GENERAL CHARACTERS OF MOLLUSCA. 



bearing a pretty close relation with the degree of locomotive 

 power. It is most evident and complete in the CEPHALOPODA ; 

 many of the animals in which class are adapted to lead the life 

 of Fishes, and resemble them in the general form of the body 

 and in the structure of many individual organs. 



865. As a group, however, the MOLLUSCA are to be 

 characterised rather by the absence, than by the possession, of a 

 definite form ; and there is a corresponding absence of any 

 regular organs of support, by which such a form could be main- 

 tained. The name they have received designates them as soft 

 animals ; and this they are pre-eminently. The Shell, where it 

 exists, Js to be regarded rather in the light of an appendage 

 designed for the mere protection of the body, and deriving its 

 shape from it ; than as a skeleton, giving attachment to muscles, 

 and regulating the form of the whole structure. Where the 

 body is entirely inclosed within it, as in the lower bivalve 

 Mollusca, no locomotive powers whatever, except such as depend 

 on the passage of water through the respiratory tubes, are 

 enjoyed by the animal. It is only where the body is uncovered 

 by a shell, or a portion of the body can be projected beyond it, 

 that any active movements can be executed ; and the muscles 

 concerned in the performance of these do not make the shell a 

 fixed point, as is done by those of Articulated or Yertebrated 

 animals in regard to their skeletons, but are entirely unconnected 

 with it. 



866. Hence we see that the shell of a Mollusk is, when 

 considered in reference to its functions, a very different organ 

 from that of a Crustaceous animal, although formed in somewhat 

 the same manner. Its frequent absence might of itself lead us 

 to suspect its want of importance to the living structure. In one 

 whole class of Mollusca it is entirely deficient ; and in three others 

 it is frequently absent. In only one it is universally present. 

 When speaking of the anatomical conformation of the body, 

 therefore, we may leave the shell pretty much out of considera- 

 tion. Before the animals which produce them had been properly 

 studied, Naturalists founded their classification of Mollusca upon 

 the shells only ; and the greatest confusion thus resulted. Shells 



