T^he Shell and its Formation 37 



lias assumed the form known as the Smooth Lmipet. 

 Exotic species of Patella that live entirely upon 

 floating seaweeds have their transparent shells like 

 those of Helcion, but those that live upon rocks 

 require thick shells to withstand the breakers. 



All the species to which we have just referred — 

 with the exception of the Limpets — have the mouth 

 of tlie shell accurately closed by a thin plate of 

 shell or chitin. It is seen closino^ the orifice of the 

 Wentletrap figured opposite, and on the end of the 

 foot of the Sting Winkle beside it. It is known as 

 the operciditin, or cover, and at one time was con- 

 sidered to be the other valve, bringing the univalves 

 into close agreement with the bivalves. But a little 

 consideration serves to show that the operculum is 

 not part of the true shell, for it is not a product of 

 the mantle, but of the foot. 



And this brings us to the consideration of the 

 enormous difference between the shells of, say, the 

 Oyster and the Whelk. In spite of the very high 

 value, commercial and epicurean, that is set upon 

 the Oyster, present-day knowledge all seems to point 

 to the probability tliat he is a degenerate from the 

 form ot* the ancestral mollusk, which almost 

 certainly had a head and something approaching a 

 masticatory apparatus. Whilst some of its de- 

 scendants in successive generations went on im- 

 proving the simple tent-like form of their shells, 

 by twisting them into spirals, and developing their 

 organs to fit them for lives of varied activity ; 

 others, finding that the sea was well provided with 

 food, appeared to think that activity involved an 

 unnecessary strain upon the organism. These simply 



