24 Our Food Mollusks 



ing by one end into the chamber in which the heart lies, 

 and by the other to the exterior of the body near the 

 base of the gills. Their position is shown in Figure i, n. 

 Waste matter is extracted from the blood as it flows 

 through the walls of these kidneys, and is discharged to 

 the exterior through the outer opening of the tube. 



These nephridia, as they are technically called, may be 

 seen directly exposed as conspicuous swellings on the 

 under side of the large adductor muscle of the scallop, 

 but in the other forms their examination is difficult. 



The Sexual Organs. As a rule, our edible mollusks 

 are of separate sexes, though there are no secondary 

 sexual characters that will enable one to distinguish 

 male from female. The small warm water scallop ( Pec- 

 ten irradians), found from Cape Cod to Texas, is her- 

 maphroditic, that is, possesses both male and female 

 sexual glands. Hermaphroditism is a very common 

 condition among lower animals and among plants, but 

 where it occurs, it usually happens that the two kinds of 

 sexual cells come to maturity at different times, in order 

 to prevent self-fertilization. The breeding together even 

 of nearly related animals, usually tends to produce weak 

 offspring. The sexes are separate in the northern or 

 giant scallop, and in Venus and Mya. This is also true 

 of the oyster, while its near relative, the European flat 

 oyster, is hermaphroditic. 



Because the losses among young bivalves are so enor- 

 mous, immense numbers of eggs are produced. Fifty 

 or sixty millions would be a conservative estimate of 

 the actual number discharged by a large female oyster 

 during a single breeding season. Though the eggs are 

 minute, they are large enough to be distinguished by the 

 unaided eye. The sexual glands constitute the greater 



