MOLLUSCA. 13 



either with horny or calcareous matter. Sometimes an 

 operculigerous lobe is developed around the operculum, 

 and secretes a coating of enamel on the upper surface, 

 as in Imperator, Turho, and Pliasianella. 



The operculum may be annular and multispiral, as in 

 the TrocMdw ; annular and paucispiral, as in Modulus^ 

 Echinella, Stomatella, and Eucliele ; or subannular and ovate, 

 as in Distortion Bursa, and Murex ; or subannular and 

 unguiculate, as in Strombus and Harpago. It may be 

 also spiral and ovate, as in Littorina and the NaticidcB ; 

 or spiral and orbicular, as in Tympanotomus and Ger%- 

 thidea. The most singular variations of this organ are 

 in Hipponyx, Calyptrwa, and lanthina. In the two 

 former genera it is a calcareous plate, and fixed to foreign 

 bodies, while in the latter it is vesicular, and serves as 

 a float. 



The mantle is a muscular sac, which envelopes the body. 

 It is usually open in front, and secretes a shell, which 

 serves to protect it. Sometimes the shell is contained in 

 the substance of the mantle, as in the Cephalopods and 

 Aplysiidtt ; and sometimes the edge of the mantle is 

 extended and covers the shell, as in Cypreeidce.. The 

 front edge is often simple and thickened, as in TrocMda; 

 furnished with filaments, as in BtromVidce ; or with com- 

 plicated fringes, as in Muricidce. In some tribes, as in 

 the phytophagous Gasteropods, the margin is entire ; but 

 in the zoophagous tribes, it is prolonged into a tube or 

 sipJion. 



The shells of Mollusca have been compared to the 

 skeletons of Vertebrate animals ; but they are obviously, 

 as shown by Dr. Carpenter, nothing but portions of calci- 

 fied skin. The mantle, which is a mucous membrane, 

 throws off an epithelium, which secretes a calcareous 



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