ROOM XII.J NATURAL HISTORY. 73 
has a shell exactly like Patella in shape, while the animal 
very nearly resembles those of the two latter genera, 
except that it has only one branchia placed obliquely 
across the back of the neck, which is exerted when the 
animal walks. The larger species of these animals are 
eaten, and also used as bait. The eggs of some, as the 
Neritce , are ovate, covered with a horny skin and attached 
to other shells, and those of the Naticce have been described 
as a coral under the name of Flustra arenaria. 
Cases 5 and 6 contain the shells of Mollusca that breathe 
free air, for which purpose they are furnished with a cavity 
over the back of the neck, which cavity is internally lined 
with a quantity of vessels. Those which live on land, have 
cylindrical, retractile tentacula, as the Slugs (Limax), which 
have no shell or only a small internal one; the Tesiacellce , 
which have a small shell on the end of the body, and the 
Snails (Helix), Bulimus, Pupa,Clausilia , Succinea , and Vitri- 
na, which have large shells, differing from each other in form. 
The animals of the last two genera are so large, as scarcely 
to admit of their being withdrawn into their shells, and that 
of the last has a fleshy collar spread over the neck before 
the shell. Those which live in water have compressed con¬ 
tractile tentacula, the eyes of some, as in the genera Auri¬ 
cula 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 Calusilicc 
and the three last-named genera have their whorls always 
turned to the left, and the Ancylus much resembles a Pa¬ 
tella 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 Bullcea , 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 gizzard covered with 
three shelly plates, by which they can crack the shells in 
the stomach, after having swallowed 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 
E 
