PHYLUM MOLLUSGA 



695 



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



posteriorly. 



In the Euthyneura (Fig. 599) except in 



Actceon and Chilina, the visceral cords are not 



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 Pecten (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 



FIG. Goo.-xervous system of Limnseus (Puimo- the pedal ganglia, but are 



nata). aba. ft. abdominal ganglion ; cer. g. cerebral 1 i . , 



ganglion; oi>hf. <j. osphradial ganglion; ped. y. always innervated trom 



geSUT lgli n ; Pl ' '* PlOUral gangli n ' (After the cerebral. An olfactory 



I'm. .">:>!). Xervous system 

 of Aplysia (Opistho- 

 branchia). a/itf. abdo- 

 minal ganglion ; cer. </. 

 cerebral ganglion ; oxj>/t.,: 

 (j. osphradial ganglion ; 

 1 1" i. ;/. pedal ganglion; 

 pi. g. pleural ganglion. 

 (After Spengel.) 



