XI.] THE FKESH-WATER MUSSEL. 109 



They are united by commissural cords which connect the 

 cephalic ganglia with one another, and with the pedal 

 and parieto- splanchnic ganglia, respectively. The only 

 sense organs which have been discovered, are a pair of 

 auditory vesicles, connected by nervous cords with the pedal 

 ganglia. 



The sexes are distinct. The testes and ovaria are similar 

 in character, being racemose glands, which, in the breeding 

 season, occupy a great part of the interior of the body. 

 There is one gland on each side, opening by a minute 

 aperture close to that of the organ of Bojanus. 



The spermatozoa have minute, short, rod-like bodies, 

 to which a long, filamentous, active cilium is attached, and, 

 thrown off in enormous numbers, make their way out with 

 the exhalent currents. 



The ova are spherical, and the vitelline membrane is 

 produced at one point into a short open spout-like tube, 

 with a terminal aperture, the micropyle, through which, in 

 all probability, the spermatozoon makes its entrance. When 

 fully formed, multitudes of these ova pass out of the oviducal 

 aperture and become lodged in the chambers of the gills, 

 particularly the external gill, which is frequently completely 

 distended by them. Here they are hatched, and give rise 

 to embryos, which are so wholly unlike the parent Anodonta, 

 that they were formerly thought to be parasites, and received 

 the name of Glochidium. The embryo Anodonta is provided 

 with a bivalve shell. Each valve has the form of an equi- 

 lateral triangle united by its base with its fellow, by means 

 of an elastic hinge, which tends to keep the two wide open. 

 The apex of the triangle is sharply incurved, and is produced 

 into a strong serrated tooth, so that when the valves ap- 

 proach, these teeth are directed towards one another. The 

 mantle is very thin, and the inner surface of each of its 

 lobes presents three papilla?, terminated by fine pencils of 



