And now to the _Pteropoda_, practically “winged feet” molluscs, the position of which in scientific nomenclature many think unsatisfactory. This is, however, of little consequence to the general reader. These curious little molluscs can pass through the deep blue seas they usually inhabit rapidly, reminding us strongly of the movements of a butterfly or some other winged insect. They can “ascend to the surface very suddenly, turn themselves in a determinate space, or rather swim without appearing to change their place, while sustaining themselves at the same height.” “If,” continues Figuier, “anything alarms them, they fold up their flappers and descend to such a depth in their watery world as will give them the security they seek. Thus they pass their lives in the open sea far from any other shelter except that yielded by the gulf weed and other algæ. In appearance and habits these small and sometimes microscopic creatures resemble the fry of some other forms of mollusca. They literally swarm both in tropical and arctic seas; and are sometimes so numerous as to colour the ocean for leagues. They are the principal food of whales and sea‐birds in high latitudes, rarely approaching the coast. Only one or two species have been accidentally taken on our shores, and those evidently driven thither by currents into which they have been entangled, or by tempests which have stirred the waters with a power beyond theirs. Dr. Leach states that in 1811, during a tour to the Orkneys, he observed on the rocks of the Isle of Staffa several mutilated specimens of _Clio borealis_. Some days after, having borrowed a large shrimp net, and rowing along the coast of Mull, when the sea, which had been extremely stormy, had become calm, he succeeded in catching one alive, which is now in the British Museum.” Professor Huxley has told us that they have auditory organs, are sensible of light and heat, and probably of odours, but that they possess very imperfect eyes and tentacles. They have respiratory organs, hearts and livers, and are undeniably social and gregarious, swarming together in great numbers. We now approach the highest class of the mollusca—on paper, only, be it observed, for in actual life most of them are either nearly unapproachable, or, at all events, are most undesirable acquaintances.