H2 THE LIFE OF THE MOLLUSC A 



So far as the shell is concerned, then, the Cephalo- 

 poda seem to have been yet more eager than the 

 Gastropoda to jettison the encumbrance, and their 

 predatory habits have obviously had much to do in 

 hastening this consummation. 



Turning next to the form of the animal itself, it 

 cannot be said that any definite line of development 

 is presented, unless, perhaps, in the case of the more 

 specialized Gastropoda, where, with the discarding 

 of the shell, the visceral hump tends to be smoothed 

 down and distributed along the dorsal keel, till the 

 true slug-like form is attained. Even this seems to 

 be due to the burrowing habit of the animal rather 

 than indicative of any definite product of development. 



Allusion has already been made (ante, pp. 33, 34) to 

 the great inequality of size the anterior and posterior 

 portions of the body present in certain forms like 

 Mytilus, and the disappearance pari passu of the 

 anterior adductor muscle in proportion as, by the 

 increase of growth in the posterior portion of the 

 body, it is brought more and more into line with 

 the hinge and posterior adductor muscle, and con- 

 sequently ceases to be needful. It is possible that 

 in the case of Mytilus the predisposing cause may be 

 due to the long-continued action of gravity operating 

 on successive generations of suspended animals, 

 aided possibly by some other morphological tendency. 

 Whether a similar tendency to monomyarianism 

 observable in forms that like Pecten, Ostrea, etc., rest 



