44 THE SCALLOP FISHERY 



turning the body with right valve uppermost (the reverse of the natural 

 position), maintains itself on the surface of the water. This habit has 

 been observed in numerous cases in the aquarium in which scallops 

 were confined. In scallops over 1 millimeter it appears to be accom- 

 plished by the spreading of the tentacles. 



Observers, as Jackson (4), have stated that the animal is assisted by 

 the tips of the tentacles in crawling, more particularly in climbing, 

 during which the tentacles cling to the sides of the glass. Whether the 

 extension and clinging of the tentacles is any great help to the foot in 

 climbing is a matter of doubt, but they undoubtedly rest on the glass 

 and are extended during both swimming and crawling. 



The chief function of the tentacles is sensory. Often the tentacles 

 of the adult do not respond to external stimuli, as would naturally be 

 supposed, and in the case of repeated stimulation often fail to react 

 at all. In scallops of 2 millimeters the tentacles may be made to contract 

 separately by mechanically stimulating one at a time. This nervous 

 reaction is not general, but if the whole animal is suddenly jarred all 

 the tentacles are withdrawn with surprising swiftness. The tentacles 

 of scallops of this size render the animal more sensitive than the 

 smaller scallops, which do not have the full development of the tentacles. 

 Thus the sensory nature of the tentacles is proven, and the subsequent 

 inactivity of the large adults must be accounted for in other ways. 



(b) The Eyes. The most prominent feature of the mantle border 

 is the fringe of brightly pigmented eyes, which are thickly scattered 

 along the edge. In the adult there is great variation in the number, 

 size and order of arrangement. These eyes are comparable with those 

 of higher animals, and evidently have a sensory function. 



As stated by Drew (1) the eyes are closely allied to the tentacles, and 

 are in fact derived from the same source, being nothing more than 

 modified tentacles. Their situation, origin, time of appearance, arrange- 

 ment, all indicate that the eyes and tentacles are fundamentally the same. 



The eyes make their appearance during phase 5, when the first or 

 primary set is developed just after the primary tentacles are formed. 

 The two lower or ventral eyes are formed first, then the eyes near the 

 hinge line, and the intermediate ones soon after, numbering seven on 

 each lobe of the mantle. The color of the eyes varies at this age from 

 a brown to a blue. As can be seen in Fig. 29, the primary eyes and 

 tentacles are arranged definitely, the eyes being situated on slight pro- 

 jections on the outer fold of the mantle between the tentacles. The 

 successive development of the eyes is like the tentacles by the formation 

 of secondary, tertiary, etc., sets between the primary eyes, at first 

 alternating with the tentacles, but later apparently without definite 

 arrangement. Although the visual function of the scallop's eye has 

 been a matter of much dispute, there is but slight question that the eye 

 has its use as a sensitory organ. 



