12 MOLLUSCA. 



Many Mollusca are furnished with numerous comjilicated 

 canals which communicate with the surroundinsf fluid and 

 serve (as in Dolium) to distend the foot by the admission 

 of water. In Cyprcza there is a long slit in the sole of 

 the foot near its middle. In Haliotis there are two or 

 three pores at each extremity, and in Doris, Aplysia, and 

 Bulla there is a series of orifices placed round its edges. 

 In many families the sole of the foot is imperforate, as 

 in Turbo, Trochus, Murex, and Purpura, when the water 

 enters by a peculiar orifice near the vent, from whence 

 it finds its way into the canals that ramify through the 

 foot. The use of this system of aqueducts, as Dr. John- 

 ston has termed it, is to distend the various organs and 

 render them more firm and capable of muscular exertions. 



The dorsal surface of the foot secretes certain horny 

 or calcareous layers, which constitute the operculum, a 

 flat body, which closes the mouth of the shell when the 

 animal is retracted. When the animal is too large to 

 enter the shell, the operculum is absent, as in Haliotis, 

 Gena, and Stomatia, or is rudimentary, as in Sigaretus.' 

 When the animal is entirely retractile within the shell, the 

 operculum is larger. It is round in the vegetable-eating 

 tribes, as in TurhinidtE, and unguiform in the flesh-eating 

 tribes, as in Muricida. When the animal envelopes the 

 shell in lobes of the mantle, there is no operculum, as in 

 Cyprtza, Marginella, Cyprcecassis, and Sycotypus. When 

 the foot is large the operculum is small, and mce versa. 

 In some tribes the operculum is stony, as in Turhinidce, 

 while in others it is horny, as in TrocMdee. In other 

 cases it is stony, in one genus of the same family, as in 

 Natica, and horny in another, as in Lunatia, or rudimen- 

 tary, as in Sigaretus. The operculum consists of a hard- 

 ened portion of the skin, the cells of which are filled 



