CLASSIFICATION. 
and consequently made to assume the form of the fig. 8 as in the 
Viviparidw. 
Fig. 3. — Nervous system of Limneea siagnalis^ as 
typical of the short looped Euthyneurous condition 
(modified after Lankester). r.b. and Lb. right and left 
buccal ganglia ; r.c. and Lc. right and left cerebral 
ganglia ; r.p. and l.p. right and left pedal ganglia, 
with the otoc^’sts on their inferior face ; r.pL and l.pL 
right and left pleural ganglia ; r.v. and l.v, right and 
left visceral ganglia, the long nerve to the osphradium 
or olfactory organ Oy is given off from the right visceral 
ganglion ; ab. unpaired abdominal ganglion. 
Fig. 4. — Nervous system 
of Vivipara as a type of the 
Streptoneurous condition(after 
Ihering). r.b. and Lb. right 
and left buccal ganglia ; r.c. 
and Lc. right and left cerebral 
ganglia ; r.p. and l.p. right and 
left pedal ganglia with the 
otocysts on short pedicels ; 
r.pl. and Lpl. right and left 
pleural ganglia ; s.b. and s.p. 
the sub and supra-intestinal 
ganglia ; ab. unpaired abdo- 
minal ganglion. 
In our British species of land and freshwater shells, the orders 
Streptoneura and Euthyneura are exactly equivalent to the groups 
Operculata and Inoperculata respectively, with perhaps the doubtful 
exception of Nerithia. The Streptoneures are divided into two Sub- 
orders, Zygobranchia and Azygobranchia, the first-named embracing 
those Streptoneui’ous species in which although the semi-rotation of 
the organs has taken place and the gills and other organs become 
transposed in position, have yet re- 
tained their bilateral symmetry, the 
common Ormer or Ear-shell, Haliotis 
tubercidata is an illustration. 
The Azygobranchiata, of which 
Vivipara and other Prosobranchs 
are more or less typical, differ from 
the preceding group, owing to the 
compression and twisting of the vis- 
cera having led to the loss of one of 
Fig. 5. — A Streptoneurous Azygo- 
branchiate Gastropod. 
Valvata piscinalis Miill. X 3, 
Sway, Hants., coll, by Mr. C. Ashford. 
ct. ctenidium or exsertile branchial 
plume ; r.ct. filiform appendage, prob- 
ably^ the vestigial rudiment of the 
primitively left branchial plume. 
