Although these results have been known for many years, it is only recently that any attempts have been made to carry out the same plan with regard to marine animals and plants in sea-water. The only plan adopted of keeping the Marine Invertebrate Animals was the laborious one of supplying them every day with water from the sea. For upwards of thirty years Sir John Dalycll carried en his observations on sea-animals in the city of Edinburgh upon this system. It was however known generally amongst naturalists, that by exposing sea water to the action of the air by pouring it from one vessel into another, it became perfectly purified. In this way many persona succeeded in maintaining alive, in the heart of London, Actinic, Star Fishes, and other low forms of marine animal life. It was however left for 3Ir.Warington to demonstrate what others had often theorised ou, and that was, that marine plants purified sea-water just as fresh-water plants purified fresh-water. That the same idea had occurred to others is proved by Mr. W. Thomson's communications to the a Annals of Natural History' (May 1853), and Mr. Gosse's account of Marine Vivaria in his Naturalist's Rambles on the Devonshire Coast' The difficulties, however, are greater in main taining the balance between the plants and animals in sea-water than in fresh. This arises from the more sluggish life, both of marine plants and animals, and the greater amount of disorganised matter which they throw from their surfaces. By care in the selection of sea-weeds, avoiding those which are large and throw off much matter from their surface, and not overcrowding the water with animal life, jars or tanks containing sea-animals and sea-plants can be easily managed. Mr. Warington recommends green sea-weeds, such as the species of Porphyra, &e. Mr. Gorse speaks favourably of C'hondrus crispus, lridtra (dulls, and the Dclesserice. In jars or tanks containing these pl ants various forms of sea-animals have been successfully kept for many months.
The greatest experiment of this kind which has hitherto been attempted is in a large glass-building that has been erected in the gardens of the Zoological Society, Regent's Park It was opened to the public in May, 1853. This building contains an area of 60 feet by 25 feet. The sides of this parallelogram are bounded by ponds of plate-glass, each being about 6 feet in length and 2 feet 6 inches in depth. They are placed at a height of about 3 feet from the ground, so that each division presents as it were a submarine picture 'on the line,' and may be approached so closely that the minutest animals not microscopic, may be watched with the most perfect success, under circumstances which differ as little as possible from those of nature. The whole of these tanks are supplied with gravel, sand, rocks, and sea-weeds, so as to imitate the rock-pools left on the sea shore by a receding tide, which indeed they may be said to represent; but with this great advantage to the observer, that instead of looking vertically into a cavity in which the light becomes less and less in proportion to the depth, he has here the means of examining each animal in its turn, under an effect which is not only most delightful in itself, but which, the water being seen in section through perfectly transparent walls, afford the best possible position for investigating the structure and functions of the living beings contained in it.
At present the water in the ponds or tanks in this establishment has not been left to the purifying influence of the plants which are placed in them ; a certain quantity of water being supplied to the freshwater tanks every day, whilst the salt-water is gradually drawn off and supplied again by dropping, so as to effect aeration by means of tubes above the tanks. This however is only precautionary, at the commencement of an experiment on so large a scale.
The tanks contain fresh-water animals and marine animals. The fresh water tanks present all the more common species of British Fishes, as the Pike, Tench, Perch, Roach, Rudd, Carp, Eel, Stickleback, Minnow, Gudgeon, &c. Some of the larger forms of fresh-water Cruatacea, as the Crawfish, have also been introduced. With these are placed a large variety of the fresh-water Mollusca, belonging to the genera Limneus, Planorhis, A nodon, Unio, &c. These tanks have been occupied since Christmas, 1852, with scarcely any loss.
The marine tanks are those which will undoubtedly always form the most attractive feature in an Aquavivarium, as we are less conversant with the habits of these creatnrea of the mighty deep, than with those of our ponds and rivers. In the establishment in Regent's Park all the classes of the Invertebrate Animals are represented as well as the'fish among the Vertebrate.
Amongst the Radiate Animals none are more remarkable for their power of resisting destruction than the A ct in iadf r, and all experimenters agree that they are amongst the animals which may be most success fully kept in the Aquavivarium. All the more common British species are now to be seen in the Regent's Park, and some of remarkable size and beauty. The Sertularian Zoophytes and the Polyzoa are also there, hut their animal inhabitants are too minute to be seen with the naked eye. Specimens of the Ecltinodermata, including several forms of Star-Fishes (A at cries), the Sun-Star, the common Sea-Egg, and a species of llolothuria, which have lived for some weeks, prove that undoubtedly these beautiful forms of animal life will live in confine ment, and lead to the hope that some of the rarer sorts from tropical oceans may find their way to our collections.
As was to be expected, the Mollusca thrive. In the sea they play the same part as in the fresh-water : they are the scavengers of the ocean. The Pinna, the Oyster, the Pecten, the Cockle, amongst bivalves ; and the Whelk, the Periwinkle, with many other univalves, have demonstrated how large a field of observation is in store for those who study the Mollusca. Several species of those gems of the ocean, the Nudibranchiate Molluscs, whose forms and colours are only known to us through the great work of Alder and Hancock, have been successfully kept alive ; whilst the red leaves of the species of Ilhodyntenia have been starred with their eggs. Various forms of Aseldian Mollusca have lived, and complete the evidence that this great group of animals may be watched in their living habits as easily as their shells may be examined in a cabinet.