xii PHYLUM CHORDATA 327 



B. THE CRANIA TA 



The Fishes, Amphibians, Reptiles, Birds and Mammals 

 are grouped together under the general designation 

 Craniata, derived from one of the features which these 

 animals have in common, viz., the presence of a skull or 

 cranium. In order to understand the general character- 

 istics of the Craniata, it will be advisable to examine and 

 compare representatives of some of the principal classes. 

 For this purpose a Dog-fish, a Lizard, and a Rabbit will be 

 a good selection. Not only must entire and if convenient, 

 living specimens be examined, but prepared skeletons of 

 all three must be available for examination, and preparations 

 showing the various systems of internal organs, notably the 

 digestive system, the heart and the brain. 



An external comparison appears at first sight to reveal 

 few points of agreement between the three selected ex- 

 amples. 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 bearing teeth ; near the 

 mouth are a pair of smaller apertures the nostrils or nasal 

 apertures, and at the sides of the head region are the pair 

 of conspicuous eyes ; while further back the pair of promin- 

 ent auricles or pinnae, with the wide apertures at their 

 bases, mark very conspicuously the position of the 

 auditory organs in the Rabbit, less clearly indicated in 

 the Lizard, and still less in the Dog-fish. On the lower 

 (ventral) surface, towards the posterior end of the trunk, 



