144 
THE GASTROPODA 
right side; the connective passes from the right pleural to the 
infra-intestinal ganglion, and may have the effect of bringing 
Fic. 124. 
Trochus cinerarius, central nervous 
system, dorsal aspect, with the anterior 
part of the digestive tract. I, salivary 
gland; II, cerebral ganglion ;_ III, 
cerebro-pleural connective ; IV, pleural 
ganglion; V, right pallial nerve ; Wile 
pedal ganglion; VII, supra-intestinal 
part of the visceral commissure ; VIIT, 
posterior part of the glandular oeso- 
phagus; IX, infra-intestinal part of 
the visceral commissure ; X, abdominal 
ganglion; XI, oesophagus ; XI, radula ; 
XIII, supra-intestinal ganglion ;; XIV, 
osphradial ganglion and branchial 
nerve; XV, left pallial anastomosis or 
dialyneury ; XVI, glandular oesopha- 
gus; XVII, left pallial nerve ; XVIII, 
buccal mass; XIX, cerebro-pedal con- 
nective ; XX, stomato-gastric ganglion ; 
XXI, snout. 
the latter ganglion between the two 
pleural centres (Fig. 123, C, 2.7.9). 
The head of Streptoneura bears only 
a single pair of tentacles (Fig. 125, q). 
The radular teeth, when there is more 
than one on either side of the median 
tooth, are of several different kinds 
in each transverse row (Fig. 74, C, F). 
The heart is almost always posterior 
to the branchia. The sub-class in- 
cludes two orders, Aspidobranchia and 
Pectinibranchia. 
ORDER 1. Aspidobranchia. 
These are Streptoneura in which 
the nervous system is still but little 
concentrated (Fig. 124). The pedal 
centres have the form of long gan- 
glionated cords, to the anterior end 
of which the pleural centres are 
attached: the cerebral ganglia are 
widely separated from one another, 
and are united by a long commissure 
lying in front of the buccal mass and 
the salivary glands (Fig. 127, c.c). 
An infra-oesophageal or “ labial” 
cerebral commissure is present. The 
osphradium is but little specialised, 
and is situated on the branchial nerve. 
The otocyst contains numerous oto- 
conia. The eye is open (Fig. 100), or 
if closed has a very small pellucida. 
The central teeth of the radula are 
multiplied. | Ctenidia are almost 
always present; they are bipectinate 
and free at their distal ends (Fig. 
81, d). Asa rule, the Aspidobranchs 
exhibit well-marked traces of the 
original bilateral symmetry, having 
two auricles to the heart and two 
kidneys (Fig. 127), the last named 
opening to the exterior at the end of short papillae (Fig. 88, fp 
The gonad has no accessory organs and discharges its products into 
