xii 



PHYLUM MOLLUSCA 



731 



downwards and forwards a stout cerebro-pedal connective, and from 

 each pleura! ganglion a pleuro-pedal connective, to a large pair of 

 closely-united pedal ganglia (Figs. 616 and 617, pcd. g.) embedded 

 in the upper layers of the muscles of the foot, to which they give 

 off numerous nerves. The right pleural ganglion gives off behind a 

 supra-intestinal visceral connective, which bends across to the left, 

 over the oesophagus, and, some distance to the left of the ali- 

 mentary canal, expands into a triangular supra-intestinal visceral 

 ganglion (supra, g.), situated below the superficial layer of muscular 

 fibres. The left pleural ganglion gives off an infra-intestinal 

 visceral connective, which passes obliquely backwards and to the 

 right, below the alimentary canal, to a ganglion situated a little to 

 the right of the middle line the infra-intestinal visceral ganglion 

 (infra, g.}. The supra-intestinal ganglion gives off" a nerve which 



ped.^ 



FIG. ill?. Triton nodifcrus. Lateral view of nerve-ganglia and related parts. Letters as in 

 Fig. 016; in addition ant. aort. anterior aorta; cr. crop; (.vs. oesophagus; sal. du. salivary 

 duct ; r. sal. <j(. right salivary gland. 



runs towards the osphradium and ctenidium, which it supplies 

 with branch nerves, and unites with a stout mantle-nerve (I. mant. n.), 

 which is given off from the left pleural ganglion. The right 

 pleural also gives off a stout connecting nerve to the infra-intes- 

 tinal ganglion. From the supra- and infra-intestinal ganglia the 

 left and right visceral connectives are continued backwards and 

 unite behind in the neighbourhood of the stomach ; each ends 

 in a triangular abdominal ganglion (I. aid. g. ; r. aid. g.), and these 

 are joined by a transverse commissure, from which a number of 

 visceral nerves (vise, n.) are given off. A remarkable torsion of 

 the nerve connectives is here to be observed, the two visceral 

 connectives becoming twisted into the form of the figure 8. 



The organs of special sense of Triton, in addition to the 

 tentacles and the osphradium, which have been already referred to, 

 are the eyes and the stalocysts. The eye (Fig. 618) is a rounded 

 invagination of the epidermis with an inner wall or retina (ret.) 



3 A 2 



