What is called the foot in the testaceous nzollusca, is an organ of motion very singular both in its structure and uses. In the gasteropoda, it consists of the fleshy mass which we have noticed above, and is generally of an oval shape, pointed at its sacral extremity, as in the snail and slug. In other gasteropoda, the muscular fi bres composing the foot are arranged so as to form a furrow by which the animal embraces the body on which it crawls. Among the acephala, the animal inhabiting the razor-shell has one of the most curious organs of this kind. It is situated at one extremity of the animal, is of a conical form, and is surrounded at its root by a peculiar fold of the skin. By means of this foot, the animal in a moment buries itself in the sand, or as speedi ly emerges from its retreat.
The muscles of Wonms very much resemble those of LARITJE and Mom.use A. In the animals below these, no muscular fibres have been discovered.
Another material difference with respect to the or gans of motion, is the presence or want of articulated members. One of the most striking varieties in this
respect consists in the number of these members.
All the MAMMALIA, nearly all the REPTILES, and all BIRDS, have four articulated members. In several spe cies of the tribes phoca, (seals,) and trichechus, (wal ruses,) these members are indeed not very evident, as the atlantal extremities are often very short, and the sa cral approach so near each other, and are thrown so far backward as to resemble the tail of a fish. Many of the MAMM ALIA have their atlantal extremities so formed that they can employ them like the hands of man.
The cetacea have not properly four articulated mem bers, as they have no bones in their tail fins ; but in their pectoral fins the bony structure is very evident, and very nearly resembles that of the atlantal extremi ties in the MAMMA LIA. This is particularly the case in the porpoise and the unicorn fish, the extreme bones of whose pectoral fins very much resemble those of the human hand.