136 FOUNDATIONS OF BIOLOGY 



Amphibian may be thought of as a Fish which early in life 

 at the end of the tadpole stage discards its gills, devel- 

 ops lungs, substitutes five-toed limbs for fins, and takes up a 

 terrestrial existence. In the same general way, a Reptile may 

 be pictured as an Amphibian which has relegated, as it were, 

 the tadpole stage to the egg, and therefore emerges with limbs 

 and lungs. Birds and Mammals may be regarded as deriva- 

 tives of the reptilian stock which have transformed the scales 

 of the reptile into feathers and hair respectively, and have 

 developed a special care for their young; the Birds by incu- 

 bation of the eggs and the Mammals byretention of the young 

 essentially as parasites within the body of the female until 

 birth occurs. It will be appreciated, of course, that other 

 important characteristics some of which will be apparent 

 as we proceed delineate these chief Vertebrate groups; 

 but there is, in fact, less diversity in structure among the 

 Vertebrates as a whole than is present, for example, in the 

 one subdivision of the Arthropods, the Crustacea, of which 

 the Crayfish is a member. Accordingly we shall confine our 

 attention largely to a description of the structure and physi- 

 ology of an 'ideal' Vertebrate, and mention incidentally, so 

 far as possible, the chief modifications of general significance 

 which appear in the different groups. 



1. Body Plan 



The ideal Vertebrate body is more or less cylindrical in 

 form, and is bilaterally symmetrical with respect to a plane 

 passed vertically through the main axis which extends from 

 the anterior to the posterior end. Three regions of the body 

 may be distinguished, HEAD, TRUNK, and TAIL. The head 

 forms the anterior end and contains the brain, eyes, ears, and 

 nostrils, as well as the mouth and throat. On either side of 

 the head is a series of openings, or GILL SLITS, leading into the 



