NERVOUS SYSTEM 281 



proper and the pleural, which are distinct in certain other bivalves 

 and in whelks. The cerebral ganglia supply the fore-part of the 

 body, and each gives off a cerebropedal commissure to one of 

 the two pedal ganglia which He 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 gangha bear the same relation to 

 the cerebral that the subpharyngeal do to the suprapharyngeal 

 in the earthworm. The pedal gangha give off several nerves to 

 the foot, and each sends a nerve to a statocyst which lies just 

 behind it. Each cerebral ganghon also gives off a visceral commis- 

 sure, which runs backwards between the kidneys to join one of 

 the visceral or parietosplanchnic gangha which he as a fused 

 pair on the under side of the posterior adductor muscle, immedi- 

 ately 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 ganghon and is known as the osphradium, and tactile 

 nerve endings in various parts of the skin. There are no eyes. 



LIFE-HISTORY 



The sexes of the swan mussel are separate. Semen is passed 

 out through the dorsal siphon and spermatozoa are drawn into 

 the female with the inward stream. The eggs are fertihsed in the 

 cloacal chamber and then passed into the space between the 

 lamella of the outer gill, where they develop. This takes place 

 in the summer. In the following spring the young are set free. 

 They are larvae, very unhke the parent, and are known as 

 glochidia (Figs. 209, 210). Each has a shell composed of two tri- 

 angular valves, hinged along the base and with the apex drawn 

 out into a strong hook. There is no posterior adductor muscle, 

 anus, or foot, but in the place of the latter is a gland which secretes 

 a long sticky thread known as the byssus, comparable with the 

 threads by which the adult sea mussel anchors itself. When 

 some small fish, such as a stickleback, passes over the glochidium, 

 the latter flaps its valves so as to drive out the byssus, which if 

 it touches the fish sticks to it. The movements of the fish now 

 bring the glochidium against its body, whereupon the hooks are 

 used to hold on to its skin. The tissues of the fish become inflamed 

 and swell up, enclosing the httle parasite, which lives for some 

 months by absorbing the juices of its host, during which time it 

 M.z. — 10 



