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 exp- 



skeleton. The shells of MoUusks 



are epidermal, and are, therefore, 



dead and incapable of true repair. 



"When broken, they can be mended 



Fig. 99.— Left Valve of iiBiviilveM<)Ilusk(C3/«Acrea Fie. 100. — Section of a Spiral 



chimie): h, hinge ligament; u, umbo; I, luuule; 

 e, cardinal, and t, t', lateral teeth ; o, a', impres- 

 sions of the anterior and posterior addnctor mus- 

 cles; Pt pallial impression ; 8, siunS} occupied by 

 the retractor of the siphons. 



Univalve i,Triton corrugatua) : 

 a, apex; 6, spire; c, suture; 

 d, posterior canal; e, outer 

 lip of the aperture ; /, ante- 

 rior caDal. 



only by the animal pouring out lime to cement the parts 

 together. They cannot grow togetlier, 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, 

 tiie 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 

 etadoskeleton, and is feebly developed in comparison. It 



