Tin: oyster. CO 



but next to the heart; one i ad is secretory, communicating with tlie 

 pericardia] cavity, while the othei is excretory ami 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 armaria, Fi 

 it consists of three pairs of small ganglia, one above (the " brain") 

 and one below the oesophagus (the pedal ganglia) connected l>\ i 

 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 winch nerves are sent to the gills and to 

 cadi division of the siphon. This last pair of ganglia can he usually 

 found with case, without dissection, especially after the clam has 

 been hardened in alcohol. The car of the clam is situated in the so- 

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



Piq. 77. — SlytUut eduh/t, common mussel, with Its siphons expanded, and an- 

 chored by its byssus. 



nerve sent off from the pedal ganglion. It is a little white body 

 found by laying open the fleshy fool 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 coal of 

 connective tissue. From the epithelium spring long hairs; the sac 

 contains fluid and a large otolith. The structure of this oti 

 ma; lie considered typical for Invertebrates. 



The ovaries or testes, as the sex of the clam may be, are bilat< I 



symmetrical, blended with the wall of the vi ceral or liver-mass, 



and are yellowish. The openings for the exit of the < mar 



the base of the foot. 



In tin 1 oyster the two shells arc 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 s« inis about in the water. 



