THE PHYLUM MOLLUSCA 255 



incurrent siphon as the clam suddenly closes the valves, squirting water 

 out of both siphons. 



From the mouth a short esophagus leads to a small stomach. A 

 long looped intestine eventually turns dorsally and runs posteriorly 

 straight through the ventricle of the heart, around the posterior adduc- 

 tor muscle, and ends at an anus over the excurrent siphon. The stomach 

 is buried in a digestive gland that opens into it, and the intestine is 

 buried in the gonads. All lour organs are bound tightly into a rounded 

 visceral mass at the base ot the toot. 



The first portion of the intestine is divided longitudinally to form 

 right and left channels. The right channel functions as the intestine 

 and is continuous with the rest of the intestine. The left channel forms 

 a tubular, blind sac that contains the crystalline style, a structure 

 unique to the pelecypods and a few gastropods. In many pelecypods 

 the sac is completely separate from the intestine. The crystalline style 

 is a gelatinous rod secreted by the wall of the diverticulum which moves 

 slowly into the stomach where the end wears away. Its function is 

 similar to that of the salivary glands in snails, since it contains enzymes 

 for the digestion of carbohydrates. Little is known of the function of 

 the digestive gland. Although its cells may secrete digestive enzymes into 

 the stomach, it is more likely that they function as in the snail, phago- 

 cytizing small food particles. 



The paired gonads open through small ducts ending on urogenital 

 papillae, one on each side of the posterior part of the foot. Sexes are 

 separate. Eggs and sperm are released throughout the summer into the 

 sea where fertilization takes place. The embryo develops into a larva 

 known as a trochophore that settles to the bottom by autumn as a tiny 

 clam. It matures in about three years. 



The circulatory and excretory systems are similar to those of the 

 snails, except that two auricles and two nephridia are present. The 

 pericardial cavity surrounds not only the heart but also a small part of 

 the intestine. The tubular portion of each nephridium opens internally 

 into the pericardial cavity as well as externally on the urogenital papilla. 



The heart of Venus is used extensively in physiological research. If 

 one valve is removed from a live specimen, the beating heart can be 

 seen in the dorsal part of the body. The only further dissection neces- 

 sary is the removal of a portion of the mantle and one wall of the 

 pericardial cavity. A small hook can then be inserted into the ventricle 

 and attached by a string to a lever, so that both strength and frequency 

 of the beat may be recorded. After a "normal" record is obtained, vari- 

 ous drugs are dripped onto the heart and the results observed. Since 

 the moUuscan heart has been found to respond to the same kinds of 

 drugs that affect the human heart, the heart of J'eniis is used in some 

 laboratories as a means of measuring the strength of various drug ex- 

 tracts. The response is very closely related to the concentration of the 

 drug administered. 



The nervous system follows the typical molluscan plan. The brain 

 and some ganglia are located over the esophagus. A pair of large vis- 

 ceral ganglia can be easily distinguished on the anterior surface of the 



