REACTIONS OF BIVALVE MOLLUSKS 311 



absolute proof could only be obtained by removing all the eyes 

 and then subjecting the animals to the same tests. Such a 

 method might also demonstrate whether the eyes alone, or other 

 parts of the mantle-border as well, are sensitive to the changes 

 in light intensity. But the operation involved would undoub- 

 tedly be a great shock to the animals and their subsequent 

 behavior might not be typical. 



b. Discussion 



The complex and highly organized structure of the eyes of 

 Pecten has attracted the attention of histologists for many 

 years. Those who have undertaken a careful study of the 

 structure have usually found many difficulties to overcome, and 

 unsatisfactory results were often obtained on account of imper- 

 fect technique. The most recent, and perhaps the most reliable, 

 detailed study of the Pecten eye is that of Dakin ('10b), to 

 which the reader is referred. 



Many of those who have studied the histology of these eyes 

 have assumed that the animals could "see" with them. The 

 following quotation from Garner ('37) indicates the typical 

 attitude among some of the earlier investigators: Garner says 

 (p. 488): "In Pecten, Spondylus, and Ostrea we find small, 

 brilliant, emerald-like ocelli, which, from their structure, having 

 each a minute nerve, a pupil, a pigmentum, a striated body, 

 and a lens, and from their situation at the edge of the mantle, 

 where alone such organs could be useful, and also placed, as in 

 Gasteropoda, with the tentacles, must be organs .of vision." 

 Similarly, on anatomical grounds, Grube ('40), Rawitz ('88), 

 Schreiner ('97), R. Hesse ('00), and others conclude that these 

 eyes are organs of vision. But very few of the histologists have 

 performed crucial experiments to determine whether or not 

 Pecten really could form an image. And some of them have 

 indulged in various speculations as to some function other 

 than vision. 



As an example of such speculations I may quote Hickson ('80). 

 He says (p. 454) : " Little is known and little can be said con- 

 cerning the function of the eyes of Pecten. The presence of 

 such a well-formed tapetum makes it probable that they are 

 capable of appreciating very diffused light. The close approx- 

 imation of the lens to the retina makes it exceedingly improb- 



