Vi PREFACE 



student may realize the unity as well as the variety in animal 

 life. 



So far as possible in a limited space, the relations of the 

 animals to other animals, to plants, and to environmental 

 factors in general are considered, and the animals of special 

 economic importance are emphasized. By this method the 

 student is brought into closer contact with and gains a broader 

 idea of natural phenomena. Questions naturally arise in the 

 student's mind, such as, " Where does the animal live ? " " What 

 does the animal do ? " and " What is this or that particular 

 organ for? " and stimulate interest in the work leading to more 

 careful observations and more accurate inferences. 



Each phylum is introduced by a more or less complete account 

 of the anatomy, physiology, and ecology of one, or in certain 

 cases, two or more types. These types were selected with the 

 following requirements in mind : (i) they must represent as 

 nearly as possible an average of the phylum; (2) they must 

 illustrate clearly the characteristics of the phylum so as to serve 

 as an introduction to a comparative study of other members of 

 the group ; (3) they must be common native species which can 

 be obtained for direct observations in the laboratory ; (4) they 

 must occupy an important position in the animal series ; and 

 (5) they must be of special importance to man. Very few types 

 fulfill all of these requirements; in several cases two types have 

 been employed because one was not considered adequate. 



It is impossible in one small volume to describe as many 

 different animals under each phylum as might be desired, or 

 to give a full classification of each group. However, a general 

 idea of the various kinds of animals and their habitats can be 

 obtained from the short account included in each chapter. 

 The species mentioned are in most cases the commonest and 

 most representative of those living in North America. 



More space has been devoted to the Chordata than to any 

 other phylum, and the classes under the subphylum Verte- 

 brata have been treated in a somewhat different manner from 

 those of the invertebrates. It is customary in studying the 



