Hitherto in our progress from the lowest animals upwards, the mind has been perpetually submerged; not only every group, but every individual that we have had occasion to consider, has been an inhabitant of the waters, and to the great body of which a fluid medium is as necessary to life and action as an aërial one is to a land animal, but now we shall be permitted to emerge occasionally, for although the largest proportion of the animals forming the great class we are now to advert to, the Molluscans, are also aquatic, yet still a very considerable number of them are terrestrial, as a stroll abroad will soon convince us, when after a shower we find we can scarcely set a step without crushing a snail or a slug.

The term Molluscan was employed by Linné to designate his second class of worms, which excluded all the shell-fish, and amongst real Molluscans included both Radiaries, Tunicaries, and Worms; it literally signifies a nut or walnut, and therefore seems more properly applied to shell-fish, than to animals which are defined as simple and naked. As now understood, it still comprehends a very wide range of animal forms, and it seems difficult to describe them by any character common to them all.