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; u, umbo; l, lumile; Univalve ( Triton corrugatus): 
c, cardinal, and t,t', lateral teeth; a, a', impres- a , apex; b, spire; c, sntnre; 
sious of the anterior and posterior adductor mus- d , posterior canal; e. outer 
cles; p, pallial impression ; s, 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 
