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.