THE EYE OF PECTEN. 103 



variation, however, in specimens of the same size renders the 

 examination of a large number a necessity, and I have been 

 unable to obtain a fraction of that number. It is possible 

 that P. jacobin us and P. maxim us are more highly 

 developed forms than P. opercularis and P. varius, for 

 they possess no byssus, though the gland is present and they 

 have passed thi-ough a byssus stage, and the retractor muscles 

 of the foot, of which one is left in P. opercularis, are even 

 more vestigial in P. maxim us. If these two forms are con- 

 sidered older we find that there has been a reduction in the 

 number of the eyes, for they are more numerous in P. opercu- 

 laris, P. tigrinus, and other smaller forms, and this reduc- 

 tion has then taken place to a greater extent on the under 

 convex valve than on the upper flat one. The increase in 

 convexity and difference betw^een the two valves, reaching a 

 maximum in P. jacobasus, has been accompanied by a 

 reduction of the eyes on the convex mantle-lobe both in 

 number and size. These are, however, only hypotheses. 

 The large number of eyes present is probably to be accounted 

 for by the reason put forward by Rawitz, namely, that the 

 actual recipient area in each eye is small, that oblique rays 

 are cut off, and that in life the eye-stalks remain still; a 

 large field of view is therefore only possible witli numerous 

 eyes. 



The presence of two series of recipient elements has not 

 been explained by previous writers and has in fact been 

 usually passed over. No experiments have enabled me to 

 state anything definitely about this, except that, as already 

 mentioned, there appears little evidence of accommodation. 

 It might be advisable to point out here that the removal of 

 an animal like Pecten from the dim regions at the bottom to 

 the daylight and shallow water of the aquarium has possibly 

 an injurious effect, and pi-obably it would be a delicate com- 

 plicated structure like the eye that would suffer most. Hence 

 it may be that our aquarium experiments are almost useless 

 in this respect. 

 L The presence of the distal layer of sense-cells as well as 



