That Pecten makes use of its ability to swim in escaping from 

 its enemies no one can doubt who has seen the shallow water 

 species, Pecten irradians, scatter when disturbed. How it 

 becomes acquainted with the presence of enemies is more a 

 matter of surmise than of experiment. That the tentacles are 

 exceedingly sensitive to touch is well known, and they may be of 

 great service in detecting disturbances in the water. That the 

 eyes are physically optical instruments that may produce distinct 

 images there is every reason to believe, but it seems to be diffi- 

 cult to get responses from specimens in aquaria that are without 

 question due to the sense of vision. No experiments definitely 

 to test their sense of vision have been devised, but it is very 

 probable that the eyes are important organs in detecting the 

 approach of enemies in shallow water where light is sufficient. 

 The shallow water scallop is known to migrate between shallow 

 and deeper water at different seasons of the year. Whether the 

 migration is due to search for food, temperature of the water, 

 enemies, or location for spawning, or to a combination of factors, 

 is not known. The giant scallops seem to shift their position 

 from time to time, but as they always remain in comparatively 

 deep water, the movement cannot be accounted for by either a 

 change in temperature or spawning. It is most likely due to 

 either continued disturbances by dredging, or enemies, or to lack 

 of food. The latter is the usual reason assigned. 



It is not entirely certain what relationship Pecten bears to the 

 usual form of lamellibranch as regards positions of parts. In 

 lamellibranchs that are supplied with two practically equal 

 adductor muscles, a .line connecting the two adductors runs 

 nearly lengthwise of the animal. In such a case the hinge line 

 is more or less dorsal, one end is anterior, and the other pos- 

 terior. When one of the muscles disappears, as is the case with 

 Pecten, one of the landmarks disappears and it becomes more 

 difficult to locate the direction of parts. Inasmuch as the hinge 

 line is usually dorsal, it is very natural to look at the hinge line 

 of this form as dorsal, and for matters of description it is con- 

 venient to so consider it. If, however, the position that the 

 anterior adductor would have occupied, had it been retained, be 

 considered, the position of the mouth, foot and heart indicate 

 that it would have to be placed much nearer the hinge line than 



