18 THE SCALLOP FISHERY 



Foot. - - The foot (f ) of the adult scallop is a small muscular organ 

 extending from the upper part of the visceral mass dorsally for about 

 a quarter of an inch. It is nearly cylindrical, with a deep groove on one 

 side and a hollow, sucker-like disc on its distal end (Fig. 42). It has a 

 slight twist which places the cleft portion in juxtaposition to the right 

 valve, instead of on the ventral border. This is of use to the young in 

 crawling, as the sucker can be put down evenly on the surface without 

 a twist of the foot to hinder the retraction. Jackson (4). The func- 

 tion of crawling is only for the young, and the adult has either lost the 

 power of locomotion through degeneration or at least does not make 

 use of it. A byssal gland (bg) is on the proximal end of the foot, and 

 secretes the bundle of threads by which the mollusk anchors itself to 

 various objects, as described under " Attachment " in chapter IV. 



The Reprodtictive Organs. -- The generative organs comprise a large 

 share of the soft parts of the scallop, and lie, surrounding the folds of 

 the intestine, in the lower portion of the visceral mass. The surface of 

 this mass, which terminates bluntly some distance below the large 

 adductor muscle, is usually covered with a black glossy pigment, which 

 is especially noticeable previous to and during the spawning season, 

 when it completely hides the bright color of the egg sac. Both the male 

 and the female organs are found in the same scallop, whereas in P. 

 tenuicostatus, the giant scallop of the Maine coast, the sexes are sepa- 

 rate. Drew (1). 



The testis (Fig. 75, ts) or male gland is a cream-colored organ lying 

 just ventral to the liver and foot and extending down the side of an 

 orange-colored sac. This sac is the ovary (ov) or female gland, which 

 during the spawning season takes on a bright orange color, presumably 

 due to the number and ripeness of the eggs. In size it is somewhat 

 larger than the testis, and lies ventral and slightly posterior to that 

 organ. During the early part of the spawning season, when full of 

 eggs and spermatozoa, these glands are well rounded and brilliantly 

 hued; but after the completion of spawning they become small and 

 lighter colored. In the ovary of the scallop previous to spawning are 

 many millions of little eggs in various stages of maturity. These eggs, 

 held in large follicles, are packed firmly in place, giving to the genera- 

 tive mass a smooth, plump appearance. Dr. James L. Kellogg has 

 kindly permitted the use of an illustration from his work on the " Mor- 

 phology of Lamellibranchiate Mollusks, 1890," which shows a section of 

 the generative organs of Pecten irradians, and supplements it with the 

 following excellent description : 



Fig. 67 represents a section passing vertically through the outer wall of 

 the visceral mass, where the testis and ovary are closely apposed. The 

 body wall is represented at ep and consists of a single layer of columnar, 

 ciliated, epithelial cells, whose nuclei are about equally distant from their 

 outer ends and the thick basement membrane (bm). In this epithelium are 



