MOLLUSCA. 201 
of muscle fibres; it opens into the left and right aortae, the 
former supplying the circular muscle. Both aortae soon termi- 
nate in lacunar spaces, from whence the blood presumably 
passes to the gills. The blood is colourless, and contains 
amoeboid corpuscles. 
The nephridia are paired, but the right is much larger 
than the left. They open to the exterior by small renal 
papillae, situated one on each side of the anal prominence, and 
also, according to some observers, internally by two minute 
pores into the pericardium. ‘The existence of the reno-peri- 
cardial openings has recently been denied, both in Patella and 
in Fissurella.  Haliotis and Trochus possess a left reno-peri- 
cardial duct only. The left kidney lies between the rectum 
and the pericardial chamber. The right kidney, which is 
aborted in other Anisopleura, occupies a large space in the 
visceral hump. In part of its course it is closely applied to 
the generative organs, and when the ova and spermatozoa are 
ripe they are stated to burst into the lumen of the kidney, and 
so to leave the body through the renal papilla on the right of 
the anus. The lumen of the kidney is much broken up by 
ridges which project into it from its walls. The ridges are 
covered with glandular epithelium, which is partly ciliated ; in 
the substance of the ridges numerous blood-vessels ramify. 
The nervous system is very complex, it comprises several 
pairs of ganglia, the most important of which are the cerebral, 
the pedal, and the pleural. The cerebral ganglia are situated 
at the base of the tentacles, they give off nerves to the eyes 
and to the tentacles. The two ganglia are united by a com- 
missure above the pharynx ; they also give off a commissure on 
each side which passes to an anterior superior buccal ganglion. 
From each buccal ganglion two commissures arise, one uniting 
it with the similar ganglion of the other side, the other pass- 
ing posteriorly to a posterior superior buccal ganglion, which is 
in its turn united with the similar one on the other side. Thus 
the buccal nervous apparatus consists of a square of commis- 
sures with a ganglion at each angle. ‘The cerebral ganglia are 
connected with one another by a commissure which runs 
underneath the buccal mass ; this bears two small ganglia—the 
inferior buccal ganglia. 
