134: 



COMPARATIVE ZOOLOGY. 



a univalve is frequently closed by a horny or calcareous 



plate, called " operculum," which the animal carries on its 



back, and which is a part of the exo- 



skeleton. The shells of Mollusks 



are epidermal, and are, therefore, 



dead and incapable of true repair. 



When broken, they can be mended 



FIG. 99. Left Valve of a Bivalve Mollnsk (C.yf/wrea FIG. 100. Section of a Spiral 



chione): ft, hinge ligament; u, umbo; Z, lunule; Univalve (Triton corrugatus) : 



e, cardinal, and t, <', lateral teeth; a, a', impres- a, apex; b, spire; c, suture; 



sions of the anterior and posterior adductor mns- d, posterior canal; e, outer 



cles ; p, pallia! impression ; s, sinus, occupied by lip of the aperture ; /, aute- 



the retractor of the siphous. 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 



