A FRESHWATER MUSSEL 97 
lies with its forward end buried in the sand and its hinder end 
with the siphons projecting into the water. 
Exercise 9. 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 larve 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. Look for glochidia in your specimen. 
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 
