rilYLOGENY OF THE PELECYPODA. 



333 



succeeded in getting very young specimens of either genus. The observations which 

 have been made on the development of these genera are given in section xvi. 



IX. Pecten. Anatomy and Habits. 



Material collected at Buzzards Bay, in 18S8 attbrded an opportunity to study Pecten 

 irradians. Lam., fig. 37, which is common on the southern shores of Cape Cod. Through 

 the kindness of Dr. Dall, I have also l^een able to study the young of several other species 

 from the collections of the Smithsonian Institntion at Washington. The consideration 

 of the anatomy and life habits of Pecten irradians is taken up in this section and a study 

 of the shell of that and the other species studied is described in section x. 



On the fourth of August, in examiningthe glass plates iumy draiu-pipe traps (descril)ed 

 in section ii), a number of extremely young Pectens were found. These specimens 

 l)roved of great interest on account of the shell foi-m 

 and structure and also on account of anatomical feat- 

 ures and the habits which I had the opportunity of 

 observing, as they were kept alive for several days. 

 Ho very young specimens were found on the sand- 

 bar immediately around the drain pipes although 

 diligently searched for, and the traps were a most 

 invaluable assistance in catching extremely young 

 molluscs which Avere easily seen on i^lates of glass by 

 allowing the light to strike them oblicpiely. 



The specimens found on the fourth of August 

 varied slightly in size as shown in PI. xxviir, figs. 

 9-12. A few of the same age were found on the glass 

 the next day; but later none of these youngest stages 

 were seen. The habits of these were alike in all 

 the individuals observed and the anatomical features were the same as far as traced. 



The young Pectens, PI. xxviii, figs. 1, 9, 12, were free, crawling on the glass, 

 byssus was observed until a considerably later stage and it is safe to say that this early 

 period has not yet developed the habit of byssal fixation. It is a free period, preceding 

 the byssal period. The young scallops were extremely active crawlers with a highly de- 

 veloped foot, which is capable of extreme prolongation, PI. xxvirr, fig. 1, and is often 

 produced beyond the edges of the valves to a length equal to the whole height of the 

 shell. "While lying on the right valve, the foot is extended thi-ough the notch close un- 

 der the hinge line and is attached by a sucker-like action to the glass, shortening its 

 length. It thus drags the shell along on its I'ight side. The motion is very rapid and 

 the little creature quickly passes over a considerable distance. Shells were fi'equently 

 observed to clap as in the adult; but they did not SAvim by this means and it is quite pos- 

 sible that they were not capable of doing so, as special features of the mantle which 

 serve the adult in swimming were not yet developed. The foot is so mobile and'exten- 

 sible that it may be twisted and turned in any direction at the will of the animal. 



Older scallops and the adults lie on the right valve, and ai-e i-arely fouiul lying on 



FKt. 37. — Adult I'eileii inadlans, L:im. 

 sizx". (From Biuney's Goukl.) 



Life 



No 



MEMOIRS BOSTON SOC. NAT. HIST., VOL IV. 



