THE CLAM AND OTHER BIVALVES 163 



which it is once more collected and carried to the purifying 

 organs in the manner already described. 



The Excurrent Tube. Besides the rows of cilia which carry 

 food from the region of the in current tube to the mouth, 

 there are rows of cilia on the body and along the mid- 

 ventral line of the mantle, and it is known that these cilia 

 wave toward the incurrent tube. Food that has been rejected, 

 or waste that has accumulated, may be carried by the out- 

 waving cilia to the base of the incurrent tube. By muscular 

 contraction of the siphon at its base these substances may be 

 expelled through the tube. The dorsal tube of the siphon, 

 however, is the customary path of exit for substances that are 

 not used by the organism. All the undigested substances that 

 pass through the intestine must leave the animal by the dor- 

 sal tube. In addition, it is likely that wastes from the neph- 

 ridia, and from the gills and mantle, may pass from the 

 mantle-cavity through a slit-like opening (Fig. 82, 26) at the 

 base of the gills, and be carried out with the unused materials. 



The Nervous System. Lying on the right and left sides of 

 the oesophagus, just posterior to the mouth, is a pair of cerebro- 

 pleural ganglia (Fig. 82, 27). They are joined by a cerebral 

 commissure running over the oesophagus. 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 near by. The cerebro-pleural 

 ganglia are joined to the pedal (foot) ganglion (Fig. 82, 28) 

 by two connectives, one on either side (Fig. 82, 29). The 

 pedal ganglion controls the movements of the foot. The vis- 

 ceral ganglion (Fig. 82, 30) is joined to the cerebro-pleural 

 ganglia by a pair of cerebro-visceral connectives (Fig. 82, 31). 

 The visceral ganglion controls the organs in the posterior 

 region of the body. 



The Reproductive System. The pair of large glands (Fig. 

 82, 32), which in male and female clams contains spermatozoa 



