xii PHYLUM MOLLUSCA 729 



mantle-cavity, to near the anterior end, where it terminates in 

 a short, freely projecting, spout-like portion, with the anus (an.) 

 at its extremity. 



The digestive gland or " liver " forms a mass of reddish-brown 

 glandular follicules which compose the greater part of the bulk of 

 the visceral coil. 



Vascular System. — Close to the base of the ctenidium, behind 

 it and a little to the right, is the heart, lodged, like that of the 

 fresh- water Mussel, in a cavity, the pericardium, lined by a trans- 

 parent membrane — the pericardial membrane. The heart consists 

 of two chambers, an auricle (Fig. 613, aur.) and a ventricle. The 

 auricle, which is the smaller of the two, is situated somewhat in 

 front of the ventricle, close to the ctenidium, from the main 

 central vessel of which it receives the blood. The ventricle (vent.) 

 is of somewhat pyramidal shape, but with the edges rounded 

 off. Its wall is extremely thick and muscular. Passing out from 

 the ventricle towards the right is a thick artery, which soon 

 divides into two, one running forwards, the other backwards — the 

 anterior (ant. aort.) and posterior (post, aort.) aortw. The former 

 is a very large trunk which runs forwards below the posterior 

 (esophagus, crop, and anterior oesophagus, giving off branches 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. 690). 



Excretory System. — There is only one nephridium (neph.), 

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

 a sac with thick, glandular, aud 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. 616 and 617) is more highly 

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

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

 together over the posterior part of the (esophagus, 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 each cerebral ganglion 

 there passes forwards a stout cercbro-buccal connective (cer. hue. 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 



VOL I 3 A 



