746 



ZOOLOGY 



In the Snails and other Puhnonata (Fig. 637) 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 visceral and pleural. A pair of small 

 buccal ganglia are connected with the cere- 

 bral 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. 638), 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 pig- 

 mented cells connected 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. 707) and in the Vertebrata, 

 instead of towards it, as in other Mollusca. The internal cavity 

 of the eye is occupied by a refractive body composed of a few 

 large transparent cells. The 

 statocysts are usually placed in 

 close relation to the pedal 

 ganglia, but are always in- 

 nervated from the cerebral. 

 An olfactory, organ is present 

 in the shape of groups of cells 

 on the tentacles, in which the 

 fibres of an olfactory nerve 

 terminate. * 



The osphradia are promin- 

 ences, usually of simple form, 

 situated close to the base of 

 the ctenidium. In many of 

 the branchiate Streptoneura 

 (Fig. 639), as already men- 

 tioned in the case of Triton 

 (see p. 726, Fig. 613), the 



Flo. 630. — Nervous system 

 of Aplysia (Opistho 

 branchia). abd. abdo 

 minal ganglion ; cer. g 

 cerebral ganglion ; ospkr 

 g. osphradial ganglion 

 ped. g. pedal ganglion 

 pi. g. pleural ganglion 

 (After Spengel.) 



osplwjr 



Fiu. 037.— Nervous system of Limnacus (Pul- 

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

 cerebral ganglion ; ospkr. g. osphradhd gang- 

 lion ; ped. g. pedal ganglion ; pi. g. pleural 

 ganglion. (After Spengel.) 



