A Typical Bivalve Mollusk 



By throwing the mantle back over the dorsal margin of the 

 valve, the left gills, two layers of brown plate-like membrane, 

 of basket-work texture, appear. They are attached along their 

 dorsal margins to each other, to the body underneath, and to the 

 mantle above them. The right gills are like them. The central 

 space between the two valves is occupied by the body of the clam, 

 the visceral mass. Its ventral part is the oblique, muscular foot. 

 Above the foot the kidney, liver, stomach and heart are embedded 

 in the fleshy mass. At the anterior end two flaps, the labial palpi, 

 conceal the mouth. 



The bilateral symmetry of this mollusk is evident. The 

 central body is flanked on each side by a pair of gills, a mantle 

 and a hard valve. Right and left, the halves of the body are 

 alike. In this particular the univalve exhibits a marked differ- 

 ence. Clams have no heads, no tentacles, no eyes. 



The alimentary canal begins with the mouth, situated be- 

 tween the flapping palpi. There is no jaw nor radula. The 

 muscular oesophagus widens into a pouched stomach, from which 

 the intestine extends downward into the foot, and after several 

 convolutions passes backward, through the heart, ending in the 

 excurrent siphon. The liver surrounds a portion of the stomach. 



The inner wall of the intestine is folded to increase its se- 

 cretory surface. In the stomach and intestine is found a gela- 

 tinous, transparent substance called the crystalline stylet, which 

 often fills the space completely. Nobody knows the origin or the 

 use of this strange body. It may be a reserve store of food, 

 partially digested. It dissolves gradually when food is withheld, 

 and reappears when feeding is resumed. It may protect the stom- 

 ach walls from injury by sharp food particles. This is but one 

 of many unsolved problems in molluscan anatomy. The digested 

 food is absorbed by the veins in the intestinal walls. 



Breathing and feeding are closely allied processes, for the 

 inflow of water supplies the gills with oxygen and the stomach 

 with food. The gills are four in number, and each consists of 

 a double fold of delicate membrane. This membrane is composed 

 of parallel gill filaments, united by connective tissue, bent back 

 upon itself and stayed with cross bands between the two walls 

 of the single gill. An intricate system of fine vessels distributes 

 the blood through the gill substance, and an equally efficient sys- 

 tem of tubes and pores admits water, so that the interchange of 



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