THE ANIMAL BODY 135 



ment of higher forms. The Earthworm is of particular 

 value since it illustrates bilateral symmetry, an alimentary 

 canal opening to the exterior by an anterior mouth and a 

 posterior anus, metameric segmentation, coelom, definite 

 organ systems for various functions, and, finally, the part 

 played in development by the mesoderm. The Crayfish 

 shows, in simple form, certain general principles underlying 

 the modification of the Earthworm type, which involve the 

 specialization of various regions in connection with the change 

 of functions of the parts to fulfil more complex life conditions. 

 The reader, however, must be cautioned against supposing 

 that there is a sort of progression through all the series of 

 lower animals up to the Vertebrates. We have selected from 

 the groups of Invertebrates certain types which illustrate 

 several of the fundamental structural principles which are 

 to be found in the Vertebrate body, but there are other In- 

 vertebrate groups that exhibit body plans which depart 

 widely from the types described. The consideration of the 

 morphology of the groups which comprise such forms as 

 the Tapeworms, Rotifers, Sea Urchins, Oysters, etc., would 

 but tend to obscure those principles which are requisite for a 

 proper interpretation of the structure and functions of the 

 Vertebrates, including Man. 



E. VERTEBRATES 



The Vertebrates form one of the most clearly defined divi- 

 sions of the animal kingdom and include all the larger and 

 more familiar animals Fishes, Amphibians, Reptiles, Birds, 

 and Mammals so that in the popular mind the words 

 animal and Vertebrate are essentially synonymous. (Figs. 

 82-87.) 



A Fish, as every one knows, is an aquatic backboned animal 

 which breathes by means of gills and moves by fins. An 



