PROTRACTILE MOUTHS 



capacity effected by a flexible connection between 

 the skeletal pieces of the jaws which allows the 

 lips to move forward slightly as the mouth opens. In 

 the carp family, the carp themselves and the barbel 

 have fleshy lips which can rise in a pout. But they have 

 not the fully specialised mechanism. Indeed, since 

 this mechanism is unique of its kind, it is remark- 

 able that we should find it occurring in three distinct 



Fig. 23. — Sand Smelt ; above, with mouth nearly closed 

 mouth open and protruded. 



below, with 



families which, otherwise, have no direct connection 

 one with another. 



The important function of securing nourishment is 

 that which most forcibly compels the creature towards 

 adaptation to its environment. It must eat to live. The 

 behaviour, relations, and mode of life of the individual 

 depend, in the first instance, on this necessity. So, if 

 we seek to understand and appreciate the implications 

 of the vital functions, we may well consider how this 

 essential one is accomplished. 



The simplest, or the most elementary, mode of secur- 

 ing food is that which works through the surface of the 

 body. The water commonly found in nature contains 

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