PHYLOGENY OF THE PELECYPODA. 337 



tion it draws out, or spins, the byssal tliread, wliich the creatui'e 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 thi'ead is spun. The second bj'ssal thread 

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

 thread; sometimes the two are sepai'ated by a distance of two or more milUmeters. 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 eleft-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, tlie foot may l)e arched np 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 

 byssal thread. While spinning the byssus the scallop is preoccupied and pays little at- 

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



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

 however, or at will without ajjparent cause, it may break the byssal attachment by cla[)- 

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

 as the impirlse moves it. If one bi'eak the byssus 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 are generally coalesced into one band, the distal 

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

 threads may measure a centimeter or more in length but ai'e 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 

 watei'. 



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. xxviii, figs. 1 and 6. Later the position of the ej'es being 

 the same, two tentacles alternate with each eye, PI. xxviir, 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 Verrill) 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 Ijorder of Pectens, 

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

 The border is produced perpendicularly in a thick marginal wall or ciu'tain wliich has 

 iiupdrtaiil ("uiiclions. This wall is highly developed along the [)rK'al('(l borders of I Ik; 

 valves but is icduced in thickness, and height as the base of llie angle forming tlie eai's, 



