THE ANIMAL BODY 117 



the more complex colonial Protozoa there are forms, as al- 

 ready noted, in which the various cells become organically 

 connected so that a primitive sort of body results, and, fur- 

 thermore, certain cells are set aside for reproduction. In other 

 words, cooperation involving a physiological division of labor 

 takes place between the individuals of a group of cells, and 

 this results in the establishment of an individual body of 

 somatic cells associated with germ cells. (Fig. 18.) 



The Metazoa proper may be divided into two large groups 

 known as INVERTEBRATES and VERTEBRATES. The former 

 group, frequently referred to as the lower animals, comprises 

 some five hundred thousand living species and exhibits an 

 enormous variety of form and complexity of structure ranging 

 from the Sponges and Hydroids to the Molluscs, Crustacea, 

 and Insects. On the other hand, the Vertebrates, or higher 

 animals, form a relatively homogeneous group of about thirty- 

 five thousand species, including the FISHES, AMPHIBIA, REP- 

 TILES, BIRDS, and MAMMALS. The Birds and Mammals in 

 contrast with all other animals are commonly referred to as 

 warm-blooded, because their body temperature is practically 

 constant and usually above that of their surroundings. 



The highly complicated and varied organization of animals 

 renders it impossible to present a concise and adequate plan 

 of a typical animal body, and it is therefore necessary in the 

 present work to select one group of animals as the basis of 

 study and then to compare with this, in so far as comparisons 

 are possible without confusion, a few of the most significant 

 morphological and physiological variations presented by 

 other groups. We naturally select the group of Vertebrates 

 for chief consideration not only because its relative homo- 

 geneity renders it the most available, but because it includes 

 Man. However, even before we focus attention on the 

 Vertebrates, it is necessary to make a brief preliminary sur- 



