THE OYSTER 



69 



but next to the heart; one end is secretorj'-, 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 (tlie pedal ganglia) connected by a 

 commissure, thus forming an (Esophageal ring; and at the middle of 

 the mantle, near the base of the gills, is a tlurd 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 alcoliol. The ear of the clam Is situated in the so- 

 called foot; it bears the name of otocyst, and is connected with a 



Fig. 77. — Mytilus eduUs, common mussel, with its siphons expanded, and an 

 chored by its bjssus. 



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. 



