46 THE SCALLOP FISHERY 



they immediately contract. If a few drops of formalin are placed in 

 the water near a small scallop a sudden clapping of the valves frequently 

 shoots out a detached portion of the gills. 



The Adductor Muscle. According to Jackson (4) the revolution of 

 the axis has brought about the loss of the anterior and retention of the 

 posterior adductor muscle in the adult Pecten. Naturally, as with the 

 oyster, there is one period of life, the early veliger stage, when the ante- 

 rior adductor is present. Then follows an intervening stage where 

 both are presumably present, and finally, by the time of the dissoconch 

 stage, the anterior adductor has disappeared. Sharp (15), like Jackson, 

 favors the view that the mechanical shifting of the axis of the shell has 

 caused the atrophy of the anterior and the subsequent enlargement of 

 the posterior adductor. In the adult the muscle is formed of two parts, 

 a large anterior and a small posterior division. The relative increase in 

 size of the muscle between 3 and 10 millimeter scallops is more rapid 

 than the formation of the shell, the muscle increasing sixteen times, the 

 shell only eleven times, in volume, and is possibly due to the need at 

 this period of a larger muscle. 



The Foot.. As the functions of the foot are given in chapter IV., 

 under " Locomotion " and " Attachment," little needs to be said here. 

 From the relatively largest organ in the scallop during its dissoconch 

 stage, the foot rapidly becomes smaller, owing to degeneration and lack 

 of use, until in the adult it is but a small projection on the antero-dorsal 

 surface of the visceral mass. 



The Visceral Mass. The degeneration of the foot marks the growth 

 of the visceral mass, which contains the reproductive organs and the 

 coils of the digestive tract. It is first noticeable to the naked eye in the 

 3-millimeter scallop as a mere speck on the ventral surface of the foot. 

 The reproductive organs are the last to mature and the last to be of use 

 to the animal before its decline, which theoretically starts at the com- 

 pletion of spawning. Even at this early stage it is covered with the 

 black pigment so prominent in the adult. A white streak running along 

 the anterior edge marks the situation of the testes. The rest of the 

 mass is covered with the pigment. The surface area of this mass for a 

 13-millimeter scallop is ten times greater than for a 3-millimeter animal. 

 The intestine does not form a part of the mass until the scallop has 

 attained a size of 8 millimeters, when the coils are enveloped. The 

 visceral mass continues to increase in size until in the adult it is the 

 largest part of the body. 



The Digestive System. 



(a) The Palps. The palps are formed soon after the disappearance 

 of the velum, and there possibly may be some connection between the 

 two as the velum disappears in the vicinity of the palps. At first they 

 are simple folds, as in the average lamellibranch, and not until later do 

 they assume the ruffled form which is characteristic of the adult. 



