42 
of my fish, though the death of some of them while he was an inmate, I 
attributed to his work. 
A very interesting class of occupants are the so-called Fresh Water 
Snails, among which I have always preferred Planorbis, whose shell 
reminds one of the fossil ammonite, Paludina, or marsh agate shell, 
Limnrea, and Physa. These are all and each a study in themselves. 
Seen only in their native state they would seem to possess few points of 
attraction, but when under observation they are very different. The 
species of Physa and Limnaea have a curious habit of floating on the 
surface with shell downwards, propelling themselves by a wavy motion 
of the wing-like apparatus that encircles their body. They all multiply 
rapidly, attaching their eggs by a transparent substance to the glass, or 
to stems of plants, but the voracity of their co-occupants, fish seldom 
allows them to come to maturity, so fhat there is an alarming table of 
infant mortality amongst these shell-fish. The utility of these as 
agents in keeping down the green growth of confervas in an aquarium is 
great and the way in which they clean the glass is most interes'.ing. 
The occupant of the shell puts forth his proboscis, turning it apparently 
inside out as we do a stocking, until the silky surface, which is the 
tongue, comes in contact with the glass. It then makes a sweep, like a 
mower's scythe, taking up into a swath all the confervas on 
that spot. The proboscis enfolds this, and the tongue takes upon it all 
the vegetation which it has collected and disappears in the animal's 
interior. A forward movement is then made and another portion of 
the glass is swept clean by the same process, so that the track of the 
snails upon the glass may be traced as distinctly as that of a mower, by 
his swath along a meadow. Although I have had what are known as 
" fresh-water clams," that is the Unios and the Anodons, as occupants, 
they are unsatisfactory, and if a person wishes to study their habits, I 
think they ought to have a receptacle for themselves. They require a 
muddy bottom in which to move, and their habits are so distinct and 
different from those of everything else that they would need segregation 
from other occupants. 
Water Beetles are another interesting branch of life under water, 
but the larger ones are too voracious to be kept any length of time. I 
introduced the large Horny-cased, Black Water-Beetles (species of 
