In the scallop, however, the organ is of a calibre that suggests 

 a sac more than a tube, that curves only to follow the curvature 

 of the muscle and opens at one end by a rather broad opening 

 into the pericardial cavity and at the other, by the slit like open- 

 ing already described, into the mantle chamber. The pericardial 

 opening of the right organ is a little more dorsal and a little 

 nearer the mantle than the other. This seems to be caused by the 

 shape of the adductor muscle which spreads out near its attach- 

 ment to the left valve so the left excretory organ is forced in 

 toward the median line of the body. 



In structure the excretory organs are practically racemose 

 glands. The pockets that are frequently found in the walls of 

 the nephridia of other forms are here greatly extended and 

 branched. The thick walls of the organs allow this without 

 causing a roughened exterior. There is no division of the organ 

 into glandular and non-glandular portions but it is glandular 

 throughout. 



The excretory organs are joined by the genital ducts near 

 their inner, pericardial ends. The relationship of the two is 

 further described in connection with the genital organs. 



GENITAL ORGANS. 



These organs occupy the greater part of the portion that has 

 been called the visceral mass (fig. 12, vm.). 



In this species of scallop the sexes are separate and may 

 easily be distinguished by the color of the sexual products which 

 give the color to the parts of the body containing them. In the 

 female the color is bright salmon pink to dull pink, apparently 

 differing with the number of eggs and possibly also with the 

 maturity of the eggs. In the male the' color is white or with a 

 tinge of yellow. Pecten irradians is hermaphroditic, with the 

 male portions of the organ dorsal, that is, near the foot, and the 

 female portion in the remaining large, ventral and posterior 

 portions of the visceral mass. The male portion is here white, 

 the female brownish yellow or orange. Both kinds of sexual 

 products are matured at the same time and there is considerable 

 reason to believe that individual fertilization may be and possi- 

 bly frequently is accomplished. 



33 



