52 THE SCALLOP FISHERY 



Lying on the right valve, the foot is extended on the surface of the dish, 

 the flattened distal portion taking a firm hold as if about to crawl. This 

 position is maintained for a moment or two and then the foot is withdrawn 

 within the body; by the motion of retraction it draws out, or spins, the 

 byssal thread, which the creature had fixed to the surface of the dish while 

 the foot was laid closely against it. Soon the foot is again extended, pressed 

 flatly against the dish, and another byssal thread is spun. The second 

 byssal thread is always attached at a point a little removed from the point 

 of fixation of the first thread; sometimes the two are separated by a dis- 

 tance of two or more millimeters. Additional threads may be spun; but 

 three was a common number with specimens in confinement. Those on the 

 bar, especially the larger individuals, frequently spun a large number of 

 threads in the byssus. The byssal gland is situated in a proximal cleft-like 

 depression in the foot separate from the more distal cleft-like depression 

 which serves the animal in crawling, so that between the two there is a 

 slight interspace without a cleft. Frequently when forming the byssus the 

 foot may be arched up in this interspace, the hold being maintained by the 

 tip of the foot and at the same time the byssal cleft being pressed closely 

 against the glass, so as to make the fixation of the byssal thread. While 

 spinning the byssus the scallop is preoccupied, and pays little attention to 

 pricks or stimuli which at other times would meet with immediate response. 



The following notes, which give additional information as to the 

 length of time, were made on a 6-millimeter scallop confined in a small 

 aquarium (Figs. 58-60). 



The scallop lay in an unnatural position on its left or upper valve 

 on the bottom of the glass dish. At 10.15 it extended its foot perpen- 

 dicular to its body, lashing it to and fro with a wavy motion, until it 

 was extended to its full length. Then, at 10.15V2, it placed the tip on 

 the bottom in a cautious manner. Soon after attaching the tip the 

 scallop contracted the foot, snapping its valves in such a way as to force 

 a jet of water from the posterior edge of the shell. This movement 

 forced the body ahead with a partial turn. The scallop thereupon with- 

 drew the foot, shooting two additional jets of water from the posterior 

 pseudo-siphon. During these maneuvers a one-stranded byssus had been 

 formed and was completed by 10.17. The byssus gland, meanwhile, had 

 been in contact with the bottom of the dish, and the thread was formed 

 by the opening of the groove and the hardening of the horny mate- 

 rial by contact with water. Another scallop of the same size was twice 

 observed to spin a byssal thread in four minutes, each time swimming- 

 through the water with foot extended in the interval between the attach- 

 ments. 



Period of Attachment. The scallop can cast off the byssus at will, 

 and soon spin another. The threads are broken off at the byssal gland, 

 where they are closely united, and left adhering to the object of attach- 

 ment (Fig. 43). This habit is altogether voluntary or under the effects 

 of external stimuli. The early life of the scallop thus consists of a 



