XII 



PHYLUM MOLLUSOA 



711 



anterior oesophagus, giving off bnmrhcs to the region of the head 

 as it goes. The posterior aorta, narrower than the anterior, passes 

 into the visceral spiral, where it breaks up into branches for the 

 supply of the various parts. The blood-system consists in large 

 measure of sinuses, as in the fresh-water Mussel, and the general 

 course of the circulation is similar to what has already been 

 described in that Mollusc (p. 672). 



Excretory System. There is only one kidney (neph.), 

 a large organ situated dorsally, behind the pericardium. It is 

 a sac with thick, glandular, and highly vascular walls, the inner 

 surface of which is thrown into numerous complex folds. In front 

 it communicates directly by a large aperture (neph. ap.) with the 

 mantle-cavity, and by a narrower passage with the pericardium. 



The nervous system (Figs. 626 and 627) is more highly 

 elaborated than in the fresh-water Mussel. Two pairs of nerve- 



cer.buc.con- 



Fio. 627. Triton rubicundus. Lateral view of nerve-ganglia and related parts. Letters 

 as in Fig. 626 ; in addition ant. aort. anterior aorta ; cr. crop ; ces. oesophagus ; sal. </". 

 salivary duct ; r. sal. cL right salivary gland. 



ganglia the cerebral (cer. g.) and the pleural (pi. g.) lie close 

 together over the posterior part of the oesophagus, just where it 

 passes into the crop. The right and left cerebral ganglia are 

 fused together in the middle line, though separated by a con- 

 striction, and the ganglia of the two pairs are placed very close 

 together, though quite distinct. From ctich cerebral ganglion 

 there passes forwards a stout cerebro-buccal connective (cer. buc. con.) 

 to a buccal ganglion (r. buc. g.) situated on the posterior surface of 

 the buccal chamber. Also given off anteriorly from the cerebral 

 ganglia are optic nerves (opt. n.) to the eye and tentacular nerves 

 (tent, n.) to the tentacles. From each cerebral ganglion passes 

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

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

 closely-united pedal ganglia (Figs. 626 and 627, ped. 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 



