In the distribution of the rocks, I would always allow at least one point to project above the water, in order to afford the opportunity to those animals whose instincts lead them to seek occasional exposure to the air, the means of gratifying it by that contrivance. I have thought, indeed, of constructing a kind of double Aquarium, and perfecting a contrivance by means of which a large portion of water should flow gradually from one tank to the other at fixed periods, in imitation of the ebb and flow of the tide. Many interesting phenomena would be exhibited in this manner, such as the closing of the _Actiniæ_ as the water receded, and their expansion as it covered them on its return. This alternation, too, might be found highly advantageous to the health and development of the animals whose natural habitat lies between high and low water-mark, and whose constitution is therefore framed to require entire or partial exposure to the air at certain intervals of time. I also prefer, as preserving a similar set of analogies, a sloping bottom, similar to that of the coast. For instance, if the slate back of the Aquarium be placed next the light, which is its proper position, as the light ought to penetrate the water entirely through its upper or horizontal surface, then I would fill the side next the slate back nearly to the top with pieces of rock, gradually reducing their height, till, at the other side, they should hardly rise above the floor of sand and pebbles, leaving, at last, a flat portion of the pebbly or sandy bottom quite level. When this form of rock-work is decided upon, the Aquarium should be of rather wider proportions than usual, in order to allow of the slope being pretty gradual. Supposing the tank to fill entirely the recess of a spare window, which is a position in which it looks exceedingly well, a solid slate back may be found to darken the vessel or the room too much; in such a case, a glass back must be preferred, which can be shaded from the direct influence of the light by a blue or green shade of calico neatly fitted to the frame; and it must be borne in mind, as essential, that the Aquarium must be so placed as to receive the direct rays of light during some part of the day, being screened by a white blind when the sun may be too powerful; as should the water become _tepid_, it would be fatal to many of the inhabitants of the miniature sea. With due observance of these precautions, the amateur may hope to frame and establish an Aquarium in a suitable form, and in a suitable position for the reception of its inmates; an account of which, and of the manner of their introduction, will form the subject of the ensuing chapters. [Illustration] ------------------------------------------------------------------------ CHAPTER IV. THE VEGETATION OF THE MARINE AQUARIUM.