£12 THE GASTROPODA 
such as Cyclophorus and Cypraea, have the form of long ganglionated 
cords with multiple commissures or anastomoses. The pleural 
ganglia are but slightly differentiated in Pleurotomaria, in which 
genus they are placed on the dorsal pedal connective, at a nearly 
equal distance from the cerebral and pedal centres (Fig. 94, pl.c). In 
forms in which they are better developed, the pleural ganglia are 
still in intimate contact with the anterior part of the pedal centres, 
and there are two long connectives, the cerebro-pleural and the 
AVII 
Be Te 
Vin 
Fic. 93. 
Central nervous system of Patella vulgata, dorsalaspect. I, tentacular nerve ; II, left cerebral 
ganglion ; III, cerebro-pedal connective ; IV, cerebro-pleural connective ; V, left otocyst ; VI, 
left osphradium ; VII, pallial nerve ; VIII, anterior part of the pedal cord ; IX, supra-intestinal 
ganglion; X, pedal cords (their posterior endings are not drawn); XI, abdominal ganglion ; 
XII, pleural ganglion ; XIII, otocystic nerve; XIV, stomato-gastric ganglion ; XV, optic nerve ; 
XVI, labial commissure ; XVII, cerebral commissure. 
cerebro-pedal, on either side of the digestive tube, the pleuro-pedal 
connective being, on the contrary, very short. This arrangement 
is known as the ‘‘hypoathroid,” and is found in Aspidobranchia 
(Fig. 93) and several Taenioglossa, viz. Ampullaria, Cyclophorus, and 
Nassopsis. As a result of specialisation we get the ‘‘ dystenoid ” 
condition, in which the cerebral centres are approximated and the 
pleural ganglia are shifted nearer to the cerebrals, so that the 
pleuro-pedal connectives are elongated (Fig. 123, A). Finally, in 
the “ epiathroid ” condition, the pleural centres are either in contact 
or are fused with the cerebrals (Fig. 123, B), as is the case in the 
