The possession, by many members of this class, of two fin-like muscular expansions attached to the side of the head induced Cuvier to give them the name Pteropoda. Prof. Owen says: "All the species of Pteropoda are of small size; they float in the open sea, often at great distances from any shore, and serve, with the Acalephæ, to people the remote tracts of the ocean. In the latitudes suitable to their well-being, the little Pteropoda swarm in incredible numbers, so as to discolor the surface of the sea for leagues; and the Clio and the Limacina constitute, in the northern seas, the principal article of food of the great whales."
Some of the least highly-organized members of this class, such as the Hyalaceos, are provided with a bivalve shell, and cannot be said to possess a head. They have a simple commencement of the alimentary canal at the anterior extremity of the body; but since this anterior extremity has no tactile appendages and no eyes, and inasmuch as it also contains no cerebral ganglia, it can have no claim to be considered as a head. Their chief nervous centre consists of a flat, somewhat quadrate, sub-œsophageal ganglion, to the anterior angles of which is attached a nervous commissure which extends upward so as to encircle the gullet, though there are no ganglia either on or at the sides of this tube in the usual situation occupied by cerebral ganglia.
In other pteropods devoid of a shell, we meet with a higher organization. Thus in Clio there is a distinct head bearing sensory appendages in the form of two tentacula and two eyes, and containing in its interior a brain. This brain is represented by two connected super-œsophageal ganglia, which are in relation, by means of nerves, with the cephalic sensory organs, and in connection with the sub-œsophageal commissure are the two pedal and two branchial ganglia. The two pairs of ganglia exist separately in Clio and its allies, though they are combined into one quadrate mass in Hyalea. In this latter there are two acoustic vesicles in contact with the anterior part of the great ganglion, while in Clio similar vesicles are in connection with the anterior pair of sub-œsophageal ganglia—that is, with the pair which corresponds with the pedal ganglia of the common bivalve mollusks.
Gasteropods constitute a class of organisms which, in point of numbers, can only be compared with the still more numerously represented class of insects. Their name is derived from the fact that these animals crawl by means of a large muscular expansion stretched out beneath the viscera. The locomotion of the members of this class may be said to be, in the main, dependent upon their own individual efforts, so that, in this respect, they differ widely from the pteropods, whose locomotions are brought about by winds driving them along the surface of the water on which they float.
Some gasteropods are terrestrial, air-breathing animals, though by