FRESH-WATER MUSSEL 213 



tributed over the body. It is collected again in a vena cava lying below the 

 pericardium and is passed through the kidneys to the gills and back to 

 the auricles. 



251. Excretory System. — The organs of elimination are two U-shaped 

 kidneys (Fig. 119) lying just below the pericardial cavity, one on each side 

 of the vena cava. From each arises a thin-walled tube or ureter which 

 opens on the lateral surface of the body mass toward the anterior end and 

 at the level of the inner suprabranchial chamber. If the inner lamella of 

 the inner gill is attached beyond the point where it opens, the opening is 

 directly into that chamber. 



252. Musculature. — The muscles include, among others, the adduc- 

 tors which serve to close the shell (Fig. 119) and the protractors and 

 retractors which cause the protrusion and withdrawal of the foot. 



253. Nervous System. — The central nervous system is represented 

 by several centers, or ganglia, scattered throughout the body in pairs. 

 Among these are the cerehropleural ganglia near the mouth, the pedal 

 ganglia in the foot, and the visceral ganglia below the posterior adductor 

 muscles (Fig. 119). The two cerehropleural ganglia are connected by a 

 commissure and each one by connectives with the other two ganglia on the 

 same side. From the ganglia nerves lead to various parts of the body. 

 There are few sense organs. An organ of equilibrium lies in the foot a 

 short distance posterior to the pedal ganglia and below them. It consists 

 of a cavity known as a statocyst, containing a mass of lime called a stato- 

 lith, the movement of which stimulates the sensory cells. On the surface 

 over each visceral ganglion is a sheet of epithelial cells which appear to be 

 sensory and form an organ known as an osphradium, the function of which 

 is uncertain. It may be used in testing the water in the mantle cavity. 

 Tactile cells are abundant on the edges of the siphons and along the mar- 

 gins of the mantle. The margins of the siphons also seem to be sensitive 

 to light. 



254. Behavior. — Since the unios move about but little and may even 

 remain for a long period of time in exactly the same place, behavior is 

 confined mostly to the opening and closing of the siphons, which are 

 stimulated both by hght and by contact. The stimulation of the osphra- 

 dium by injurious substances dissolved in the water may cause the valves 

 to close. The sense of equilibrium would function in enabling the animal 

 to assume an upright position in case conditions forced it to take up a new 

 location. The anodontas move about freely and when turned upon one 

 side soon right themselves. 



255. Reproduction. — In mussels the sexes are separate. The 

 reproductive organs are situated in the body mass just above the foot, and 

 the vasa deferentia in the male, or the oviducts in the female, open just 

 in front of the opening of the ureters. The sperm cells when passed out 

 through the vas deferens into the suprabranchial chamber escape from the 



