THE MARINE AQUARIUM. 57 
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 sup- 
porting 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 
