338 ROBERT TRACY JACKSON ON THE 



fig. 37, p. 333, is reached. It is richly i)ig-mented on its outer face and marked by per- 

 pendicular folds and ridges. Just below the free, inner margin of the mantle wall it is 

 thickly set, with small equidistantly placed, arcuate tentacles, diagrammatically shown in 

 PI. xxA'iii, fig. 4, which may be called guard tentacles from their functional use. These 

 tentacles are yellowish, not papillose, incapable of extension and move onl}- in one plane. 

 When the scallop is at rest the guard tentacles stand upright. In young individuals the 

 tips of the tentacles of either mantle wall come in apposition, so that a grating or guard 

 is formed between the separated mantle walls. In virtue of this grating no large body 

 can pass into the gill chamber, while the animal is feeding with valves open, without 

 striking the guard tentacles. Collision with these tentacles commonly causes the an- 

 imal to shut its valves quickly. I say commonly, for irritation sometimes fails to pro- 

 duce the expected result. When swimming, the guard tentacles are turned outward and 

 laid flatly against the outer border of the mantle wall. In adults the valves frequently 

 are so widely separated that the tips of the guard tentacles are not in apposition, and 

 the tentacles are less sensitive to stimuli than they are in young scallops of two or three 

 centimeters in height. The guard tentacles are arranged equidistantly on the borders 

 of the mantle walls from the base of the ventral to the base of the dorsal ear. The re- 

 maining portions of the mantle walls which are relatively slight and low, bear no guard 

 tentacles. It is to be noticet!, that the regions free from guard tentacles are the regions 

 marked by the extension of the foot when young on one side and on the other marked 

 by the excurrent flow of eftete water, as well as by the flow of water when swimming. 



The mantle borders of young and adults are separated for the greater part of their 

 extent, being confluent on tlie hinge line and for a slight distance toward the byssal notch, 

 on the ventral border. Dorsally they are continent to the point marked as the inner 

 angle of the ear, fig. 37, p. 333, being a somewhat greater distance than that of the ven- 

 tral confluence. This relative confluence is correlated with the functional needs of the 

 animal, as excurrent action takes jilace doi'sally over such an area that it is not affected 

 by the partial union of the mantle boi-dcrs, whereas ventrally the foot actively swings up 

 to the hinge line, requiring and maintaining a greater separation of the borders to that 

 point. The mantle border is very broad and strong, and numerous, well-developed, radial 

 muscles which bifui-cate and anastomose freely serve to retract it. The muscles are more 

 highly developed in Pecten than in any other Pelecypod studied, which fact is probablj'^ 

 correlated with the active swimming habit. 



It is stated that Pecten swims by clapping the valves, but how the motion is effected 

 and the direction in which the animal moves has not been studied so far as I am aware 

 and seems to be commonly misunderstood. The swimming habit of Pecten is one of 

 the most striking features of this genus, and it is accomplished with such energy and 

 rapidity that the creature attains great speed. In swimming, as wlien at rest, the left 

 valve is always uppermost, and the plane of the edges of the valves is inclined to the 

 surface of the water at an angle of about 4:5°} 



It is best to study the swimming in young Pcctens some 3 cm. high, as at that age 

 it is more easily seen than in adults, and does not dilFer from what may be observed in 



' Till-' I'l'cli-iis ill wliicli llic swiiniiiiiin iialjit \v;is sl.iiilicil ;iii;;U> lihi'ii and soiiif oUit-i' di-lails iniglil vary slightly i( 



were couliucd in .small aijiiaria, and it is possible that the swiiiiiiiing freely in the ojjeii water. 



