2. The Brachiopoda. These Molluscs are furnished with a pair of shells, within which the animal lives; one of these shells (the ventral one) has a small hole in it, close to the hinge, and through this a long tendinous cord passes, which fixes the creature to some stone or rock, hence the name Brachiopodous, which means arm-footed. The young of this Mollusc are not fixed, but float about. The Lingula has horny shells, and the foot passes out between them; these creatures were some of the first created, if not the very first; the shells of Terebratula are found in vast quantities in the oldest fossiliferous strata. [Illustration: FIG. 34.--COCKLE (Cardium edule).] 3. The Lamellibranchiata include a great many of our most ordinary Mollusca, commonly known as "shell fish." They have a pair of shells, and are thence called "bivalve." The Oyster (Ostrea edulis), Mussel (Mytilus edulis), Cockle (Cardium edule), Scallop (Pecten), belong to this class. Oysters form a considerable article of commerce, thirty or forty thousand bushels are brought each season to London; they are dredged up from "beds," where they are found in great quantities. They spawn in May and June, and are not then good. There is an old and a well-known saying, that "Oysters are not fit to eat, unless there is an R in the month," all the names of the months containing an R but May, June, July, and August. The spawn is collected and placed in artificial beds, consisting of shallow places or hollows in the sea, where the tide will not wash them away, and whence they can be easily removed when sufficiently grown, which is in five or six years; these creatures have no powers of locomotion, but remain where the tide washes them; but Scallops, Cockles (fig. 34), and Mussels (fig. 33), have the power of fixing themselves to any substance they wish, by means of the "byssus" or beard, which is a tuft of fibres passing out from between the shells; and it is said, the Scallops have the power of progression, by suddenly opening and shutting the shells. 4. The Pteropoda are Mollusca which have no shell, or a very thin one; the Clio borealis (fig. 33), which forms the chief food of the Whale, is found in great multitudes in the Arctic seas, it swims about by means of two extensions, similar to wings. The Hyaloea has a small round transparent shell. [Illustration: FIG. 35.--SNAIL (Helix aspersa), AND SLUG (Limax cinerius).] [Illustration: FIG. 36.--WENTLE-TRAP SHELL (Scalaria).]