70 
NATURAL HISTORY. (UPPER FLOOR. 
formed of concentric rings. The young of the former are 
born alive, being then covered with bands of cilia. Then fol¬ 
low the Foolscap Limpet, (Cap-ulus,) which is attached, by 
the back of its foot, to shells and other marine bodies, on 
which it forms a smooth disc, either by dissolving the sur¬ 
face, or by depositing on it a shelly plate. Lastly, succeed 
the genera Velutina, Crepidula, and Calyptrea which have 
no opereula. The branchiee of the animals of the last 
two genera are formed of long filiform processes, placed 
at a very oblique angle across the back of the neck. 
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. 
Case 5 contains the shells of those animals which have 
their hranchise 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 prey on 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 
animals which have their branchiee placed on the right 
side, in a groove between the body and the foot, as the 
Pleurobranchus; and lastly, those in which the branchiae 
are placed along both sides on the inner edge of the man¬ 
tle, 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. 
Cases 6 and 7 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 Testacellce , 
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. 
