The Holothuriadæ, or Sea-Cucumbers, are very singular creatures; their form, as it floats in the waters, exhibiting as good a miniature representation of a small pickled Gherkin as can be conceived, except in colour, the shells or cases of these animals being generally white. One of the species, Hyalina, has a case which seems formed of crisp rice-paper, and is covered with spines of the same colour and texture. The tentacula, or breathing apparatus, eight in number, are curiously branched, and, when expanded, have the appearance of a skeleton flower, of which the figure in [Plate IX.] will convey a tolerable idea. The functions of this flower-like set of organs are probably the same as those of the Nudibranch class of Molluscs, which, though generally considered as being a breathing apparatus, are, probably, at the same time food-collecting organs, as all the creatures thus furnished are liquid feeders.
Thyone papillosa, one species of Sea-Cucumber, has ten branches to this set of organs, which it seldom displays when in captivity; but a little gentle motion artificially imparted to the water, as suggested in another place, would probably produce the kind of excitement requisite for their expansion, as the introduction of fresh water to the tank seldom fails to produce this effect for a time. When irritated, these creatures have the capacity of committing self-destruction in a most determined and complete manner, by expectorating the whole of the intestines and leaving their case or shell bare and empty. But Sir J. Dalzell has observed that the shell thus deprived of its living inmate, must be much more intimately connected with its life and organization than the shells of the Molluscs; for after a considerable lapse of time, he observed that the rejected parts have been renewed by gradual growth. The introduction of a single drop of fresh water will at once drive the creature to this summary mode of putting an end to the inconvenience.
OF THE NAKED OR SHELL-LESS MOLLUSCS.
The Sea-Lemon, Doris tuberculata, is one of the most attractive. It derives its popular name from its peculiar form, which is like that of half a Lemon cut longitudinally. It is generally of a yellow tone of colour also, which greatly adds to the fancied resemblance. It has its breathing apparatus exposed externally, like other Nudibranchs, spreading over the mantle, near the head, in a flower-like shape; and, as it moves slowly round and round the Aquarium, forms a very singular object.
The Doris pilosa is a pretty white species of the same order; and the little black shining Nudibranch, Runcina Hancoci, is a pretty and interesting creature. But the handsomest of the Nudibranch or naked-gilled tribe, as the term might be Englished, is the Eolis corronata, which forms certainly a splendid ornament to the Aquarium. Its general colour is a pellucid indistinct tone, of pinkish hue, the papillæ or branchiæ are in clusters, and the central canal is of a rich crimson. Different parts of the surface reflect the brightest metallic colours, and the whole creature has a very gem-like appearance. In captivity it is very active. Another species of Eolis has the power of making a singular clicking noise, like the Tritonia.
The Aplysiæ, or Sea-Hares, have been unenviably celebrated among their congeners as containing a virulent poison. The species common in the Mediterranean, A. Leporinæ, furnished the venom with which the infamous Locasta destroyed the enemies of Nero; and with which she eventually prepared, at the tyrant’s request, a draught for himself, but which he had not the courage to swallow. The British species, A. hybrida, might probably be kept in confinement.
Many of the Sea-Worms are very beautiful. The Nereis bilineata is very brilliant, with its crimson body brightly marked by two white longitudinal stripes.
The Phyllodoce are a class of Sea-Worms, somewhat resembling the land Centipede, which form curious objects of observation when they are in search of food. Instead of spreading a set of tentacles, like some of the Zoöphytes and Molluscs previously described, they have the faculty of turning the cavity which forms the stomach inside out, like a stocking, the inverted organ protruding from its mouth to a considerable distance, which, when it becomes sufficiently covered with the minute Infusoriæ which form its food, is drawn in, assuming gradually its natural position, where it remains till the nutriment so introduced has been absorbed, when the operation is repeated.
The Sea-Mouse, one of the largest and commonest of our marine worms, is of a flattened and somewhat oval form, pointed at each end, its general colour being pale brown. The clothing of silky hairs, however, with which it is invested, is so splendid, glittering in iridescent colours like the plumage of a humming-bird, that Lamark has appropriately named it Halithea, or Sea-Goddess—Linnæus having previously given it the name of Aphrodite, the Marine Venus. When, indeed, it receives the rays of light, and reflects them from the depths of the sea, rich with prismatic hues, the effect has been compared in splendour to that of the peacock’s tail when outspread in the sunshine. When in the Aquarium it crawls restlessly to and fro, as though anxious to exhibit its splendours in every possible point of view; the metallic tinges, changing with every position, being most magnificent by candle-light, when red and orange hues predominate, while by day-light pearly greens and blues are most frequent.
The bodies of many of the Euricidæ and Nereidæ exhibit changing colours of similar character, though less splendid; but many of them possess an opal-like tenderness and delicacy almost equally attractive.