PROTRACTILE MOUTHS 



the mouth alone. The rapid snapping carried out in 

 a single movement, the sort of sucking action which may 

 accompany it, make up the principal method, especially 

 in deep-water fish. Usually, the teeth play a secondary 

 part. Their appearance should not disguise from us 

 that their presence, their condition, are often apparently 

 in contrast to the masticating function we are tempted 

 to attribute to them. Sometimes we find them strong 

 and numerous in species that live on the bottom and 

 feed upon dead bodies; sometimes we find them small 

 and few in hunting species which would seem to have 

 the more need of them. 



The protractile mouth is an obvious improvement. 

 In those aquatic creatures which have no prehensile 

 organs, it makes up for their absence and, by its own 

 effectiveness, enlarges the field of action. It takes on 

 their duties while still performing its own. When 

 the fish thus equipped opens its jaws, it projects them 

 at the same time, pushes them forward, so extending 

 the scope of the mouth considerably. This is a definite 

 step forward of relatively rare occurrence, and we see 

 it developed independently in different groups, just as 

 in mankind we sometimes observe chance resemblances 

 between unrelated individuals. 



Things are different in land animals. The seizing 

 of food by a sucking mouth is only known in a few 

 instances and to a very limited extent. A new circum- 

 stance operates in their case — the force of gravity. 

 The prey is too large and the mouth too small to 

 permit of such a mechanism. There is not the sur- 

 rounding environment of water to lessen the specific 

 gravity. Then the prehensile organs and the teeth 

 become functionally important, and we find a degree 

 of specialization greater than that exhibited in other 

 instances. The limbs effect movement, but they often 

 serve to hold, or to seize, in addition. The muzzle 

 becomes a snout. The jaws in birds and turtles become 

 beaks. Tongues and salivary secretions, in certain 

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