PHYLUM MOLLUSCA 



745 



Opisthobranchia, as already mentioned, it lies in front of the 

 ctenidia ; in the Streptoneura at the side or behind. Given off 

 from the apex of the ventricle is a large vessel which soon bifur- 

 cates to form anterior and posterior aortte. These are the main 

 trunks of the arterial system, which is more highly developed than 

 in the Pelecypoda ; the finest branches terminate in sinuses, as in 

 the latter class. 



The nervous system varies considerably in the different 

 groups in regard to the arrangement of the ganglia and their 

 commissures and connectives. 



In the majority the arrangement is nearly that which has been 

 described as occurring in Triton. There is a pair of cerebral 

 ganglia usually closely united, but in Patella (Fig. 635) widely 

 separated, situated over the 



cer:^ 



ospJi-. 



TTiantn 



gullet, and giving off behind 

 a pair of visceral nerve-cords, 

 in the course of which there 

 is placed laterally a pair 

 of pleural ganglia, which 

 are united together behind 

 in a median abdominal gang- 

 lion (or a paired ganglion, as 

 in Triton). In the course 

 of these visceral cords there 

 is a pair of visceral ganglia. 

 A pair of pedal ganglia 

 united together by a trans- 

 verse commissure and joined 

 to the cerebral ganglia by 

 connectives, give off behind 

 one or two pairs of pedal 

 nerves, as already mentioned. 

 A pair of buccal ganglia are 



connected by slender nerves with the cerebral. At the base of 

 each osphradium is usually a small osphradial ganglion connected 

 by a slender nerve with the visceral. In most Streptoneura 

 (Fig. 619), in accordance with the displacement of the anus, the 

 visceral cords are twisted, as already described in the case of 

 Triton, into a figure of 8. 



In Patella (Fig. 635) the pedal ganglia (ped. g.) give origin to 

 a pair of elongated pedal nerve-cords. In Haliotis and Fissurella 

 there is a similar pair of pedal cords which are connected together 

 by transverse commissures, and, in the latter genus, join one 

 another posteriorly. 



In the Euthyneura (Fig. 636) except in Actaion and Ghilina, 

 the visceral cords are not caught up in the twist of the visceral 

 mass, and do not cross one another. 



[G. 635. — Nervous system of Patella, cer. g. 

 cerebral ganglia; maiit. n., mantle-nerves; 

 OHph. (J. osphradial ganglia ; ped. (j. pedal 

 ganglia and pedal nerve-cords; j)t.g. pleural 

 ganglion. (After Bpengel.) 



