134 



COMPARATIVE ZOOLOGY. 



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 MoMusk (Cytherea Fig. 100. — Section of a Spiral 



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



c, cardinal, and t,t\ lateral teeth ; a, a', impres- a, apex; &, spire; c, suture; 



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



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



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 



