400 PHILOSOPHY OF ZOOLOGY. 



the body, separated from the back by a slight groove, and 

 containing the mouth and tentacula. 



In many of the animals of this division, the different 

 members of the body are in pairs, and are arranged, in re- 

 ference to a mesial plane, into right and left. In some, 

 part only of the organs respect a mesial plane, other 

 parts being single, or in unequal numbers. In other spe- 

 cies, the organs, which are not in pairs, are arranged round 

 a central axis, and give to the external form a radiated ap- 

 pearance. But these characters are exceedingly variable 

 and uncertain, as indicating the limits of particular tribes ; 

 since, in different parts of the same animal, modifications of 

 all these forms may be readily distinguished. 



The sJiin of molluscous animals is more simple in its 

 structure, than the same organ in the vertebral animals. 

 The cuticle is here very distinct ; and, as in other classes, 

 it is thick and coarse where much exposed, but thin and 

 delicate in its texture, where it lines the internal cavities. 

 A mucous web may be detected in the cuttle fi.?h and slug, 

 but of great tenuity. The cerium is destitute of a villous 

 surflice ; and on its central aspect it is so intimately united to 

 cellular substance, that its fibrous structure can scarcely be 

 distinguished. The imiscidar web, may, in general, be 

 readily perceived. Its fibres proceed in various directions, 

 according to the kind of motion to be executed, and extend 

 «r corrugate the skin at pleasure. 



The appendices of the skin in this class of animals ought 

 to be carefully studied, as they furnish the most obvious 

 marks for distinguishing species, and for constructing divi- 

 sions in their systematical arrangement. The appendices 

 of the cuticle are few in number, and perhaps ought to be 

 considered as limited to hairs. These, in some species, in- 

 vest the surface regularly and closely, and may be observed 

 on those which live on land, as well as those Avliich reside 



