COLLEGE ZOOLOGY 



such as population growth, disease, the ef- 

 fects of radiation on hfe, and man's survival 

 in outer space. 



To provide a background for the study of 

 animal life, a brief consideration is given to 

 each of the following topics: 



1. The name and distinguishing characteristics 

 of each large group of animals. 



2. The features common to all animals, with 

 emphasis on the unity of animal life as 

 shown by the universal presence of the liv- 

 ing substance, protoplasm. 



3. Conditions under which animals live, habi- 

 tats. 



4. The value and method of classifying ani- 

 mals, classification. 



5. The scope of zoology. 



6. The scientific method and how it aids in 

 formulating scientific principles. 



7. The influence of zoology on intellectual 

 progress, and its practical value. 



VARIETY AND UNITY 

 OF ANIMAL LIFE 



Variety of animal life 



Everyone is familiar with many of our 

 common animals and knows something 

 about where and how they live; but few 

 people realize how many different kinds of 

 animals there are and how greatly they 

 vary in size, shape, structure, and habits. 

 It is easy to observe the larger types such 

 as cats, birds, frogs, and even some of the 

 smaller ones such as earthworms and flies, 

 but a considerable part of the animal king- 

 dom consists of forms so minute that they 

 can be seen only with the aid of the micro- 

 scope. Then there are forms that live in the 

 soil, in the ocean, or in other places where 

 we do not ordinarily see them. 



No one knows exactly how many different 

 kinds of animals there are now in existence, 

 but we do know that more than one million 

 have been described by zoologists. Fortu- 

 nately for us, although they differ from each 



other sufficiently to be recognized as dis- 

 tinct kinds (species), they possess char- 

 acteristics in common and can be arranged 

 in groups. The principal groups are called 

 phyla (singular, phylum). Zoologists are not 

 in agreement with respect to the number of 

 phyla into which the animal kingdom should 

 be divided, but usually 11 are studied in 

 some detail in a beginning zoology course. 

 Representatives of some of the phyla are 

 shown in Fig. 430. Besides these, there are a 

 few groups of animals of more or less uncer- 

 tain relationships such as the Rotifera and 

 Br}Ozoa. 



For each phylum, in the brief outline pre- 

 sented here, the approximate number of 

 known living species is given. Figure 1 shows 

 that the Arthropoda comprise about three- 

 fourths of all the species of animals. We 

 shall find later (Chap. 16, Fig. 130) that 

 about 97 per cent of the Arthropoda are in- 

 sects. Among the other phyla, the Mollusca 

 (snails, clams, etc.), Chordata (fish, birds, 

 mammals, etc.), and Protozoa (one-celled 

 animals) are the most numerous. The num- 

 bers given are estimates by specialists, but no 



Figure 1. There are approximately 1,116,300 

 known living species in the entire animal kingdom. 

 Of these, 875,000 or approximately 78 per cent are 

 arthropods, leaving 241,300 species to account for 

 the other animals. 



