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PROCEEDINGS OF SOCIETIES. 71 
ing; this should never be done until late in the spring, and 
on no account in the autumn, as it leaves the tank with a 
weakened vegetation at the very time that its healthy func- 
tions are most required. 
The vegetation of the ocean is of a totally different cha- 
racter and composition, being very rich in nitrogenous con- 
stituents. There are three distinct coloured growths—the 
brown or olive, the green, and the red. For the purposes of 
the aquarium, where shallow water subjects are to be kept, 
the best variety is the green, as the Ulve, the Enteromorpha, 
Vaucheriz, Cladophora, &c. These should be in a healthy 
state, and attached to rock or shingle when introduced. We 
shall have occasion to notice the rhodosperms under the head 
of light. 
Scavengers —A most important element in establishing 
and maintaining the permanent balance between the animal 
and vegetable life; without which no healthy functions can 
be secured, and the aquarium must become a continued 
source of trouble, annoyance, and expense. The mollusc 
which was first employed, the Limnea stagnalis, was found to 
be so voracious, as it increased in size, that it had to be 
replaced by smaller varieties of Limnez, by Planorbis, and 
other species of fresh-water snail. The number of these should 
be adjusted to the quantity of work they are required to per- 
form. In the marme aquarium, the common periwinkle 
fulfils the required duties most efficiently, and is generally 
pretty active in his movements. The varieties of trochus are 
also most admirable scavengers ; but it must be borne in mind 
that they are accustomed to mild temperatures, and will not 
live long im a tank lable to much exposure to cold. The 
nassa reticulata not only feeds on the decaying matters ex- 
posed on the surface of the rockwork and shingle, but bur- 
rows below the sand and pebbles with the long proboscis 
erected in a vertical position, like the trunk of the elephant 
when crossing a river. But in the ocean there are innu- 
merable scavengers of a totally differmg class, as the anne- 
lids, chitons, starfish, nudibranch molluscs, &c.; thus afford- 
img a most beautiful provision for the removal of decaying 
animal matter, and converting it imto food for both fish 
and man. 
Light.—It is most probable that the greater amount of 
failures with the aquarium have arisen from the want of a 
proper adjustment of this most important agent; the ten- 
dency being generally to afford as much sun’s light as pos- 
sible ; but, on consideration, it will be found that this is an 
erroneous impression, When the rays of light strike the 
VOL. VI. G 
