336 COMPARATIVE ANATOMY AND PHYSIOLOGY. 



Vertebrates, in which only those persist that take part 

 in the formation of the nasal cavities. 



In the voracious sharks the mouth is of large size, 

 but the character of their food is related rather to the 

 size of their teeth than to the extent of their mouth ; 

 when the mouth is not terminal but ventral in position, 

 a shark has to turn on its side to seize its prey. 

 In some Rays, such as Pristis (the saw-fish) the 

 snout is produced into a long flattened weapon of 

 attack, which is armed at the sides with spinous 

 processes, and serves as an organ by means of which 

 the body of its prey may be torn open. In the 

 Teleostean sword-fishes, where maxillae and premaxill<e 

 are present, these bones are produced into a long 

 stabbing-organ, not only strong enough to pierce the 

 bodies of whales, but even the planks of wooden 

 ships. In many Ganoidei, such as the sturgeon, the 

 snout is of considerable length ; the function of this 

 organ is not completely understood, and the most 

 plausible hypothesis is that of von Martens, who 

 ascribes to the long snout of Polyodon the function 

 of a tactile organ, the necessity for which is to be 

 explained by the turbidity of the rivers in which it lives. 



In the Teleostei the mouth may be very large, as 

 in the Angler (Lophius), or exceedingly small, as in 

 Chsetodon and Diodon ; in Chelmo, a form allied to 

 Chsetodon, the mouth is prolonged into a snout which 

 possibly serves as an apparatus for drawing from holes 

 or crevices the small animals on which it feeds 

 (Giinther). Where the teeth are of great size, and 

 adapted, say, for crushing shells, as in the sea-cat, the 

 jaws which carry them are of corresponding strength; in 

 the wrasses, with somewhat similar habits, the upper 

 pharyngeal bones are articulated with the basi-occipi- 

 tals, and no doubt afford a firmer fulcrum for the jaws. 



In bony fishes the eye-ball is sometimes provided 

 with separate bony pieces for its protection. 



