2928 DIVISION Il. MOLLUSCOUS ANIMALS.—CLASS III. GASTEROPODS. 
The eyes are very small, sometimes placed upon the head, sometimes at its 
base, either to the side, or at the tips of the tentacula. The class is divided 
into five orders, the characters of which are drawn from the position and 
form of the branchix. 
“The PuLMONEA breathe the atmosphere, receiving the air within a cavity 
whose narrow orifice they can open and close at will: they are hermaphro- 
ditical, with reciprocal copulation: some have no shell, others carry one, 
which is often truly turbinate, but never furnished with an operculum. 
“The Nuprprancuiara have no shell, and carry their variously-figured 
branchiw naked upon some part of the back. 
“The INFEROBRANCHIATA are similar, in some respects, to the preceding, 
but their branchiw are situated under the margins of the cloak. 
“The Trormrancnrata have their branchiw upon the back, or upon the 
side, covered by a lamina, or fold of the cloak, which almost always contains 
a shell more or less developed; or sometimes the branchiw are enveloped in 
a narrow fold of the foot. 
“These four orders are hermaphroditical. 
“The Hererorops carry their branchie upon the back, where they form a 
transverse row of little tufts, and are, in some instances, protected, as well 
as a portion of the viscera, by a symmetrical shell. What best distinguishes 
them is the foot compressed into a thin vertical fin, on the margin of which 
a little sucker often appears — the only trace left of the horizontal foot of 
the other orders of the class. 
“The PkCTINIBRANCHIATA have the sexes separated: their respiratory 
organs consist almost always of branchisw composed of lamelle united in a 
pectinated form, and which are concealed in a dorsal cavity, opening with a 
wide gape above the head. Nearly all of them have turbinated shells, with 
the mouth sometimes entire, sometimes emarginate, sometimes produced into 
a siphonal canal, and generally capable of being more or less exactly closed 
by an operculum attached to the foot of the animal behind. 
“The Scurrprancuiata have branchixw similar to those of the Pectini- 
branchiata, but they are complete hermaphrodites, and require no union with 
a second to effect impregnation: their shells are very open, and in several 
like a shield; they never have any operculum. 
“The CycLoprancHtaTa are hermaphrodites of the same kind as the 
Scutibranchiata, and have a shell, consisting of one or several pieces, but in 
no case turbinate nor operculate: their branchiw lie under the margin of 
their cloak, as in the Inferobranchiata.” 
“Nature,” it has been well remarked, “never passes abruptly from 
one type of organization to another ;” and thus we find a long series of 
