INTRODUCTION. 



A glance at a typical Fish and a typical Cetacean (whale, 

 dolphin or porpoise) reveals at once a general similarity in 

 bodily form. This resemblance, however, as might be expected 

 in two animals with such markedly divergent ancestral his- 

 tories, is a purely superficial one, and a closer study of their 

 anatomy reveals a number of important differences. The 

 fish has always lived in the water, but the whale is a mammal 

 that has exchanged a terrestial for an aquatic life, a change that 

 has been accompanied by the assumption of a stream-lined 

 and superficially fish-like form, designed to slip easily through 

 the water. The whale has also undergone a number of other 

 important modifications in connection with its adopted 

 environment. 



For convenience, as well as for descriptive purposes, we 

 may divide the body of a fish or cetacean into three regions : 

 a head, a trunk and a tail (Figs, i, 2). As a general rule there 

 is no trace of the neck so characteristic of most land Verte- 

 brates, and the outline of the head passes into that of the trunk 

 smoothly and without any sign of constriction. Similarly, 

 the trunk itself merges imperceptibly into the tail, the line of 

 division between the two regions being denoted by the vent. 

 In fishes, the last of the external gill-clefts in the Selachians 

 (sharks and rays), and the hinder edge of the bony gill-cover, 



