SYSTEMATIC DISTKIBUTION. 
3 
vered with two separate shelly valves : they 
live attached to other marine bodies. 
IV. Pteropoda or Pteropods, having a promi- 
nent head, with one or two pairs of fins on 
the sides of the neck, by means of which 
they swim about on the ocean ; body is often 
covered with a thin glassy conoidal shell. 
Y. Cephalopoda or Cephalopods^ which have a 
large distinct head, furnished with eight or 
ten arms, by means of which they walk head 
downwards. 
The terrestrial and fluvlatile Mollusca, of which 
alone we have to treat in this little work, are con- 
fined to the two first of these classes. 
The SHELL, which is peculiar to this division of 
the animal kingdom, may be seen covering the young 
animal in the egg, before it has gained all its organs, 
as was observed by Swammerdam, and verified by 
the more extended observations of Pfeiffer, Turpin, 
and others. This may easily be seen in the egg of the 
Limnodi^ Physce^ Ancyli^ and BithinicPy which have a 
transparent coat. 
The shells of the newly-hatched animals have 
been frequently considered as distinct species ; and 
some very thin shells of land Mollusca, such as 
VitrincE, have been taken for the young of other 
well-known species, as H. hortensis. These young 
shells are easily known by their always being of a 
pale horn colour ; the whorls are generally rather 
irregular, and enlarge very rapidly; and the apex of 
the whorl which was first formed is generally large 
