MOLLUSCA 



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

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

 part only of the organs has respect to a mesial plane, other 

 parts being single, or in unequal numbers. In other species, 

 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 skin 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 fish and slug, 

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

 surface ; and on its central aspect it is so intimately united 

 to cellular substance, that its fibrous structure can scarcely 

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

 readily perceived. Its fibres proceed in various directions, 

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

 or 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, 

 invest the surface regularly and closely, and may be ob- 

 served on those which live on land, as well as those which 

 reside in water. In some cases the hairs may be considered 



