_Ostrea_ (oysters) are well known bivalve mollusks. The shells are so irregular in surface and shape that it would be impossible, as it is unnecessary, to describe them. The animal itself is very simple in structure, proverbially stupid, low in the scale of animal life, but highly esteemed as a delicious article of food. They are found in almost all seas, and in water of from two to six fathoms, but never very far from some shore. They multiply so rapidly that, though the consumption increases with the increase of population and the facilities for distributing them, the supply is equal to the demand. Boston is mostly supplied from artificial beds, and the flats or shallow bays in the vicinity of our maritime cities abound in such “plants.” Baltimore and New York have each an immense local trade, and the oysters exported from the Chesapeake Bay fisheries amount to about $25,000,000 annually. CLASS II.—_Gasteropoda_ (stomach-footed). This class, including the great snail family, is a very large division of terrestrial, air-breathing mollusks. Their light shells vary much in form; when spiral and fully developed they have as many as five or six whorls, symmetrically arranged. The shell can contain the whole body, but the animal often partially crawls out and carries the shell on its back. The locomotion is slow and accomplished by the contractile muscle of the ventral foot. CLASS III.—_The Cephalopoda_ (head-footed) have distinctly formed heads, large, staring eyes, mouths surrounded with tentacles or feelers, symmetrically formed bodies wrapped in a muscular covering; they have all the five senses, and are carnivorous. This class is entirely marine, and breathes through gills on the side of the body. The naked _Cephalopoda_ are numerous, and furnish food for many other animals. Those living in chambered shells, once numerous, are now known principally by their fossils, the pearly _Nautilus_ (sailor) being their only living representative. This has a smooth, pearly shell, and is much prized as an ornament. It is a native of the Indian Ocean, dwelling in the deep places of the sea, and at the bottom. The shell is too well known to need description, and but few specimens of the living animal have been obtained. [Illustration: NAUTILUS.] The following facts as to the physical organization and habits of this interesting species of univalves are gleaned from the American Cyclopædia, and given for those who have not access to any extensive work on the subject; they were mostly copied for the Cyclopædia from Professor Owen’s celebrated memoir, and the “Proceedings of the Linnæan Society of London:”