CHAPTER VII.
THE ASCIDIANS, BARNACLES, SEA-CUCUMBERS, NAKED MOLLUSCS, SEA-WORMS, ETC.
A group of _Ascidians_ forms a very curious object for the Aquarium, their forms being singular, and sometimes delicate as a transparent egg-shell. The group delineated in Plate VII. will convey some idea of the general appearance of these creatures, whose habitations might be taken for a store of fairy pitchers, placed snugly in their submarine china-closet for extra safety, and partially covered with sea-weed as a further means of concealment. There are above fifty native species, varying greatly in appearance. They may be found at the extreme verge of low water, many having the aspect of pellucid bags, formed of a substance between jelly and leather; while others present a far more robust and rugged appearance, both in form and texture. Some are very dingy in colour, but a few species—more rarely found—are very attractive, and sometimes brilliant, in their hues.
The Barnacles must not be omitted in furnishing an Aquarium, nor the fable connected with the Common Barnacle (_Pentelasmis anatifera_), in which it is affirmed that the Barnacle Geese were their offspring. Our old naturalist Gerard not only gives a detailed account of the transformation by which this wonder of the good old times was accomplished, but positively illustrates his description with an engraving, in which the metamorphosis is seen in progress.
The tube-like cells of the _Serpulæ_ have some resemblance to the cells of the Common Barnacle, but that of the solitary Serpula, _Serpula tubularia_, is much taller, often rising a foot from the substance it adheres to. The fan-like feathers forming the feeding, and, perhaps, also the breathing apparatus, of _Serpula contortuplicata_, are exceedingly rich in colour, as is also the member which acts as a “cork” to the tube when the feather-like tentacles are withdrawn, and which is familiarly termed the “stopper;” for when, on alarm, the feathers are suddenly drawn in, the “stopper” immediately follows, shutting up the opening of the tube in a very perfect manner. This organ is often of a rich orange, and the feathers a brilliant scarlet, though they are sometimes pale, or nearly white, as shown in Plate X. These fan-like organs, termed feathers, appear to act as breathing organs, by separating the oxygen from the currents of water which pass between their fibres. The _Sabella_, an allied tribe, forms its tube of mud; while that of the _Serpula_ is always of hard shell. The golden-combed worm, _Amphitrite auricoma_, another singular creature of this class, may be best alluded to in this place. Just below the cork-like head, when it leaves its tubular shell, are the scarlet gills, slightly resembling those of fishes, and across the head the golden comb-like appendage is expanded, from which it derives its popular name. When the animal retires within its tube, the upper part of the head has, like the _Serpulæ_, all the appearance of a cork or small stopper. This creature is one of the most curious of its class.
The _Balani_, or Acorn-shells, which are generally parasitic, fixing themselves to the shell of the Whelk or some other univalve, spread their crimson tentacles when seeking food exactly in the manner of the _Serpulæ_, the feathery filaments forming a kind of living casting-net, as it has been observed, in which the minute _Annelid_ or _Infusory_ is entangled and devoured. Two _Balani_ are represented in Plate VII. on the shell of a common Whelk.
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.
The little Sea-Slug, _Ægines punctiluceus_, is a brilliant little creature, well worthy the trouble of being permanently established in our Aquaria. Its general colour is pale reddish-brown; but, with the aid of a moderate lens, it exhibits a number of small black tubercles, in the centre of each of which is a speck of resplendent blue or green, forming a succession of gem-like ornaments that have been compared, by enthusiastic naturalists, to sapphires and emeralds; but, without exaggeration, the jewelry of this little creature may be said to surpass that of the Diamond Beetle, notwithstanding his superior reputation.
A few _Chitons_—a sort of Sea Wood-Louse—would do well in a tank; and a specimen of _Gastrophæna modolina_ is said to have thriven well during many months.