GENERAL CHARACTERS OF MOLLUSCA. 329 



of very similar aspect are often produced by animals extremely 

 unlike each other,* and living in different conditions as, for 

 instance, fresh and sea water. And shells of very dissimilar 

 character in the eye of the mere Conchologist, often belong to 

 animals closely allied. In fact, the form of the shell taken 

 alone is a character as purely artificial, as the number of stamens 

 and pistils in a flower ; and will lead to a classification as far 

 removed from a natural plan. But when the principal divisions 

 have been formed upon other grounds, the conformation of the 

 shell will often afford valuable subordinate characters ; and the 

 Naturalist seeks to employ these as much as he safely can, on 

 account of the facility with which he can apply them to the study 

 of those fossil remains, from which all traces of the animal itself 

 have disappeared. The softness of the entire body of the Mol- 

 lusca prevents us from recognising its form and structure after 

 death, in any other way than by the shell ; but upon this, it 

 must be remembered, entire reliance cannot be placed, since it is 

 liable to great variation, in accordance with the circumstances of 

 the individual^ whilst it is by no means certain that there are 

 constant differences in its form in distinct species. 



867. The only tribe of Mollusca which presents anything 

 that corresponds to the internal skeleton of the Yertebrata, is 

 that of the Cuttle-fish. There is in their bodies a cartilage, 

 partly inclosing the nervous centres in the head, and sending 

 prolongations along the back, for the protection of the large 

 nervous cords which traverse it, and for the attachment of the 

 muscles by which it is moved. These last are especially deve- 

 loped, where the body is spread out into fin-like processes, 

 resembling those of Fishes, by the stroke of which active move- 

 ments are produced (Fig. 540). This skeleton is almost as 

 highly organised, as are the lowest forms of that internal skeleton 

 which is characteristic of Vertebrata ( 585). 



868. In all the Mollusca, the soft body of the animal is 



* The impossibility of founding a correct classification upon the characters 

 afforded by the shell only, is made at once apparent by comparing the shell of a 

 Serpula (Fig. 528), with that of a Vermetus (Fig. 583). Between these two 

 shells, there is no essential difference ; yet the animals which form them belong, 

 not merely to different families, but to different sub-kingdoms. 



