160 PHILOSOPHY OF ZOOLOGY. 
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 amimals belongmg to the 
class termed Annelides by Cuvrer, as on some of the spe- 
cies of the Lmnean 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, 
or supply the deficiency, 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 im form, texture, and thickness. 
In some cases, they are thi semi-transparent 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 are not exclusively the production of the animals 
termed Mollusca. They occur, likewise, as a covering to 
several of the Annelides, as the Serpula and Dentalium. 
