31 



cockles, mussels, scallops, and, above all, our excellent friend 

 the oyster. 



The sixth and last class is that of the Tunicata, a sort of 

 sea slugs, to whict), as they have no shells, we have nothing 

 to say. The shells of the testaceous molluscs are for the 

 most part of such capacity that the animals can be entirely 

 contained within them, and of such solidity that they are 

 rather their habitations than their clothing. The bodies 

 of the Gasteropoda are exceedingly retractile, so that al- 

 though they are attached to the shell by their hinder parts, 

 the bulk of the body can be protruded beyond the aper- 

 ture, and withdrawn to some depth within it, when the 

 animal re-enters. In some species there is a horny or 

 shelly plate called the Operculum, attached to that part of 

 the animal which is last drawn in, fitting the mouth of the 

 shell, and closing it up, whereby the seclusion and protec- 

 tion of the mollusc becomes complete. Every one who has 

 eaten a periwinkle, or seen one eaten, knows the circular 

 scale which must be picked off, before the animal can be 

 drawn out, after the most approved method by means of a 

 pin, and this is the Operculum. In some large species of 

 the genus Turbo, the Operculum is a solid calcareous stopper. 

 As the molluscs grow and increase in bulk, so does the 

 shell advance in capacity, differing in this respect 

 from the crustaceans, the lobster for instance, 

 which casts its shell during its growth, and is 

 again clothed with a new one, but the shell of 

 the mollusc does not grow in the same sense as that in 

 which the animal grows ; the animal constructs it, though 

 not in the same sense in which the bee constructs its comb, 

 since the action on the part of the mollusc is functional and 

 involuntary. The body of the mollusc is covered by an in- 

 tegument called the pallium or mantle, and by this organ 

 the material is secreted, which forms the shell. When a 

 snail is taking.his walks abroad, you will observe that the 

 blicll rests altogether on the back of the animal, so that 

 every part of the aperture is in contact with the mantle. 



