XII 



PHYLUM MOLLUSCA 



UU5 



transverse commissures, and, in the latter genus, join together 



posteriorly. 



In the Euthyneura (Fig. 599) except in 



Actccon and C/iilina, the visceral cords arc n<>t 



caught up in the twist of the visceral mass, 



and the crossing of the cords does not occur. 

 In the Snails and other Pulmonata (Fig. 



600) 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 and at 



least two pairs of ganglia representing the 



visceral and pleural. A pair of small buccal 



ganglia are connected with the cerebral by 



means of slender nerves. 



The organs of special sense are the 



eyes, the otocysts, and the osphradia. In 



nearly all cases there are two cephalic eyes 



(Fig. 601), the position of which has already 



been referred to in the account given of the 



external characters. In structure they are 



the simplest in Patella (A), where each 



consists of a pit-like depression, lined by 



pigmented cells connected with nerve- 

 fibres. In the majority they have the 



structure described in the case of Triton. 



In certain species of Onchidium, 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 direct- 

 ed away from the centre of 

 the eye, as in Pecteii (see 

 p. 658) and the Vertebrata, 

 instead of towards it, as in 

 other Mollusca. The in- 

 ternal cavity of the eye is 

 occupied by a refractive 

 body composed of a few 

 large transparent cells 

 The otocysts are usually 

 placed in close relation to 

 the pedal ganglia, but arc 

 always innervated from 

 the cerebral. An olfactory 



FIG. 599. Nervous system 

 of Aplysia (Opistho- 



bnuichia). nlxl. abdo- 

 minal gang-lion ; c ( ,-. </. 

 cerebral ganglii >n ; o.</</( >' 

 g. osphraclial ganglion ; 

 ped. <j. pedal ganglion ; 

 /''. .'/. pleural ganglion. 

 (After Spengel.) 



'19 



FIG. Goo.-xervous system of Limn se us (Puimo- 



nata). a/n/. .</. abdominal ganglion ; c t r. ;/. cerebral 

 ganglion; ospfcr. ^ osphradiaT ganglion ; //.-/. 



i n ; ** * 1>leural gangli n ' (Aft " 



