GASTEROPODA. 
9 
a kind of lid or operculum to the aperture of the 
other valve. 
Sub-class I. Gill comblike^ formed of one or two 
series of lamellae ^ under the mantle on the back 
of the neck^ or rarely round its edge^ in an 
open gill cavity. Adult and larva shell- 
bearing ; larva furnished with deciduous cepha- 
lic fins. Unisexual or hermaphrodite^ and self- 
impregnating : Ctenobranchiata. 
Order I. Pecthstibranchiata.— Gills comblike, 
formed of one (or rarely two) longitudinal 
series of laminge^ or rarely of branched ves- 
sels^ on the left side of the back of the mantle 
over the back of the neck. Animal unisexual ; 
shell spiral. 
Order II. Scutibranchiata.^ — Gills consisting 
^of two series of lamellae, forming one or two 
series over the back of the neck, or under 
the edge of the mantle round the foot. Ani- 
mal hermaphrodite, self-impregnating ; shell 
conical, spiral, or symmetrical ; rectum often 
traversing the heart. 
Sub-class II. Respiratory organs variously formed^ 
gills exposed, or only slightly covered by a, 
fold of the mantle, or in the form of a closed 
lunglike cavity. Hermaphrodite, with reciprocal 
impregnation : Heterobranchiata. 
Order III. Pleurobranchiata.^ — Gills forming 
a tuft on the sides under a fold of the mantle. 
