14 GASTEROPODA. 
junction, at will, anteriorly, a sort of siphon, by which water 
is conveyed to the branchie, Pl. VII. fig. 8. A similar effect, 
with slight modifications, takes place in the Purpuradee, by a 
folding of the mantle, which is considerably produced on the 
left side, into a more distinct siphon, situated in the channeled 
process, which is more or less developed in all the indigenous 
genera of that family, on the left side of their shell (excepting 
in those whose whole system is reversed or heterostrophe), by 
means of which they are enabled to respire, when their shell is 
closed by the opercule, Pl. II. fig. 10, ¢7. 
The anterior part of the back is sometimes furnished with 
tentacula, which are stationary or retractile. There are two, 
which are retractile within sunken or slightly prominent tubes, 
in Doridide and Tritoniade. The Eolidiadze have four of 
these tentacula, two of which are certainly incapable of re- 
traction. 
The foot is as long as the body in Limacide, Pl. VIII. 
fig. 1,2; longer than the body in Aplysia, and not capable of 
changing its form materially in the Cyclobranchia, Pl. II. 
fig. 11. 
The branchiz are subject to many modifications in form and 
position. They are spread over the surface of the inside of a 
sac-like cavity, in the orders Saccobranchia and Antrobranchia. 
They are foliaceous and placed im a sac-like cavity in the order 
Phyllobranchia, or upon the back, beneath the shell, in the 
order Aspidobranchia. The branchize are naked in Gymno- 
branchia ; they are branched and encircle the anus in the 
family Dorididze; they are branched and arranged along the 
sides of the back in Tritoniadz ; and are foliaceous and ar- 
ranged like tiles, over each other, on the sides of the back 
in Eolidiade, Pl. VII. fig. 3. In the order Stegobranchia 
they are situated on the sides, or sides of the back, and are 
covered by a reflection of the mantle. They are placed on the 
right side, between the junction of the mantle and the foot, in 
the family Pleurobranchide, Pl. VII. fig. 5; placed on the 
right side of the back and covered by a testaceous operculum, 
which is enveloped in the mantle, in Aplysiadee, Pl. VII. fig. 8; 
situated on the right side and covered by a shel im Marse- 
