MOLLUSCA 145 



dorsal exhalant siphon or cloaca. The wall of the rectum has a 

 longitudinal ridge or typhlosole (as in the earthworm). Two similar 

 ridges begin in the stomach and are continued into the first part of 

 the intestine. An oyster is said to strain 135 liters of water daily 

 to obtain its oxygen and food. GaltsofF, P. (1928, Bull. Bur. of 

 Fish., vol. 44), has shown that for the North Atlantic oyster the 

 maximum flow of water is 3.9 liters per hour at 25° C; for the Gulf 

 of Mexico oysters, it is nearly twice as much, 7.5 liters per hour at 

 25° C. 



Circulation. — The heart consists of a ventricle., surrounding part 

 of the rectum, and two auricles. The ventricle contracts and drives 

 the blood through the anterior and posterior aortae. Some of the 

 blood goes to the mantle where it is oxygenated and then returns 

 to the heart but not to the kidneys. The rest circulates through 

 the body and is finally collected by the vena cava just beneath the 

 pericardium. From the vena cava, the blood passes into the 

 kidneys and gills to the auricles and into the ventricle. Oxygen 

 and dissolved food are carried to all parts of the body, CO2 to the 

 gills, and other wastes to the kidneys. 



Respiration takes place through the surface of the mantle, and 

 by means of a pair of branchiae or gills made up of two lamellae on 

 each side, united at the edges except dorsally. A lamella is com- 

 posed of gill folds supported by chitinous rods and covered with 

 cilia. The cilia of the gills produce a current which sets in through 

 the inhalant siphon into the mantle cavity and through the ostia 

 and the water tubes into the suprabranchial (epibranchial) chamber 

 and out at the exhalant siphon. The ingoing current carries oxygen 

 for the aeration of blood, and also brings food such as diatoms and 

 Protozoa, which pass into the mouth between the ciliated labial 

 palps. The outgoing current carries excreta from the blood and 

 feces from the cloaca. 



Excretion. — The nephridia (organs of Bojanus) are a single pair, 

 one on each side of the body just below the pericardium. They are 

 U-shaped tubes bent on themselves and opening at one end into the 

 pericardium and at the other on the external surface of the body. 

 There are two parts — a brown, spongy, glandular kidney, and a thin- 

 walled non-glandular bladder with ciliated epithelium which com- 

 municates with its mate anteriorly by a large oval aperture. The 

 kidney receives excreta from the pericardium by cilia. Waste is 

 carried out through the exhalant siphon. The bladder receives 



