726 



ZOOLOGY 



SECT. 



left becomes displaced to the right and comes to lie below the 

 alimentary canal (infra-intestinal). 



In Patella (Fig. 645) 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. 646), except in Actceon 

 and Chilina, the visceral cords are not caught 

 up in the twist of the visceral mass, and do 

 not cross one another. They unite behind in 

 a ganglion (abd. g.) which represents both the 

 abdominal and the supra-intestinal visceral. 



In the Snails and other Pulmonata (Fig. 647) 

 the ganglia of the nervous system are more 

 closely aggregated together. A pair of cerebral 

 ganglia overlie the oesophagus, and below it is 

 a mass of ganglia in which are to be made out 

 a pair of pedal ganglia and at least two pairs 

 of ganglia representing the pleural and visceral. 

 A pair of small buccal ganglia are connected with 

 the cerebral by means of slender connectives. 



The organs of special sense are the eyes, 

 the statocysts, and the osphradia. In nearly 

 all cases there are two cephalic eyes (Fig. 648), 



the position of which has already been referred to in the account 



given of the external characters. In structure they are simplest 



in Patella (A), where each con- 

 sists of a pit-like depression, 



lined by pigmented cells con- 

 nected with nerve-fibres. In the 



majority they have the structure 



described in the case of Triton. 



In certain species of Oncidium, 



a littoral Pulmonate, there are 



numerous eyes of a simple type 



scattered over the dorsal surface. 



In this case the optic nerve 



pierces the retina and the cells 



of the latter have their free ends 



directed away from the centre of 



the eye, as in Pecten (see p. 690) 



and in the Vertebrata, instead of 



FIG. 646. Nervous sys- 

 tem of Aplysia (Opis- 

 abd. 



ganglion ; cer. g. cere- 

 bral ganglion ; osphr. 

 g. osphradial ganglion ; 



(After Spengel.) 



-, . . -i -ii 



towards it, as in Other MolluSCa. 



Tile internal cavity of the eye 



FIG. 647 Nervous system of Limnaeus (Pui- 



monata). abd. g. abdominal ganglion ; cer. g. 

 cerebral ganglion ; osphr. g. osphradial gang- 



* 1 *' ' pleural 



