Chap. 31 MOLLUSKS — specialists in security 639 



ing the mantle returns through the kidneys and gills where waste substances 

 are eliminated and oxygen received. Many animals must hunt for their food; 

 clams are relaxed receivers lying quiet while cilia-driven currents of water serve 

 them. Most of the water coming into the mantle cavity enters the gills. The 

 smaller particles of food become entangled in mucus on the outer surfaces of 

 gills and are propelled by cilia to the lips (labial palps). These are remarkable 

 sorting mechanisms that separate out the usable particles which are turned 

 into a groove between the lips and from thence go directly to the mouth. 



Food and Digestion. As before stated, clams feed upon bacteria and micro- 

 scopic plants and animals. Ciliated lips surround the mouth which opens into 

 a short passageway leading to the stomach that is surrounded by a greenish 

 black gland, the so-called liver, whose ducts empty a digestive secretion into it. 

 The intestine extends from the stomach into the foot where it is coiled about 

 the ovary or the testis as the case may be, then turns toward the dorsal side of 

 the body, extends through the heart and opens into the excurrent siphon. This 

 curious route is necessitated by the close quarters of the shell (Fig. 31.9). 

 Food is digested by secretions such as the enzyme amylase of the liver and also 

 within cells. Throughout the digestive canal ameboid cells are common. From 

 microscopic examinations it is believed that such cells make their way through 

 the walls of the canal, ingulf food and digest it, then leave the intestine and 

 return into the spaces between the tissues. Similar intracellular digestion occurs 

 in hydra and other invertebrates including starfishes. 



Excretion. The two kidneys are close to the heart (Fig. 31.9). They are 

 difficult to understand without special study but two important facts can be 

 made out. They are tubular and they are closely associated with the blood 

 vessels. Thus they conform in essentials with other kidneys. 



Coordination. The nervous system is mainly composed of three pairs of 

 ganglia and their connectives: one pair, the brain or cerebropleural ganglia, is 

 above the mouth, the usual location of a brain; the pedal ganglia are in the 

 foot; and the visceral ganglia just below the posterior adductor muscle (Fig. 

 31.9). The different ganglia of each side and the members of each pair are 

 joined by nerves. Small branches extend from the ganglia to muscles and sense 

 organs. 



Sense organs are few and their functions uncertain, as might be expected of 

 an animal living in unusual security. A minute structure near the pedal ganglia 

 is a typical organ of balance, a cavity containing a bit of lime surrounded by 

 sensory ceUs. The edges of the mantle contain cells pecuUarly sensitive to 

 touch, those of the siphon to touch and light. 



Reproduction and Development. In some bivalves, male and female organs 

 are in the same individual; in fresh-water clams, they are in separate ones. The 

 reproductive organs are in the foot packed between the coils of the intestine. 

 The sperm cells are shed into the excurrent siphon and carried into the open 



