100 PHILOSOPHY or ZOOLOGV. 



stances, however, where they are remarkably thick, they are 

 placed laterally, and have a covering of the epidermis spread 

 over them. They may be observed on many insects, exhi- 

 biting great varieties of form. What are termed feathers 

 on the wings of butterflies, seem to be a variety of scales. 

 They are found, likewise, on the animals belonging to the 

 class termed Annelides by CUVIEII, as on some of the spe- 

 cies of the Linnsean genus Aphrodita. 



5. Shells. Shells consist of layers of an earthy salt, with 

 interposed membranes of animal matter, resembling coagu- 

 lated albumen. They grow by the addition of layers of 

 new matter to the edges and internal surface. When bro- 

 ken, the animal can cement the edges, and fill up the crack, 

 oj supply the deficiency r when a portion is abstracted. 



In general, they occur as a covering to the corium, and 

 are protected by the cuticle, as in the shell of the snail ; 

 but, in other cases, they are imbedded in a cell of the co- 

 rium itself, as appears to be the case with the shield which 

 protects the pulmonary cavity of the slug. In some cases, 

 the corium adheres to the shell by a considerable extent of 

 surface, as in many bivalve shells ; while, in the univalves, 

 the body only adheres by one or more muscles inserted in 

 the pillar. 



Shells differ remarkably in form, texture, and thickness. 

 In some cases, they are thin semi-transpancati and elastic, 

 as the shield of the snail and aplysia; while, in others, as 

 the volutes, they possess the opacity and hardness of marble. 



They occur simple, like a scale, tube, or cone variously 

 twisted ; or in the form of double cups, simply opposed, or 

 variously articulated. In other cases, where the shells are 

 numerous, they cover the corium in an imbricated form, or 

 are distributed over it in detached scales. 



Shells arc not exclusively the production of the animals 

 termed Mollnsca. They occur, likewise, as a covering to 

 several of the Annelides, as the Serpula and DentaKum. 



