24 THE ANIMAL ORGANISM 



food. Such series of dependencies are called food chains. In the 

 protoplasm of the animal these organic substances undergo 

 destruction, in consequence of which there are set free carbon 

 dioxide and simple nitrogen compounds. Thus plants provide 

 food and oxygen for animals, while animals, destroying this food, 

 provide simple nitrogen compounds and carbon dioxide for the 

 use of plants. Since the nitrogen compounds actually produced 

 by animals can mostly not be used by plants, the presence of 

 bacteria is necessary to change them to nitrate, the form in which 

 nitrogen is generally absorbed. 



ZOOLOGY : PLAN OF STUDY 



Biology comprises botany, which deals with plants, and zoology 

 which deals with animals. Now an organism may be regarded 

 from two points of view according as attention is concentrated 

 upon its structure or its functions, though of course these two 

 are so closely connected that it is impossible to study structure 

 intelligently or function at all without reference to the sister 

 topic. The sciences of zoology and botany are correspondingly 

 divided each into two subordinate sciences, anatomy or 

 morphology, which deals with the structure of the bodies of 

 organisms, and physiology, which deals with their functions. 

 Anatomy is sometimes used as a synonym for morphology, but 

 is often, especially by botanists, taken in a slightly more restricted 

 sense to mean the detailed study of structures, morphology 

 being restricted to general form. 



In this book we shall approach zoology chiefly from the 

 anatomical side, partly because our knowledge of the physiology 

 of animals in general is still fragmentary, but chiefly because 

 knowledge of physiology must of necessity be grounded in 

 previous knowledge of anatomy. Wherever possible we shall con- 

 sider the functions of the structures we find, but we shall avoid 

 imputing imaginary functions to structures which have not 

 been adequately studied. We shall begin with some of the 

 structurally simplest animals, and proceed through the animal 

 kingdom in what is now generally considered to be an order of 

 increasing complexity. In so doing we shall be following what, 

 according to the hypothesis of organic evolution, is roughly the 

 line which animal life has taken in its temporal development 

 through the ages. Before we consider the evidence for evolution in 



