474 The Animal Kingdom 



supplies the rectum and mantle. Blood is pumped both backward and 

 forward. Some of the blood is oxygenated in the mantle and returned 

 directly to the auricles. In the organs the blood is circulated through 

 veins and sinuses. From the organs other than the mantle, the blood 

 passes through the kidney into the gills where it is oxygenated and then 

 returns to the auricle. The blood is colorless and contains leucocytes. 

 Respiratory exchange takes place in the gills, each of which consists of 

 two very thin sheets or lamellae. Above the gills is a special chamber, 

 the suprabranchial chamber. Water coming in through the ventral 

 siphon passes into the gill lamellae which are divided by partitions into 

 many vertical water tubes. The water is finally collected in the supra- 

 branchial chamber and passed out through the dorsal siphon. Small food 

 particles are also carried in by the incoming currents. 



Fig. 154. — Diagram of circulation in the clam; arrows indicate 



direction of flow. 



Two kidneys are located just below the heart. Each is a tube 

 which removes wastes from the blood by a glandular structure and 

 from the pericardial cavity by a ciliated opening. The wastes are dis- 

 charged through an opening into the suprabranchial chamber. 



The nervous system is composed of three pairs of ganglia con- 

 nected by commissures. One ganglion, the cerebral, is located near the 

 esophagus ; another, the pedal, is in the foot ; and a third, the visceral, 

 is near the posterior adductor muscle. Several sense organs are as- 

 sociated with this nervous system. There are light sensitive regions 

 in the margins of the siphon, a pair of statocysts in the foot, chemical 



