MOLLUSCS: THE MOLLUSCS 243 



Beneath the posterior adductor muscles note a small 

 white spider-shaped body, the more or less united visceral 

 ganglia of the nervous system. Posteriorly these ganglia 

 give off nerves to the mantle and gills, while anteriorly 

 there proceed two nerves, the ccrcbro-visceral connectives, 

 running forward, one on either side of the foot close to 

 the visceral mass, to the cerebro-pleural ganglia, paired 

 ganglia lying near the mouth. A delicate commissure 

 running over the gullet connects these ganglia. 



TECHNICAL NOTE. Cut away the skin and outer muscular layer 

 from the left side of the foot. 



Note the large stomach-cavity, surrounded by the 

 digestive gland. Trace the convolutions of the alimentary 

 canal through the foot to the anal exit. Note in the 

 anterior portion of the foot a fused pair of ganglia similar 

 to the visceral ganglia. These are the pedal ganglia, 

 which are connected by a pair of delicate commissures, 

 the cerebro-pedal connectives, with the cerebro-pleural 

 ganglia. Note the glandular tissue which fills the cavity 

 of the foot and surrounds the loops of the alimentary 

 canal. This is the reproductive organ, which has its exit 

 leneath the gills on each side of the foot. The sexes of 

 ihe mussel are separate, but the reproductive organs are 

 very similar. 



Life-history and habits. The eggs (ova) of the female 

 pass first into the supra-branchial chamber, whence, after 

 being fertilized, they drop into the outer pair of gill- 

 chambers. These outer gills serve as brood-pouches, and 

 here it is that the embryonic stages are passed through. 

 The embryo when ready to issue has a soft body enclosed 

 in two triangular valves. At this stage it is called a 

 glochidium. The glochidium on being discharged through 

 the exhalant siphon of the parent fails to the bottom, 

 where it remains for a time, when it attaches itself to some 



