OF MASSACIU'SKTTS. :>7 



mav have in climb to get their proper positions. I !' il were not for 

 tlu> ability lo climb upon the eel grass again, many detached scallops 

 would undoubtedly perish. 



Titrnhii/ Over.- -The young- scallop ( Figs. 49-51) possesses several 

 resources by which it may orient itself \vhen placed on its upper valve. 

 There are two general moans, one by help of the foot and the other by 

 the clapping of the valves. With the small scallop the use of the worm- 

 like foot is the primitive method of getting into a natural position. 

 When scallops from 1 to H/2 millimeters are placed on their left valve, 

 they at tirst appear uneasy. After a few moments the animal thrusts 

 out its foot, waves it around as if seeking a foothold, and finally ap- 

 plies the cleft tip to the bottom of the glass dish with a twisting motion. 

 By this movement the shell is pulled so that the hinge line is resting 

 on the bottom of the dish (Fig. 50), and the scallop pries itself over in 

 the opposite direction, naturally falling into its right position. This 

 operation is frequently aided by a slight opening and shutting of the 

 valves. The quickest way of turning over in case of the older scallops 

 is by dapping the valves, which flips the animal from one side to the 

 other. The animal may turn in a lateral direction on the hinge, but 

 the usual turn is anterior or posterior, either toward the foot or away 

 from it. Another means is to swim with foot extended, usually landing 

 proper side up. 



Eate of Traveling. Observations were made on the rate of travel- 

 ing of 3-millimeter scallops, placed both on the right and left valves. 

 Scallop A, resting in an unnatural position on the upper (left) valve, 

 compared with scallop B, resting on the lower valve, did not exhibit 

 as great speed or travel so great a distance. Although young scallops 

 do not seem to be as uncomfortable as the old when placed on the 

 left side, they do not move so rapidly as in a natural position. Out 

 of 19 consecutive moves scallop B showed 11 greater and 7 less than 

 scallop A. while one was the same distance. Scallop A traveled a 

 total di.-tance of 190 millimeters in thirty minutes, but actually gained 

 only 37 millimeters, owing to its random movements. The same ir- 

 regularity, in spite of its greater speed, which is accounted for by the 

 foot having a more direct line of tension for the single left retractor 

 when the animal lies on the right side, was noticed on scallop 15. The 

 animals may move for some time without going far, and may even 

 return to the exact place of starting. The distribution of scallops at 

 this age is probably determined more by wind and current than by any 

 movements of the animal itself. 



Swimming. - - The first attempt at swimming occurs when the sur- 

 face cells of the scallop embryo acquire cilia. The succession of rotary, 

 circular and straight line movements of the larva have already been 

 described for this period, and likewise for the early veliger. with 

 its ciliated velum or swimming organ. Also, the change from the 



