12 THE LIFE OF THE MOLLUSCA 
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 
