COLLEGE ZOOLOGY 



CHAPTER I 

 INTRODUCTION 



i. GENERAL SURVEY OF THE ANIMAL KINGDOM 



ONE who is not a naturalist or who does not have access to 

 the apparatus necessary for the examination of minute objects 

 usually becomes acquainted with only a few of the many kinds 

 of animals that inhabit the earth. The most familiar of these 

 are the comparatively large four-footed beasts, the fish, the 

 frogs, the snakes, the birds, and the insects. The majority of 

 animals are never seen by most people, and perhaps never even 

 heard of. This is true of the microscopic parasite which is pres- 

 ent in the blood of malaria patients, of the coral polyp (Fig. 87) 

 which builds up entire islands in the sea, of the Trichinella 

 (Fig. 113), a parasitic worm which sometimes causes a human 

 disease called trichinosis, and of a host of others. 



Scientists have found it convenient to separate all animals 

 into two groups, the vertebrates and the invertebrates. The ver- 

 tebrates possess a backbone or vertebral column consisting of a 

 linear series of bones called vertebrae (Fig. 418); the inverte- 

 brates have no vertebral column. The vertebrates are better 

 known than the invertebrates, since they are usually large and 

 include most of the domesticated animals. The invertebrates, 

 however, are much more numerous both in regard to the number 

 of kinds and the number of individuals. Thus of the eleven main 

 groups (phyla) of animals recognized in the classification adopted 

 in this book only part of one group, the CHORDATA (Chap. XIV) , 

 deals with the vertebrates, whereas the rest of this group and 



