 295 
 Rhopalocera, 103 
 Rhynchocephalia, 257 
 Rhynchophora, 116 
 Robin, 274 
 Rodentia, 292 
 
 Rotifera, 188 
 Round-worms, 183 
 Ruminant, 295 
 
 Salamanders, 250 
 
 Salientia, 246, 251, 255 
 
 Sarcodina, 135 
 
 Sauria, 257, 261 
 
 Saw-fly, 116 
 
 Scale insects, 97 
 
 Scallop, 223 
 
 Scolex, 180 
 
 Scorpion-fly, 102 
 
 Scorpionida, 130 
 
 Scyphozoa, 170 
 
 Sea-cucumber, 201 
 
 Sea-squirt, 226 
 
 Sea-urchin, 200 
 
 Seal, 292 
 
 Sense organs, 79, 80, 205, 206, 244 
 
 Serpentes, 257, 261 
 
 Serricornia, 115 
 
 Seta, 203 
 
 Sharks, 232 
 
 Ship worm, 219 
 
 Shrew, 291 
 
 Silkworm, 105 
 
 Siphonaptera, 112 
 
 Sirenia, 296 
 
 Skeleton, 18, 19; vertebrate, 229 
 
 Skink, 258 
 
 Sleep, 303 
 
 Sloth, 291 
 
 Slug, 16 
 
 Snail, 213 
 
 Snake, 262 
 
 Snake star, 199 
 
 Snipe, 273 
 
 Society, 320 
 
 Soma, 140, 145, 148, 352 
 
 Somatic variations, 351 
 
 Songs, bird, 283 
 
 Specialization, 47 
 
 Spicules, 158 
 
 Spider, 123, 124, 129 
 
 Sponge fishery, 161 
 
 Sponges, 157, 158, 169 
 
 Spontaneous generation, 25 
 
366 
 
 INDEX 
 
 Sporocyst, 179 
 Sporozoa, 141 
 Squamata, 257, 261 
 Squid, 220 
 Squirrel, 293 
 Starfish, 192, 197 
 Stegocephalians, 331, 332 
 Sting ray, 234 
 Stone-fly, 71 
 Stone-lily, 202 
 Stork, 272 
 Symmetry, 197 
 Symphyla, 65 
 
 Tabanidse, 109 
 Tapeworm, 180 
 Tapir, 295 
 Teeth, 234, 287 
 Teleostomi, 235 
 Teredo, 219 
 Termite, 72 
 Tern, 273 
 Tertiary Age, 334 
 Testudinata, 257, 264 
 Tettiginse, 76 
 Threadworms, 183 
 Thrips, 72 
 Tick, 132 
 Tipulidse, 106 
 Tissues, 148 
 Toad, 251 
 Torpedo, 234 
 
 Tortoise, 264 
 Tracheae, 68 
 Tree frogs, 256 
 Trematoda, 178 
 Trichina, 187 
 Trichoptera, 102 
 Trilobites, 328, 329, 330, 332 
 Trimera, 116 
 Triploblastic, 16 
 Trochophore, 190 
 Tryaxalinae, 76, 77 
 Tube-foot, 192 
 Tunicata, 226 
 Turbellaria, 174 
 Turtle, 264 
 
 Variation, 350 
 Vertebrata, 228, 230 
 Vesalius, 8 
 Vitalism, 30 
 Vorticella, 143 
 
 Walking-stick, 74 
 Walrus, 292 
 Wasp, 118 
 Weismann, 351 
 Whale, 296 
 White ant, 72 
 
 Zoogeography, 5 
 Zoology, 5 
 
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