Some of the Trochus tribe of shells look pretty in an Aquarium, but at present their treatment is so imperfectly understood, that they do not seem to do well. The specimens can, however, be renewed as required.
Trochus ziziphinus, the pearly Trochus, the animal of which is of a rich orange colour striped with black, moves freely about, and forms a very attractive object. The animal of T. granulatus is larger and handsomer, but shy, and displays little activity in confinement. The small Trochus, T. cinerarius, if placed in an Aquarium, may be observed rasping down the minute Confervæ that grow on the inside of the glass; and the curious method of the operation, and the singular instruments with which it is performed, may be observed by the aid of a small pocket lens.
Limpets—those curious bonnet-shells, as they are termed in some places, which are found in the form of a flattened and inverted funnel, adhering closely to the flat rocks of the sea-shore in all the European seas—are more curious than they appear at a first glance, and have characteristics that well repay the labour of persevering observation. I should always place a few in an Aquarium.
The common Limpet (Patella vulgata) has a power, which appears extraordinary when the soft substance of its body is considered, of excavating, more or less deeply, a portion of the rock which it makes its home. It is supposed to leave its hollow in the night, returning infallibly to its home in the morning. This habit might be watched in an Aquarium, and, if verified, a very interesting fact would be established, which at present remains somewhat doubtful, although Mr. Lukis, of Guernsey, marked a Limpet, and found it return to its haunt. These creatures belong to the order named Cyclobranchiata, from the breathing apparatus being arranged in a circle round the body. The pretty British shell, vulgarly called the Ark of Noah, but which is the Arca tetagrona, should be tried in Aquaria, as well as the elegant Heart-shell of the beautiful genus Isocardia; the movements and habits of the latter, as described by the Rev. J. Bulwer, being very curious and interesting.
The pretty little Cowry is an object that must not be passed over in silence, when treating of objects fitted for the marine Aquarium, although, in a little book of this extent, many others must of necessity be omitted. This beautiful little creature, Cypræa Europæa, carrying its porcelain-like dwelling on its back, is enabled to move steadily along by the action of its single foot. When in action, the mantle, as it is termed, which is the general covering of the body, is greatly distended, and protrudes from the shell, which it perfectly encloses, folding up at the sides, and meeting at the top, the joining being scarcely perceptible, and the whole surface fitting so tightly to the shell, that the little ribbings are seen distinctly through it. It is curious to observe the act of respiration, and all the associated phenomena of this wonderful little animal. The foot is pale orange, the mantle delicate olive, spotted with black and studded with protruding glands of yellow. It is, in short, when in a state of activity, a most curious and beautiful creature, of whose appearance and habits thousands, who only know and admire the deserted shell, can have no idea.
The bivalves, of which all are acquainted with at least one kind—the delicious edible Oyster—offer many animals suited to Aquaria. The curious Razor-shell, but for its habit of burrowing, would form a very curious object; and the Cockles, from the rich colour of their beautiful fringes, when the shells are partially opened for feeding and breathing, are very beautiful objects.
The means of movement of the common Scallop, or Cockle, and other bivalves, by means of a single fleshy “foot,” have been described in speaking of Molluscs in general; but the spinous Cockles, Cordium aculeatum and C. tubercutum, have been termed the aristocracy of the Scallop tribe. The valves of the largest open three-quarters of an inch, and the visible portion of the spongy-looking fleshy mantle is of a pellucid orange colour; at the end is protruded a double tube, thick and short, enveloped in a fringe of cirrhi or tentacles. The foot, which has been compared to a tongue, is smooth, glossy, and semi-transparent, like scarlet cornelian, and enables the creature to move about with great activity in an Aquarium; some that were sent, by the Rev. C. Kingsley, to Mr. Gosse, having startled that gentleman by the noise they were creating among the pebbles and other objects of his tank, by their rapid movements.
Many other kinds of shells might be mentioned if space permitted, but I must content myself with mentioning, en passant, the polished Donax, which, when the animal displays itself, exhibiting its bright yellow colouring, with its curious stripes and gay pink fringes, would form a real ornament to the drawing-room sea; and just hinting at the introduction of a specimen of the Tritonia Hombergi, remarkable for a power of producing an audible sound like the click of a steel wire.
The pretty little bivalve, the Lima hians, also forms a very attractive addition to the Aquarium, especially in motion, when its long orange fringes form a train or tail like that of a fiery comet, as Dr. Landsborough has observed, as it glides along, propelled by the discharge of a jet of water, the mechanism for the propulsion of which forms its swimming apparatus.
