THE SWAN MUSSEL 



323 



which lie side by side in the foot, just above the muscular 

 region, about one-third of the length of the organ from its 

 front end. The pedal ganglia bear the same relation to the 

 cerebral that the subpharyngeal do to the suprapharyngeal 

 in the earthworm. The pedal ganglia give off several 

 nerves to the foot, and each sends a nerve to a statocyst 

 which lies shortly behind it. Each cerebral ganglion also 

 gives off a visceral commissure, which runs backwards 

 between the kidneys to join one of the visceral or 

 parietosplanchnic ganglia which lie as a fused pair on the 



/b m, 



Fig. 231. — A glochidium larva in ventral view. — From Latter. 



6., Byssus (cut short) ; d., future mouth ; m., adductor muscle ; s. t sensory cells ; 

 t. t main teeth and denticles on ventral edge of each valve. 



under side of the posterior adductor muscle, immediately 

 within the skin. The sense organs are inconspicuous. 

 They include the statocysts, the tentacles of the ventral 

 siphon, a sensory epithelium, believed to be olfactory, which 

 covers the visceral ganglion and is known as the osphradium, 

 and tactile nerve endings in various parts of the skin. 

 There are no eyes. 



The sexes of the swan mussel are separate. Sperm is 



passed out through the dorsal siphon and 



y ' spermatozoa are drawn into the females with 



the inward stream. The eggs are fertilised in the cloacal 



chamber and then passed into the space between the lamellae 



