88 
collar spread over the neck before the shell Those ROOM XII. 
which live in water have compressed contractile ten- Nat> Hist. 
tacula, the eyes of some, as in the genera Auricula 
or Carychium , being placed on the inner part, and 
of others, as in the genera Lymnea, Physa, Planorbis , 
and Ancylus, on the outer side of their base. The 
Calusilioe and the three last-named genera have their 
whorls always turned to the left, and the Ancylus much 
resembles a Patella in shape, but has a notch in the 
muscular scar on the left side, where the hole is placed 
that leads to the lungs. Cyclostoma differs from all the 
other land mollusca, in having an operculum, and in the 
breathing cavity being open in front. 
Case 7 contains the shells of those animals which have 
their branchiae placed on the side of their back, under 
a kind of lid, as the Bulla and Bullosa, which have 
the body divided into two portions, and no tentacula. 
Some of the animals are very voracious, and eat shell¬ 
fish, for which purpose they are furnished with a giz¬ 
zard covered with three shelly plates, by which they 
can crack the shells in the stomach, after having swal¬ 
lowed them whole. Others, as the Aplysia, have 
tentacula; and emit a great quantity of a purple fluid. 
In this case are also the shells of those animals which 
have their branchiae placed on the right side, in a 
groove between the body and the foot, as the Pleuro- 
branchus; and lastly, those in which the branchiae are 
placed along both sides on the inner edge of the mantle, 
as the Patella , which has a simple conical shell, with 
its apex bent toward the head of the animal, and Chiton , 
which has the body covered by a hard cartilaginous 
shield, into which eight valves, laid one over the other, 
like plate armour, are inserted. 
Case 8 contains the Bivalve shells, the animals of 
which are compressed with a variously shaped foot, 
inclosed between the two-leaved mantles, having their 
two laminar branchiae placed on each side between the 
mantle and the foot. 
In some, the two leaves of the mantle are united 
Q 2 together 
