The Cephalophora fall naturally into two fairly well-defined groups, which we may describe as the air-breathers and the water-breathers. The former breathe air direct from the atmosphere through an aperture on the right side of the body, the air passing into a pulmonary organ or lung, in the walls of which the bloodvessels ramify, and they include all the land snails and slugs. The latter breathe by gills which are more or less prominent on the sides of the body, and include all the fresh-water snails, as well as the marine species which fall within our special province. We shall first consider the class _Pteropoda_ or Wing-footed Molluscs, so called from the wing-like appendages that are attached to the side of the mouth, or to the upper side of the foot, which is either very small or altogether wanting. These Pteropods are in many respects lowly organised as compared with the higher molluscs; and as they spend the whole of their existence in the open sea, they can hardly be considered as falling within the scope of the sea-side naturalist’s work. Yet since their shells are occasionally drifted on to the shore, and because a knowledge of them is essential to the student of the mollusca, we shall briefly note their principal characteristics. The pteropods are extremely abundant in some seas, occurring in such vast numbers that they discolour the water for miles. They swim about by flapping the pair of wings already referred to. They are known to form an important article of the diet of the whale, and are also devoured in enormous numbers by various sea birds; and they are themselves carnivorous, feeding on various smaller creatures that inhabit the open waters. [Illustration: FIG. 160.--PTEROPODS] In appearance they much resemble the young of higher species of molluscs. The nervous system consists of a single ganglion situated below the gullet, and the eyes and tentacles are either rudimentary or absent. The digestive system includes a muscular gizzard provided with teeth for the mastication of food, and a digestive gland or liver for the preparation of a digestive fluid. The heart has two cavities, and respiration is effected by a surface covered with minute cilia. This surface is either quite external or is enclosed in a chamber through which water freely circulates. The shell is very different from that of a typical head-bearing mollusc, for it generally consists of two glassy, semitransparent plates, situated dorsally and ventrally respectively on the body of the animal, with an opening for the protrusion of the body, and others at the sides for processes of the mantle; and it terminates behind in one or three pointed processes. Sometimes, however, its form is conical or spiral, with or without an operculum. We append illustrations of a few pteropods, selecting for our purpose species that have been found in the Atlantic.