312 D. H. WENRICH 



able that any image is formed upon the latter. A few experi- 

 ments have been made on the extent of their visual power, 

 which make it very doubtful whether they are of much value 

 to the animal in avoiding its enemies. The most reasonable 

 theory of their function seems to be that, when on the ebbing 

 tide a probability arises that they will be left high and dry on 

 the shore, they can appreciate the fact by the growing intensity 

 of the light, and by the peculiar flapping motion of their valves 

 the Pectens are so remarkable for, move away into deeper 

 water." 



Sharp ('87) criticises this idea of Hickson, calling attention to 

 the well-formed eyes of the species found in the Mediterranean, 

 where scarcely any tide exists. Earlier ('84), he had said (p. 

 458): ' Now, I am inclined to believe, as has been suggested 

 by Hensen ('65), that the pigmented epithelial cells that cover 

 this organ and the stalk on which it is situated, and the only 

 place where we find a marked deposit of pigment, is the seat of 

 vision for these animals and that the so-called eye may be an 

 organ of some other sense. . , If the organ in question is 

 so highly developed, we must necessarily conclude that vision, 

 if that be the sense, is also highly developed, which does not 

 appear to be the case. ..." But in 1886 he offers the sug- 

 gestion that they are phosphorescent organs, saying (p. 61): 



It is not unreasonable to suppose that organs for emission of 

 light would be constructed on the same principles as those for 

 the admission of light." Dakin ('106) tested out this phos- 

 phorescent theory and obtained only negative results. 



Patten ('86) regarded the eyes of Pecten and Area as organs 

 for the absorption of energy from the sun, and termed them 

 "heliophags." This idea has received no support from later 

 investigators. 



Attention should be called to the fact that Pectens, as well 

 as other bivalves, fail to give consistent responses unless they 

 are in good physiological condition. I found this to be true of 

 all the different species experimented with, and in the case of 

 Pecten I was able to get consistent responses only from those 

 collected in very shallow water. Animals collected from a 

 depth of 10 to 12 meters gave irregular responses and usually 

 died at the end of two to four days in the laboratory aquaria. 



As already stated, Rawitz i^^) claimed that no reactions were 



