have opportunity afforded them of living their own life in a state of confinement.
Rock work is generally considered an essential of a marine tank, but experience has convinced me that the less we have of it the better. I have, in describing the fresh water tank, given instructions for its formation, and here desire only to caution the beginner to repress, as much as possible, any desire for mimic arches, caves, and grottos. In the first place, it must be remembered, that every cubic foot of rock work displaces a cubic foot of water, and reduces the capabilities of the tank for supporting a number of creatures. The more rock the less water, and the less water the fewer animals. Beside this, it is questionable if the use of cement of any kind is advisable; free lime may be expected to dissolve out of it, however much seasoning it may have previous to the introduction of the stock; and as marine creatures are more delicately constituted than fresh water ones, the subsequent loss of many may fairly be attributed to the presence of cement.
In the place of built-up arches, a few rough pieces of stone tastefully disposed at the bottom may be made to produce a good effect: a rough block of granite, or a stem of branching coral in the centre will be far preferable, except for vessels of large size, in which pyramids and arches may be less objectionable. It must be borne in mind, however, that rock work in some form or other is useful, as affording shelter and shade to such animals as love seclusion, and that, in a well managed tank, the rough blocks often get coated with a vegetable growth that
