102 . W. J. DAKIN. 



development, especially for a Lamellibranch, and the com- 

 plexity of structure, together with the large number of eyes, 

 has been a difficulty felt by most writers who have sought for 

 an explanation of these organs. Patten put forward an 

 extraordinary theory, calling the eyes " heliophags." It is 

 hardly necessary to go into this here, since a criticism 

 appeared in the ' Quarterly Journal of Microscopical Science,' 

 vol. 27, which may be referred to. 



The eyes have shown no evidence of being phosphorescent 

 oi'gans, though I have observed and stimulated them at niglit 

 and in the dark. A shadow thrown on to the eyes of an 

 open Pecten causes a closure of the valves, and this reaction 

 usually takes place very rapidly, though very often the per- 

 ception of light stimuli does not appear to be any better than 

 by Area with very simple eyes or others with pigment spots. 

 If, however, the shadow thrown on to a Pecten does not 

 extend over a number of eyes there appears to be no reac- 

 tion, and, just as Rawitz observed some time ago, a small 

 object quite near produces no effect unless its shadow falls 

 on a large number of eyes in quick succession. No evidence 

 of accommodation could be obtained experimentally. Further- 

 more, it is hardly possible to correlate the presence of these 

 structures with the active habits of the animal, e.g. swim- 

 ming, for Lima swims just as well as Pecten, but has ex- 

 tremely simple eyes. Again, Spondylus has eyes practically 

 identical with those of Pecten, but does not swim, and the 

 same thing applies to the only other Lamellibranch with an 

 eye approaching that of Pecten in structure, namely Cardium. 

 In the latter case the eyes are confined to the tentacles of the 

 siphons. It would be interesting to determine by biometric 

 methods whether these organs were still being kept up, or 

 were degenerating, especially in forms like P. Jacob a3us 

 and P. maxim us, where there exist very small eyes side by 

 side with the large ones. 



These may be growing, or they may be eyes which have 

 retained their young form, have not grown, and will not 

 grow. They agree with young eyes in structure. The 



