PHYLUM MOLLUSCA 253 



with the foot. The other nerve cord extends backward, terminat- 

 ing in a visceral ganglion which is usually located just ventral to 

 the posterior adductor muscle. The visceral as well as the pedal 

 ganglia are united. 



The sensory organs of the clam are primitive. Covering each 

 visceral ganglion is a patch of sensory epithelium called the os- 

 phradium, the function of which may be to test the purity of the 

 water brought in through the respiratory system. A short distance 

 back of each pedal ganglion is a statocyst which functions in equi- 

 librium. It is composed of a small calcareous concretion, the 

 statolith, which is surrounded by sensitive cells. In addition to 

 the sensory organs named, there are many sensory cells distributed 

 along the mantle edges and elsewhere which probably react to light 

 and touch. 



Excretion 



Paired kidneys lie on each side of the body just below the peri- 

 cardium. Each consists of a glandular portion which excretes 

 waste, and a thin-walled bladder that is connected with an ex- 

 cretory pore through which wastes are discharged to the outside. 



Reproduction and Life Cycle 



The small bivalves belonging to the family Sphaeriidae (Sphae- 

 rium) are hermaphrodites, but in the larger ones the sexes are usually 

 separate. The paired gonads are situated in the foot; the testis is 

 usually whitish in color and the ovary reddish. A short duct leads 

 from the gonad and opens just in front of the excretory pore. 

 Sperm are passed to the outside through the dorsal siphon and 

 enter the female clam through the ventral siphon. The ova, having 

 been discharged through the genital apertures, become lodged in 

 various parts of the gills, depending upon the species. Within the 

 gills the eggs are fertilized. Thus, the gills serve as brood pouches 

 or marsupia and may become greatly distended due to the tremen- 

 dous number (as many as three million) of developing embryos. 



The small bivalve larva, which ranges in size from about 0.05 to 

 0.5 millimeter in diameter, is called a glochidium and has a single 

 adductor muscle for closing the valves which may or may not be 

 hooked. Extending out from the center of the larva is a long secre- 

 tory thread, the hyssus. In most clams the glochidia are discharged 

 to the outside through the dorsal siphon. They fall to the floor of 



