THE CLAM AND OTHER BIVALVES 195 



that are not used by the organism. All the undigested sub- 

 stances that pass through the intestine must leave the ani- 

 mal by the dorsal tube. In addition, it is likely that wastes 

 from the nephridia, and from the gills and mantle, may 

 pass from the mantle cavity through a slit-like opening 

 (Fig. 101) at the base of the gills, and be carried out with 

 the unused materials. 



The Nervous System. In contrast with the nervous sys- 

 tem of the arthropods with its long series of ganglia dis- 

 tributed through the length of the body, this system in the 

 mollusks consists of but three pairs of large ganglia. Lying 

 on the right and left sides of the esophagus, just posterior 

 to the mouth, is a pair of cerebropleural ganglia. They are 

 joined by a cerebral commissure running over the esophagus. 

 As the name of the ganglia implies, there are two ganglia 

 joined in each nerve mass. One pair controls the "head' 

 region ; the other pair controls the sides of the body close 

 by. The cerebropleural ganglia are joined to the pedal 

 (foot) ganglion by two connectives, one on either side. The 

 pedal ganglion controls the movements of the foot. The 

 visceral ganglion is joined to the cerebropleural ganglia by a 

 pair of cerebrovisceral connectives. The visceral ganglion 

 controls the organs in the posterior region of the body. 



The Reproductive System. The pair of large gonads, which 

 in male clams contain spermatozoa and in the females the 

 eggs, lies in the midst of the coils of the intestine. Each 

 gland has a short, slender tube, with an external opening 

 (not shown) near the opening of the nephridium, just be- 

 low the attachment of the gills. 



Development. The very early history of the young Mya 

 arenaria is incompletely known. We know, however, that 

 after a short period of development and growth young long- 

 neck clams swim about on the surface of the water. Soon 

 after the appearance of their shell they sink to the bottom. 



