PHYLUM CHORDATA 



329 



Not only must entire and, 

 if convenient, living speci- 

 mens be examined, but 

 prepared skeletons of all 

 three must be availa- 

 ble for examination, and 

 preparations showing the 

 various systems of inter- 

 nal organs, notably the di- 

 gestive system, the heart, 

 and the brain. 



An external comparison 

 appears at first sight to 

 reveal few points of agree- 

 ment between the three 

 selected examples. The 

 skin, the general shape, 

 the movements, are all 

 widely different. A few 

 features common to all 

 three are, however, to be 

 recognised. It will be 

 observed that in all three 

 are distinguishable a head 

 region, in front, a trunk 

 region (by far the largest), 

 in the middle, and a tail 

 region, differing greatly in 

 its development, behind. 

 The head region bears 

 anteriorly the opening 

 of the mouth, bounded 

 above and below by jaws 



