182 THE GASTROPODA 
often reduced ; the latter may be partially covered over, or internal, 
or even absent. There is never an operculum in the adult, except 
in Amphibola, and an operculum is only found during development 
in the Auriculidae, Siphonariidae, and Oncidiidae, all of which are 
marine forms. In the pallial cavity the interior wall of the mantle 
is traversed by vascular arborisations (Fig. 86, X), and thus con- 
stitutes a pulmonary organ adapted for breathing air. In the 
Janellidae the pulmonary cavity is prolonged into fine respiratory 
canaliculi (Fig. 90, fr), and thus becomes a tracheal lung. It is 
much reduced in the Oncidiidae, and in Ancylus and the Vaginulidae 
it disappears as a consequence of the complete abortion of the pallial 
cavity. In some rare cases the pulmonary cavity may be filled 
with water, and then its wall may give rise to a secondary branchia 
which is not the equivalent of a ctenidium (Siphonaria, Fig. 174, 
III). In other cases the inferior pallial lobe, situated beneath the 
pulmonary orifice of the Basommatophora, may be transformed 
into a branchia (Planorbidae, Figs. 89, g, and 175, br). The auricle 
of the heart is usually anterior (Fig. 86), as is the case in the most 
archaic. Opisthobranchs, and it is only 
in the excessively detorted forms such 
as Testacella and the Oncidiidae that 
the ventricle lies in front of the 
auricle. The kidney usually has a 
more or less elongated duct or 
“ureter ” (Stylommatophora, Fig. 
86, V). In the nervous system, as a 
rule, all the ganglia are concentrated 
round the oesophagus and are closely 
apposed to one another (Fig. 146), but 
this is not the case in some archaic 
3asommatophora such as Chalina, 
Auricula, Latia (Fig. 96). In the 
Auriculid Pythia, the spermiduct re- 
tains the character of an open ciliated 
groove leading from the hermaphro- 
dite aperture to the penial orifice 
Fic. 171. (Fig. 171, ci): in other Auriculidae 
Reproductive apparatus of Pythia. this groove is simply closed to form 
Seen ania ea te a canal extending from the herma- 
Hest ar gg, ONO tees eo PUOOILe tO the male orifice. In all 
phrodite orifice ; 7.0, male orifice ; ‘ 
muc, mucous gland; pe, penis; ret, ree Other Pulmonates there is no longer 
tractor muscle of penis ; 7.s, recepta- : : Pi 
culum seminis; sp, spermiduct; spo, & Common genital orifice, but the 
Drove; Uh seminal vesicle hermaphrodite duct bifureates to 
form a distinct oviduct of greater 
or less length, and the primitive hermaphrodite aperture becomes 
the female orifice. As a result of secondary changes, the orifices 
