60 THE SCALLOP FISHERY 



portion of the valves. The distance covered on the back dart was about 

 10 millimeters. This observation was made several times on different 

 scallops, and is interesting, as it shows that the scallop can force water 

 from different parts of its shell, in this case exactly at right angles to 

 its usual direction. Darts can likewise be made in either an anterior 

 or a posterior direction, showing that the body can be forced in any 

 desired course by changing the point of expulsion of water from the 

 shell. This habit is of a protective benefit to the animal, as the ordinary 

 method of locomotion would be such as to carry it to an advancing 

 enemy rather than allow its escape by a backward " shoot." This 

 method is closely associated with the tactile functions of the tentacles, 

 and it is only when stimulated that the scallop makes use of it. 



RESTING. 



The young scallops as well as the old have periods of rest, during 

 which they probably feed or merely lie inactive. There are three kinds 

 of rest: (1) the scallop attached by the byssus; (2) lying unattached on 

 the bottom; (3) floating on the surface of the water. 



Attachment. --This position has been defined under " Byssal At- 

 tachment," and only the appearance of the scallop needs description. 

 The animal is probably in a feeding position, the mantle with its tenta- 

 cles is extended, the heart beats slowly, and the food particles rotate 

 in the digestive tract. In this position the animal is keenly sensitive 

 to stimuli and if touched closes its valves at once. 



Resting on the Bottom. During the intervals of crawling the young- 

 scallops often rest on the bottom for a long time. Even in this resting 

 position part of the internal anatomy is constantly moving. The cilia 

 in the gills, in the digestive tract and on the foot are always lashing, 

 while the foot is often restless and writhes within the shell. The tenta- 

 cles are generally extended and the shell gapes slightly open. The heart 

 action is less rapid than in crawling and at certain times seems to have 

 ceased. 



The natural resting position of the adult scallop is on the right valve 

 on the bottom. Very seldom, and then only owing to accidental over- 

 turning, does it rest on .the left valve. Often on a coarse sand bottom, 

 especially in winter, the scallop excavates a shallow hole in the sand 

 and lies passive, half concealed in its burrow. This habit may be pro- 

 tective in severe winters. 



Drifting. Scallops frequently drift just below the surface of the 

 water, with the right valve uppermost, some with shells nearly closed, 

 others with tentacles and foot extended (Fig. 47). The foot evidently 

 needs to grasp some object before the animal can control the direction 

 of the motion. If one of the drifting animals is jostled with a needle 

 it sinks to the bottom, probably taking in a little water. Scallops as 



