Wherever the above phenomenon is apparent, rest assured that the aquarium is in good condition. It is, in fact, to the oxygen thus given out by the plants and infant vegetation that the animals owe their existence. If no algæ were introduced, the water would become impure, and unless changed often, your little colony would surely die,—at least those of its members who were unable to rise above the fluid, and occasionally breathe the fresh air.
The secret herein involved, that animal and vegetable respirations counterbalance each other, has only of late years been discovered; yet it is apparent to any observing eye at the sea-shore; there we never meet with a rock-pool containing living animals, that is not more or less adorned with sea-weeds.
The green Lettuce Ulva, so abundant in rock-pools, the sea-grass, which covers almost every fixed object at the sea-shore, or the well-known dulse or Chondrus Crispus, form the only sea-weeds that it is necessary to introduce into an aquarium. In fact, one or two fronds of the Ulva Latissima alone, will answer perfectly well to purify the water of even a comparatively large tank. I have often been surprised to find how small a quantity of algæ was required for the purpose mentioned. After allowing a single frond to float for a few days in a tank, in which some sea-water was newly deposited, I took it out, and for an entire twelvemonth the water remained healthy and as clear as crystal.
The arrangement of the 'stock' of an aquarium is quite a matter of taste; perhaps no two persons adopt precisely the same plan. It may, therefore, be advisable, as this matter is so arbitrary, for the writer to state how his own tanks are mapped out, leaving it to his readers to imitate the arrangements, or adopt a style of their own as they may think proper.
At one time I used to make a grounding of sand, but this plan is not to be recommended, even though it be one highly approved of by several species of crabs, &c. White pebbles do very well, but I now prefer to cover the base of the tank with crushed shells, washed very clean.
The following is a sketch of one of my tanks as it at present stands:—
In the centre of the vessel is a semi-circular arch, formed of pure white Sicilian marble, which has to my eye a most pleasing appearance. Around it, and indeed over the entire floor of the tank, are strewn chippings of the same material as the centre piece itself.[21] From the arch, at certain intervals, hang various sized specimens of the Mytilus edulis, which have gradually advanced to their more or less elevated positions entirely by their own unaided exertions. Near hand a hardy A. mesembryanthemum has taken up his abode, and sits with ever expanded tentacles, motionless and happy. On either side of the Anemone is deposited a riband of Doris spawn, that undulates to and fro whenever by any chance the water is in the slightest degree disturbed. Several soldier crabs, of course, act as sentinels of the tank, and appear to be ever 'on duty,' marching about in all parts of their subaqueous habitation; while beneath the marble fragments repose, each with his 'weather eye' open, a small Maia squinado, two long-armed crabs, and a small Carcinus mænas. On the sides of the vase rest a Limpet, a Trochus, and two fine Periwinkles, with skin of glossy blackness. The shells of either 'Buckie' is covered with myriads of quicksilver globules, that rest on the tips of the young and rising vegetation like dew upon the bladed grass. As I write, upon the inner surface of the water, like a fly upon the ceiling of a room, an Eolis and two pearly white Dorides lie idly floating in close companionship. Beneath them, upon the verge of the aperture of a large empty whelk shell, sits a pretty, cream-coloured Plumose Anemone (A. dianthus). On two blocks of stone repose several specimens of that mysterious animal the Pholas, who, by my unkindness, are thus made to become members of the marine 'houseless poor.' Several young specimens of these bivalves are seated, in a piece of rock, and daily engaged in 'boring.' A stick of wood, formerly the slender twig of a tree, is thickly clustered with fairy-handed acorn barnacles and serpulæ, and being placed against the glass, the movements of these singularly beautiful creatures can be watched with ease. Then there are two Star-fishes, a pack of three little Blennies, and a Five-bearded Rockling, whose singular movements I have previously alluded to. Against the arch some fronds of ulva are anchored, while at chosen spots specimens of delicate sea-weeds are also fixed—these rising up, and being magnified through the sides of the vase, have a pleasing effect, even to the eye of a child.
It is a pretty sight to watch the fishes glide under and around the marble arch, or throw themselves upon its highest point, there to enjoy the fresh air, and have a pleasant 'crack' together. This expression is literally correct, for the Blennies, when thus situated, usually make a kind of noise not inaptly expressed by snapping the nail of the thumb and finger together.
The foregoing animals which constitute the entire stock of one tank, are, I am proud to state, all in a healthy condition, and if we may judge by appearances, all contented and happy. It will be from no fault of mine if they do not long continue thus, and exhibit no signs of yearning for their native haunts by the sea-shore.
'Those gay watery grots—
Small excavations on a rocky shore,
That seem like fairy baths or mimic wells,
Richly embossed with choicest weed and shells,
As if her trinkets nature chose to hide
Where nought invaded but the flowing tide.'