GASTEROPODA. 13 
The animals of this class are denominated in common lan- 
guage Univalve Shells, Slugs, and Sea-Worms. 
As their name imports, they move by a foot, which is placed 
on the lower aspect of their body ; this part is more or less 
developed in the different families and genera, as will be shown 
hereafter. _ 
The head (caput) is situated at the anterior aspect of the 
body, and is generally furnished with tentacula, which are 
either retractile or stationary. 
The families Limacide and Helicidee have four tentacula, the 
upper ones being longest, and bearing eyes at their extremities ; 
PYLE? fig? 1; oc. 
The orders Cyclobranchia, Phyllobranchia, Antrobranchia, 
and all the families of Saccobranchia, excepting the two men- 
tioned above, have but two tentacula, and when they have eyes, 
they are situated at their base ; internally, Pl. II. fig. 4 ; exter- 
nally, Pl. Il. fig. 10; above, Pl. II. fig. 7; or at their extre- 
mities, as in the newly discovered genus Assiminia, Pl. IX. 
fig. 4. 
The mouth (os) is simple and transverse in Helix; circular 
and tubular in Buccinum, Pl. II. fig. 10, os; vertical in Aply- 
slade. It is armed with jaws in Tritoniade, and with a tongue 
bristled with hooks in Aplysia. The lips (labia) are some- 
times produced into lobes laterally, and are liable to be mis- 
taken for tentacula. 
The body is elongate and capable of contraction in Limacide, 
Dorididze, &c.; spiral in all such as are covered by a spiral 
shell, and incapable of changing its form in the Cyclobranchia. 
The anterior part of the back in Limacide is formed into a 
sort of thoracic dise containing the heart and respiratory or- 
gans. The back is generally covered by the mantle (pallium), 
which is very variable in its form. In Pleurobranchidee it is 
widely dilated laterally, Pl. VII. fig. 4, 5, exceeds the foot in 
breadth, and covers the branchize above ; in Cyclobranchia it 
is attached to the shell within the margin on every side, Pl. II. 
fig. 11, pl. ; it is produced on each side into two membranes, 
which meet over the back in the Aplysiadze, covering the cavity 
that contains the organs of respiration, and forming by their 
