The aquaria at Brighton and the Crystal Palace are very differently constructed and managed. At the former there is no actual circulation of water from one tank to another, but [pg 115]it can, if necessary, be renewed from the sea; the mass of the water in the reserve tanks is small as compared with that in the show tanks, and aëration is effected by pumping air into the tanks through tubes of large diameter. At the Crystal Palace aquarium a constant circulation is maintained from one tank to another; the bulk of water in the reservoir is five times as much as that in the show tanks, while aëration is accomplished by carrying a main over their entire length, from which, under pressure, a small stream of water pours from a tap into each, breaking the surface of the water, and carrying down to the bottom of the tanks and distributing over the body of their contents myriads of minute bubbles of air, which present an enormous oxydising surface to the water, rendering it bright and sparkling. It does not answer to change the water too constantly, while some obnoxious specimens, like the flat-fish, foul it greatly, the remedy for which is found in putting animals in who in the economy of Nature act as scavengers. Various small animals have to be supplied as food for the larger ones. “As the animal life and vegetable life mutually support each other, the kind of material necessary for maintaining the ‘compensating system’ must be watchfully supplied. Mr. W. R. Hughes, of Birmingham, recommends the growth of sea lettuce (Alva latissima) in tanks, as suitable both for oxygenating the water and for food for the fishes; the stock plants being introduced in the autumn months, when they are loaded with spores.” The writer of the article in the “Encyclopædia Britannica,” from which most of the above is taken, ventures to hope that the aquarium may become useful in a practical sense, and may determine many questions in regard to fish life and growth concerning which we are ignorant to-day. “It would,” says he, “tend to the better regulation of our fisheries and to the augmentation of our food supplies, if we knew as much about the herring or the haddock as we do about the salmon.” It is well known that fish, valuable as food, are too often captured at improper seasons and in a wasteful manner.
Passing on to higher forms of animal life, the polyps and acalephæ of the older authors, now classified as the Cœlenterata, we find creatures of a superior organisation to those lately under notice. Regarded generally, their bodies are soft and gelatinous, they possess alimentary canals and digestive apparatus, and in nearly all cases the sexes are separate, generation being sometimes sexual and sometimes by gemmation or budding. This brief introduction to the subject must be taken only in a general, not a special sense, for there are numerous exceptions to be found among the animals classified as Cœlenterata.
“The sub-kingdom Cœlenterata naturally divides itself into two groups—that of the Hydrozoa, and that of the Actinozoa. The fresh-water hydra will serve as an example of the first, and the common sea-anemone of the second group. The essential difference between the two is, that in the former the stomachal cavity is not separated from the general cavity of the body, and the reproductive buds are external; while in the latter the stomachal cavity is let down, as it were, as a partially-closed sac, into the general cavity of the body, and the reproductive buds make their appearance between the walls of the general cavity of the alimentary or stomachal sac, and consequently internally. But in both there is a free communication between these two cavities—a communication obvious in the Hydrozoa, and which may be often verified in the case of the sea-anemones, by the young [pg 116]anemones making their appearance at the mouth of the parent anemone, having just escaped from the general cavity out into the alimentary cavity of its body.”
MEDUSÆ.
The class Hydrozoa includes seven orders, first and principally of which let us speak of the Medusæ, of which the ordinary “jelly-fish” is a familiar example. This great order (Medusidæ) is characterised by having a disc, more or less convex above, resembling a mushroom or expanded umbrella, the edges of the umbrella, as well as the mouth and suckers, commonly having tentacula, or feelers, and cilia. Taken from the sea, a Medusa weighing fifty ounces will rapidly dissolve away to a few grains of solid matter. These floating umbrellas or mushrooms, as they might be termed, are of many forms, but they are all to be counted among the most beautiful works of the Creator. Sometimes the animal is transparent as crystal, sometimes opaline, now of a delicate rose or azure-blue colour, now yellow, now violet, and now, again, reflecting the prismatic colours. “The [pg 117]Medusæ are animals without much consistence, containing much water, so that we can scarcely comprehend how they resist the agitation of the waves and the force of the currents; the waves, however, float without hurting them, the tempest scatters without killing them. When the sea retires, or they are withdrawn from their native waters, their substance dissolves, the animal is decomposed, they are reduced to nothing; if the sun is strong this disorganisation occurs in the twinkling of an eye, so to speak.” If they are touched ever so lightly while swimming, they contract their tentacula, fold up their umbrella, and sink into the depths of the sea. At one period of the year the Medusæ are charged with numbers of minute eggs, which are suspended in festoons—crystalline roes they might be termed—from their bodies, and which in due time become living organisms.
After all, it is to the poets we must go if we would describe the beauties of Nature aright. Michelet, speaking of the Medusa, says:—“Why was this name, of terrible associations, given to a creature so charming? Often have I had my attention arrested by these castaways, which we see so often on the shore. They are small, about the size of my hand, but singularly pretty, of soft light shades, of an opal-white, where it lost itself as in a cloud of tentacles; a crown of tender lilies—the wind had overturned it; its crown of lilac hair floated about, and the delicate umbel, that is, its body proper, was beneath; it had touched the rock—dashed against it; it was wounded, torn in its fine locks, which are also its organs of respiration, absorption, and even of love.... The delicious creature, with its visible innocence and the iridescence of its soft colours, was left like a gliding, trembling jelly. I paused beside it, nevertheless; I glided my hand under it, raised the motionless body cautiously, and restored it to its natural position for swimming. Patting it in the neighbouring water, it sank to the bottom, giving no sign of life. I pursued my walk along the shore, but at the end of ten minutes I returned to my Medusa. It was undulating under the wind; it had really moved itself, and was swimming about with singular grace, its hair flying round it as it swam; gently it retired from the rock, not quickly, but still it went, and I soon saw it a long way off.”
The Medusæ are found in all seas, and usually inhabit the depths, although often seen on the surface. They voyage usually in considerable battalions, and sometimes cover miles and [pg 118]miles of sea. They constitute one of the principal supports of the whale. They are themselves singularly voracious, and snap up their prey—small molluscs, young crustaceans, and annilids—at a mouthful. Their mouths are in the centre of the lower side of the umbrella. They vary from a very small size to as much as a yard in diameter, while to describe the known varieties would occupy the remainder of this volume, so numerous are they. It has been ascertained that these jelly-like creatures breathe through the skin, have a distinct circulation and some nervous sensations. Most of them produce a stinging pain when they touch the human body, and until lately they were, adopting Cuvier’s classification, designated Acalephæ, or “sea-nettles,” in consequence.
PRAYA DIPHYES.