New-York daily tribune. [volume] (New-York [N.Y.]) 1842-1866, 1847-11-01

Let that be n water-basin. and here the level OF
the water We have here the pressure Of the at
mosphere upon the water. Let now fresh water
rnousca Of this class b. placed at the bottom Of the
vessel IN this position. II will rise % the surface
by expansion It will diminish its own weight
90 much that the animal will rise 20 the surface.
How is that possible The animal sinks because
IL s heavier than water. Butts weight IS sonear-
ly equal to that OF the water that the moment It by
CiiatlOn assumes larger size, than u dsplaces
greater quantity Of water and becomes therefore
comparatively lighter than the fiuid, and by this
process it slowly comes u the surface Of the water.
The moment il contracts II oil course again SINKS.
When az the surface Of the water the leet VIII be
upward. and by the contraction of the feet the pres
sure OF the air upon the water is sufficient resist
ance for this animal % walk along on the surface
OF the water, aid any one may have the pleasure
OF seeing that By observing the motions OF any OF
this species common 10 all our iresh-water
ponds. Nor is the motion OF this animal very slow.
/ have seen some not more than ma an lnch IN
length Walk iOOtin this way IN minute. This
shows that the resistance must be considerable.
All the castro OF eda, have that muscular disc at
the lower portion Of the animal, and have been 50
called because the disc is below the yisceral cavity
or abdomen
Here | take occasion 20 remark that many impro-
per names have been give % animals Of these lower
groups. because the animals themselves have not
been sufficiently understood still lt Is perhaps al
Nleahie retain iNf names rs ~ the uOuC OF th.ri
hcaton were 10 be- changed o. meet every fresh ac-
cession 10 our knowledge. the memory would l.
emit arassed and endless confusion be produced.
Most Of the castro opHin have the body covered
with shells but great number Of them are naked
Here are some very beautiful from the variety Of
their coiors. The size snd thickness OF the shell
vary very much Ia some the shell is so minute
that 1t covers only a small portion Of the animal
In others the shell IS entirely concealed in the skin
aud is seen only when the skin ls cut open. The
shell cannot be considered as a very important part
of the animal. There are almost many OF the
mousca destitute OF shells as there are possessed
Of them. Again. all those without shells bad
them when young. fthe shell were Of AS great
importance as il is generally deemed by concholo-
gisrs We would not see so many OF these animals
baked Though very beautiful and diversified the
shells then are Of comparauvely little Importance.
Yet the shells after al not without their value
in the study OF Natural History for II Is by means
Of them that we can study The mOllnsca OF ancient
geological times I'm n fossil state there have been
lound a great many of these univave shells. But
we DO not know how many or what sort OF naked
shells existed in those epochs simultaneously with
the others IL is only from those which had n
hard coat that we can form an idea of the CASTRO
opmdp OF ancient times.
The shell OF this class is often single fiat disc
without any circumvolutions. In that case II IS
formed Of successive layers. growing larger and
larger and forming cOncentric lines as you see
here The mode OF deposition Of this shell is l