COMPARATIVE ZOOLOGY. 
134 
a univalve is frequently closed by a horny or calcareous 
plate, called “ operculum,” which the animal carries on its 
Fig. 99. —Left Valve of a Bivalve Mollnsk (Cytherea Fig. 100. — Section of a Spiral 
chione ): h, hinge ligament; «, umbo; l, lunule; Univalve (Triton corrugatus): 
c, cardinal, and t, t\ lateral teeth; a, a', impres- a, apex; b, spire; c, suture; 
sions of the anterior and posterior adductor mus- d, posterior canal; e, outer 
cles; p, pallial impression ; *, sinus, occupied by lip of the aperture; /, ante- 
the retractor of the siphons. rior canal. 
only by the animal pouring out lime to cement the parts 
together. They cannot grow together, like a broken bone. 
Imbedded in the back of the Cuttle-fish is a very light 
spongy “ bone,” which, as already observed, is a secretion 
from the skin, and therefore belongs to the exoskeleton. 
It has no resemblance to true bone, but is formed, like 
shells, of a number of calcareous plates. Nevertheless, 
the Cuttle-fish does exhibit traces of an endoskeleton: 
these are plates of cartilage, one of which surrounds the 
brain, and hence may be called a skull. To this cartilage, 
not to the “ cuttle-bone,” the muscles are attached. 
In Vertebrates, the exoskeleton is subordinate to the 
endoskeleton, and is feebly developed in comparison. It 
