[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.
The full-grown Cuttle is, nevertheless, so curious a creature that, in a tank prepared with that special view, his habits might furnish food for much curious observation—indeed, carefully fed up, he might form very excellent food himself; his German name, Kuttel, signifying tripe, the flavour of which his flesh is said to resemble. The common Squid, which is eaten by the poor of our coasts, is a kindred species, and is also said to have a similar flavour. Molluscs of this class, as well as the disgusting-looking Poulp, or many-feet, are seen in profusion in the markets of the south of Europe, and are as highly prized as the Oyster with us. The ancients carried their taste for them so far as to feed them up artificially; and at the nuptial feast of Iphicrates, one hundred Polypi and Sepiæ, as we are informed, were disguised with different sauces, each imparting a different flavour. The land Molluscs were also much sought as a table delicacy, a species of the large Garden-Snail being bred for that purpose, and fed upon a prepared paste, which so accelerated their growth that we learn, from the industrious Pliny, of their attaining to enormous dimensions; the shells of some of the finest being capable of holding eighty measures of water, called quadrants. But in speaking of Molluscs, I must principally confine myself to such as are suitable for an Aquarium.
Among the Sea-Snails of our own coast, which are still eaten by the lower orders, is the Periwinkle, considered by some superior in flavour to the Oyster or Shrimp. This creature, the Littorina littorea, is one of the most useful creatures in an Aquarium, cleansing it from all decaying vegetation, which is its natural food. The Periwinkle varies much both in size and colour, the ground tone of the shell being sometimes red, orange, or even scarlet, sometimes with and sometimes without handsome black bands. Such as are coloured in this attractive manner should obviously be selected as inmates of the Aquarium, in preference to the dull-coloured varieties; and a few of the small yellow kind, Littorina littoralis, may be added by way of variety, though they do not succeed so well in confinement as the other species.
The Whelk, Buccinum undatum, another of the snail-like Molluscs of our coasts, which is considered good eating by the lower orders, and often seen on fish-stalls at particular seasons, is well worthy a place in our miniature sea; especially under certain circumstances, when the shell of this creature assumes a most singular aspect, well calculated to excite the wonder of the young naturalist. It is sometimes found surmounted by a mass of living substance, which might be taken for the body of the creature, residing in preference on the roof of its dwelling during the summer months, as it may be observed spreading a set of tentacles, from a mouth-like orifice, for the collection of food. Within, however, a pair of protruding eyes are seen glaringly on the watch for prey, and another set of food-clutching machines may be noticed beneath them, ready for their work, and only awaiting the opportunity. They look much like the claws of a lobster, and if any suitable object comes within their reach, it is seized by one or both of these two-fingered hands, and carried to the yawning mouth beneath; but before it reaches that evidently impatient receptacle, a brightly-shining crimson finger, ornamented with two white stripes, darts from beneath those claws and mouth, and, snatching away the rich morsel, disappears as suddenly as it came, leaving the expectant mouth and astonished claws both empty. The mystery of this seemingly compound creature having, as it were, a first-self living outside the house and getting a separate living, a second-self located in the front parlour, and prevented from eating its own dinner by a third-self residing in the back parlour, may be easily explained, now that the persevering observations of our naturalists have solved it. It is as follows:—