PHYLOGENY OF THE PELECYPODA. 337 



tion it draws out, or spins, tlie byssal thread, which the creature had fixed to the surface 

 of the dish wliile 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 b3'^ssal thread 

 is always attached at a point a little removed from the point of fixation of the first 

 thread; sometimes the tAVO are sejiarated by a distance of two or more millimeters. Ad- 

 ditional threads may be spun; but three was a common number with specimens in con- 

 finement. Those on the bar, especially the larger individuals, frequently spun a large 

 number ot 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, 

 PI. XXVIII, fig. 7; 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 

 bj'ssal thread. While spinning the byssus the scallop is preoccupied and pays little at- 

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



The attached scallop commonly remains fixed for a considerable time. If disturbed, 

 however, or at will without apparent cause, it may break the byssal attachment by clap- 

 ping its valves. It then commonly swims about, soon renewing, or not, the attachment 

 as the impulse moves it. If one break the b3'ssus loose it is torn out from the foot and 

 left clinging to the sides of the dish. On examining separated threads we find that the 

 proximal portions of the several threads ai"e generally coalesced into one band, the distal 

 portions being separately attached to the glass by expanded disc-like extremity. The 

 threads may measure a centimeter or moi'c in length but are more often shorter. In at- 

 tempting to preserve specimens of Pecten in alcohol with the byssus intact it was found 

 that they immediately detached themselves. Success was attained however, by first 

 paralyzing them with cocaine, adding the solution gradually to a small volume of sea 

 water. 



The mantle of Pectens 2-3 cm. high has acquired adult characters. In young stages, 

 as shown, the eyes are situated one in each plication of the shell and a single tentacle 

 alternates with each eye, PI. xxvni, figs. 1 and 6. Later the position of the eyes being 

 the same, two tentacles alternate with each eye, PI. xxviii, fig. 2. In the age under con- 

 sideration, as in adults, the eyes are for the most part situated in the plications of the 

 shell, but in addition to one eye in each plication, new eyes have arisen between those 

 first formed. The new eyes, however, do not appear at regular intervals as did the first- 

 formed eyes. In each mantle border may be counted (as noted by Veri-ill) about forty 

 eyes, and between them are a large number of tentacles instead of only two as in the 

 earlier stage. These marginal tentacles, like those in the young, PI. xxviii, figs. 1 and 

 G, are papillose, extensible and active, frequently winding about a small object in a pre- 

 hensile manner. 



Besides an increase in the number of eyes and tentacles, the mantle border of Pectens, 

 of the age mider consideration, has developed new features not seen in earlier stages. 

 The border is produced perpendicularly in a thick marginal wall or curtain Avhich has 

 important functions. This wall is highly developed along the plicated borders of the 

 valves but is reduced in thickness, and height as the base of the angle forming the ears, 



