The swimming power of this race of creatures is equally curious. The _Cephalopoda_, by the ejection of a jet of water, propel themselves rapidly in the opposite direction, and by the repetition of the jet at regular intervals, a beautiful power of motion is obtained, as regular, and with less labour than that of ordinary swimming by means of the action of fins or other oar-like limbs adapted to the purpose.[3] The _Pteropoda_, however, in their little shells, translucent as glass, swim by the action of small fin-like paddles placed near the head. [Footnote 3: Some species effect leaps by an analogous contrivance—collecting water within the closed mouth, and then emitting it at a gush from a small portion of the aperture, suddenly opened, which propels the creature to a considerable distance, as it were, at a single bound.] The _Bivalves_ do not make so clever a use of their single foot as the _Univalves_. The foot in this tribe appears to be furnished with a terminal hook, which, when the foot is protruded, clings to some substance, and the animal is drawn up to that point, when the operation has to be repeated; this appears likely to produce but a slow rate of progress, yet some of the sand-boring _Bivalves_ manage, when alarmed, to conceal themselves with great rapidity by that means. The _Mollusca_, as feeders, are divided into three classes—those which take only liquid food, the vegetable feeders, and the carnivorous species. Those which are only able to take food in a liquid form, are such as have no means of seizing prey, their food consisting of the countless myriads of infusorial animalcules which float in the sea-water, and which are carried into the orifice of the stomach or mouth by the current. Of these, the _Dunicata_, _Brachiopoda_, and _Conchifera_, are examples. The liquid feeders exhibit a very low form of molluscous life, but other classes are furnished with means of defence and aggression, equal to those of terrestrial quadrupeds, and much more extraordinary in their form. Some of the carnivorous _Univalves_, for instance, feed upon the _Bivalves_ by drilling a hole through the solid shell, and withdrawing the animal piecemeal, as required. The _Eolis papillosa_ has been observed tearing away the tentacles of different species of Sea-Anemone with extraordinary voracity, and the tribe must therefore very evidently be excluded from the Aquarium. The Cuttles, also, are to be avoided from the same cause; they are fierce tyrants of the deep, that would make sad havoc among the delicate creatures with which we delight to furnish our tanks of glass. The curious substance termed Sea-Grapes, which are the eggs of this creature, might, however, be placed in the tank, and the progress of development watched, without fear of injury to the other inmates.