112 



THE GASTROPODA 



such as Cydophorus 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,^.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 



Central nervous system of Patella vulgata, dorsal aspect. I, tentacular nerve ; II, left cerebral 



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, CydopJwrus, 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 



