14 
Another important point is the gravity of the sea water; this should 
be very carefully regulated, for it must be borne in mind that many of 
the marine creatures are supplied by a permeation of water through 
their tissues or over their delicate and beautiful organs. ‘The specific 
gravity should not rise above 1.026 at 60° Fahr., and a small hydro- 
meter should be at short periods introduced to ascertain that this 
point is not exceeded, particularly during the hot months of summer. 
The reduction to this gravity can be readily effected by the addition 
of rain or distilled water. Many of the creatures will of themselves 
afford indications of this increase of density ; some of the Actiniz will 
remain closed and become coated with a white slimy covering within 
which they remain for a length of time, and if the specific gravity of 
the water be lowered this is very soon ruptured by their expansion, 
thrown off, and the tentacula become soon extended. 
All putrescent matter or excess of food or rejecta of the Actiniz 
should be carefully removed from the water, as the noxious gaseous 
compounds generated by the decay of such matters appear to diffuse 
themselves rapidly through the water, act as a virulent poison, and 
speedily destroy the vitality of the occupants. Thus many beautiful 
subjects were lost in a few hours from the introduction, into a small 
glass jar, of a large Pecten shell, encrusted with corallines, which had 
become loaded with putrescent matter by partial submersion in a foul 
muddy bottom. 
Great care should also be taken in moving the Actiniz that the foot 
or sucking disk with which it attaches itself to the rocks, stones, 
or weed, be not injured, as, when this occurs, they rarely survive, but 
roll about without attaching themselves, and gradually waste away 
and die. 
With these exceptions then, everything bas gone on very satisfac- 
torily, care being always taken not to overload the water with too 
large a proportion of animal life for the vegetation to balance, 
as, whenever this has been inadvertently attempted, the water has 
soon become foul, and the whole contents of the tank, both animal and 
vegetable, have rapidly suffered, and it has required some time before 
the water could be restored to its former healthy condition. 
In one of the numbers of the ‘ Zoologist’ of last year, I stated that 
besides the Ulve, Enteromorphe and Cladophore, I had found the 
Zostera marina a very useful plant for oxygenating the sea water; but 
this observation has reference only to the case of a tank supplied with 
a ground where its roots will find a sufficiency of food for its growth, 
as in a clear shingle or sand it soon decays; and it should be asso- 
