342 



COMPARATIVE ZOOLOGY 



ing in a spiral form. The ribs, ridges, or spines on the 

 outside of a shell mark the successive periods of growth, 

 and, therefore, correspond to the age of the animal. 

 Figures 296 and 297 show the principal parts of the 

 ordinary bivalves and univalves. 

 The valves of a bivalve are gen- 

 erally equal, and the umbones, or 

 beaks, a little in front of the cen- x 

 ter. The valves are bound to- 



FlG. 296. Left Valve of a Bivalve Mollusk (Cy- FIG. 297. Section of a Spiral 



therea chione}: h, hinge ligament; , umbo; Univalve ( Triton corrugatus} : 



/, lunule; c, cardinal, and /,' t' ', lateral teeth; a, apex; b, spire; c, suture; 



a, a', impressions of the anterior and posterior d, posterior canal; e, outer lip 

 adductor muscles; /, pallial impression;, s, sinus, 

 occupied by the retractor of the siphons. 



of the aperture ; f, anterior 

 canal. 



gether by a ligature near the umbones, and often, also, 

 by means of a " hinge " formed by the " teeth " of one 

 valve interlocking into cavities in the other. The aper- 

 ture of a univalve is frequently closed by a horny or 

 calcareous plate, called "operculum," which the animal 

 carries on the back of the hinder portion of its foot, 

 and which is a part of the exoskeleton. The shells of 

 mollusks are epidermal, and are, therefore, dead and 

 incapable of true repair. When broken, they can be 

 mended only by the animal pouring out lime to cement 



