THE 0T8TBR. 
69 
but next to the heart; one end is secretory, communicating with the 
pericardial cavity, while the other is excretory and opens into the 
cavity of the body. 
The nervous system can be, with care and patience, worked out in 
the clam or fresh-water mussel. In the clam {Mya arenaria, Fig. 76) 
it consists of three pairs of small ganglia, one above (the brain") 
and one below the oesophagus (the pedal ganglia) connected by a 
commissure, thus forming an oesophageal ring; and at the middle of 
the mantle, near the base of the gills, is a third pair of ganglia 
(parieto-splanchnic), from which nerves are sent to the gills and to 
each division of the siphon. This last pair of ganglia can be usually 
found with ease, without dissection, especially after the clam has 
been hardened in alcohol. The ear of the clam is situated in the so- 
called foot; it bears the name of otocyst, and is connected with a 
Fig. *17.—Zliitiliis edulis, common mussel, with its siphons expanded, and an- 
chored its byssus. 
nerve sent off from the pedal ganglion. It is a little white body 
found by laying open the fleshy foot through the middle. Micro- 
scopic examination shows that it is a sac lined by an epithelium, 
resting on a thin nervous layer supported by an external coat of 
connective tissue. From the epithelium spring long hairs; the sac 
contains fluid and a large otolith. The structure of this otocyst 
may be considered typical for Invertebrates. 
The ovaries or testes, as the sex of the clam may be, are bilaterally 
symmetrical, blended with the wall of the vi-ceral or liver-mass, 
and are yellowish. The openings for the exit of the eggs lie near 
the base of the foot. 
In the oyster the two shells are unlike, the lower shell 
being usually larger than the upper. A single oyster may 
produce over a million young. In six hours after develop- 
ment begins, the ciliated germ swims about in the water. 
