638 



EVOLUTION OF ANIMALS 



Part V 



Muscle refracts foot 



Muscle 



Muscfe retracts foot 



closes shells 



ncurrer^t 

 siphon 



Foot 



Fig. 31.7. The left mantle cavity of a clam. Movements of cilia on the gills and 

 surfaces of the mantle cause the currents (marked by heavy arrows) that carry 

 particles of food toward the mouth. Other cilia create currents (marked by 

 lighter arrows) that carry rejected particles outward over the folds of the mantle 

 as the clam lies with shells partly open when feeding. This occurs likewise in the 

 right mantle cavity. 



slightly bluish watery blood. At the same time metabolic waste is collected 

 from the tissues. The heart composed of two auricles and one ventricle is in 

 the pericardium near the hinge of the shell (Fig. 31.9). When the ventricle 

 contracts it forces blood forward through the anterior aorta and backward 

 through the posterior one each leading to the intestine and other organs of 

 the body. It is finally returned to the auricles. All the blood except that reach- 



Buprabranchial 

 chamber 



inner gill 



opening of 

 water tube' 



dorsal siphon 



mantle/'^ outer/ 

 cavity gUl 



ventral 

 siphon 



Fig. 31.8. Diagram of the circulation of water through the gills of a fresh-water 

 clam. Movements of cilia cause continual currents of water to pass into the hun- 

 dreds of pores in the gills, through the water tubes, and finally out of the dorsal or 

 excurrent siphon. (Courtesy, Brown: Selected Invertebrate Types. New York, John 

 Wiley and Sons, 1950.) 



