334 ROBERT TRACY JACKSON ON THE 



the left side. This hitter position, is due probably to an accidental overturning, which 

 the scallop alters at an early opportunity. Professor Hyatt informs me in a letter that 

 he has examined over three hundred specimens of Pecten irradians, all of which lie 

 habitually on the right valve. The upper, left valve very commonly is more or less over- 

 grown with algae, but I have never observed them on the lower right valve. This habit 

 of lying on the right valve is characteristic of many related genera as Perna, Spondylns, 

 Plicatula, Hinnites, and Anomia. Patten (53) in his studies of Pecten Jacobceus, says 

 that this species which has a deeply concave liglit valve and a flat left valve, lies habit- 

 uall}' on the right side, and if turned on the left side in an aquarium, soon rights itself. 

 The studies of young Pecten irradians were interesting on this point. I turned them 

 several times on the left valve intentionally and in every case they almost immediately 

 extended the foot with which they laid hold of the glass, and with a sudden jerk righted 

 themselves, showing that they Avere uneasy in the revei-sed position. While actively 

 crawling, the heart action is vei-y rapid as ma}^ be seen through the translucent valves. 

 The foot of Pecten is marked by a deep groove in its distal portion and by means of 

 this groove is capable of laying hold of objects of support by a distinctly sucker-like ac- 

 tion. By this means they are capable of suspending themselves and I have inverted a 

 glass slide on which a 3'oung Pecten was crawling and seen it hang perpendicularly sus- 

 pended by the foot. This only ajjplies to the very young, as older individuals have not 

 the power in sufficient degree to support their weight. The crawling action may, how- 

 ever, be seen in oldei" specimens more easilj^ than in the very young, as the}^ are large 

 enough to be observed without the aid of the microscope. 



The foot of a young Pecten has a well developed posterior retractor muscle, pr, PI. 

 xxYiii, fig. 1, on the left side. It takes origin in the proximal portion of the foot and is 

 attached to the left valve on the dorsal border of the adductor muscle. A dissection of 

 a Pecten of the same age as that of PI. xxviii, fig. 2, showed the posterior retractor in- 

 serted behind the adductor in its normal position. An anterior retractor was also observed 

 attached to the shell in the umbonal region. It is an iutei'esting fact that in P. irradians 

 there is only one postei'ior ]-etractor of the foot, and that one, the left retractor, whereas 

 normally there are two posterior I'etractors in Pelecypods, one of which is inserted in either 

 valve on the dorsal border of the posterior adductor muscle. This important peculiarity 

 of the absence of a right retractor is apparently due to the fixct that Pecten habitually 

 lies on the right valve. "When lying in this position it is obvious that the left retractor is 

 in the direct line of traction, while crawling with the foot extended at an angle so as to 

 reach the ground; whereas the i-ight retractor, if it exists, would not be in the line of 

 traction and has disappeared i)robably from relative disiise and atrophy.' In adult Pecten 

 mayeHanicas both posterior retractor muscles are wanting, and it is stated as character- 

 istic of the genus that the posterior retractoi'S are absent or present only on the left 

 side. Another featiu'e due to this changed position in the mechanism of parts is the 

 relative situation of the cleft in the foot. In Pelecypods which have a cleft in the foot 

 and also crawl on the ventral border of the valves, the cleft is in the ventral border of 



' A similar disappearance of tlie riglit retractor muscle cribes the loss of the rislit retractors to disuse and at- 



exists in Anomia, PI. xxix, figs. 1-2. Lacaze-Duthiers has rophy brought about by the peculiar position of the mus- 



demonstrated it in adult Anomia epkij^piitm and he there as- cles and their relations to the byssal plug. 



