STRUCTURE OF THE MOLLUSCA. 183 



ment of natural history must, therefore, be more or less 

 injurious ; but the adoption of even a very imperfect system 

 by an individual, is preferable to having no system at all. 

 It is evident that an arrangement which refers only to some 

 particular organs or parts of animals cannot afford a correct 

 idea of their nature, and that a rational system must have 

 reference to all their organs. Conchological systems, having 

 reference merely to shells, are of this imperfect kind ; for 

 shells are not animals, but parts, and not even essential 

 parts, of animals. A slug, which is destitute of a shell, or 

 at least of an external shell, is most closely related to a 

 snail, which has a shell; and there are very numerous marine 

 naked, slug- like Mollusca, which cannot be separated from 

 similar species furnished with shells. He who would know 

 the nature of shells, must know first the nature of the ani- 

 mals of which shells form a part. 



According to many modern authors, the Mollusca may be 

 conveniently arranged into six classes, of which three have 

 the head distinct, or more or less obvious, while in the other 

 three it is not apparent, or not well defined. 



Mollusca having the head distinct. 



I. Cephalopoda. The body enclosed in a bag formed 

 by the mantle, from which projects the head, surrounded 

 by filaments or arms, which are at once organs of prehension 

 and locomotion. Fig. I. 



II. Pteropoda. The body not enclosed in a bag, nor 

 the head surrounded by filaments ; but the sides of the neck 

 furnished with membranous appendages, like wings or fins. 

 Fig. 2. 



III. Gasteropoda. None of the characters of the pre- 

 ceding classes; but at the lower surface ot the body a fleshy 

 contractile and expansile mass, or foot, enabling the animal 

 to crawl or swim. Figs. 3 and 4. 



Mollusca destitute of a distinct head. 



IV. Acephala. Four thin expanded branchiae distinct 



q2 



