146 MANUAL OF ANIMAL BIOLOGY 



sists of two parts : first, a glandular portion which lies ventrally 

 and, second, a storage portion, or bladder, which lies dorsal to 

 the glandular portion and in close contact with the ventral wall of 

 the pericardial cavity. These two portions are separated for a con- 

 siderable distance by a thin wall of tissue, but posteriorly they are 

 in communication through a small opening. The glandular por- 

 tion of the kidney opens into the pericardial cavity at the anterior 

 end through a small duct, the reno-pericardial aperture. The 

 bladder opens also anteriorly through a tiny opening, the renal 

 aperture. The nitrogenous wastes collected by each kidney 

 leave through the renal aperture and thence are swept out of the 

 animal with the current of water through the exhalent siphon. 

 (W. f. 49.) 



The kidney apparently functions in two ways. In the first 

 place, it is believed that certain wastes, present in the pericardial 

 cavity, can be drawn directly into the glandular portion of the kid- 

 ney by the cilia lining the tube which opens into the pericardial 

 cavity. However, the greater portion of the work of excretion is 

 done by the glandular portion of the kidney in taking the liquid 

 wastes from the blood passing through it. It is generally stated 

 that the kidney of the Clam represents a greatly modified type of 

 nephridium such as was noted in the Earthworm. Since the peri- 

 cardial cavity of the Clam represents the greatly reduced coelom, 

 we have, therefore, much the same situation as in the Earthworm 

 with the nephridium opening directly into the coelom. 



5. Reproductive System 



In most species of Clams the sexes are separate, although a few 

 species are hermaphroditic. The general structure of the male and 

 the female gonads is much the same. They are paired organs, 

 situated in the dorsal portion of the foot. The intestine which runs 

 through this region is more or less surrounded by the gonads. 



The eggs produced in the ovaries of the female pass out through 

 a small opening, known as the genital aperture, which lies 

 almost directly ventral to the anterior end of the pericardial cavity 

 and just below the renal aperture. In the fresh-water Mussel the 

 eggs then pass to the gills. The particular portion of the gills which 

 retains these eggs — it varies in different species of Mollusca — is 

 known as a brood pouch. The sperm of the male, produced in the 

 testes, passes out through a similar opening. The sperm, how- 



