A FRESHWATER MUSSEL 91 



lies with its forward end buried in the sand and its hinder end 

 with the siphons projecting into the water. 



Exercise 10. Draw a diagrammatic view of the digestive system. 



The reproductive system. The sexes are separate. The repro- 

 ductive glands (testis or ovary) are very similar to each other and 

 consist of a pair of cream-colored masses which fill a greater 

 part of the visceral mass. They communicate with the outside 

 through a pair of small openings, one on each side of the 

 visceral mass just below and in front of the opening of the 

 ureter. The openings can often be located by pressing out 

 from them eggs or sperm. The eggs, as soon as laid, pass into 

 the interlamellar space of the outer gills of the mother, where 

 they hatch. The young larvae are very immature and are 

 called glochidia ; they leave the mother and attach themselves 

 to the sides of fishes by means of a pair of sharp projections on 

 the ventral edges of the valves of the shell, where they lead a 

 parasitic life. While here they undergo a metamorphosis and 

 finally attain the adult structure, when they detach themselves 

 and drop to the bottom. 



The nervous system consists of three pairs of ganglia the 

 cerebral ganglia or brain, the pedal ganglia, and the visceral ganglia, 

 and the nerves proceeding from them ; each of the last two pairs 

 is joined with the brain by a pair of nerve-connectives. 



First find the visceral ganglia. They are a small grayish or 

 pinkish mass on the ventral surface of the posterior adductor 

 muscle with nerves radiating in all directions. Two of these 

 nerves, the cerebro-visceral connectives, will be seen passing forward, 

 one on each side of the visceral mass. 



Find next the brain. It consists of a pair of ganglia situated 

 above the mouth, just behind the anterior adductor muscle. The 

 two ganglia are not so close together as those of the visceral 

 pair; they lie on either side of the muscle and are united by a 

 commissure. Each ganglion sends out three large nerves the 



