104 . W. J. DAJilN. 



that of the rod-cells and rods may be a device for increasing 

 the area of the recipient elements without increasing to any 

 extent the size of the retina, but more probable is perhaps 

 the following view. There has not yet been definitely proved 

 to exist any special apparatus for accommodation in the eye 

 (though Hesse's theory has not been disproved). Now 

 it may be that the two layers of recipient cells are for 

 the reception of images of objects situated at different 

 distances from the eye, which are focussed at different 

 distances from the lens. Thus the image of near objects 

 would be focussed on the rods and that of distant objects on 

 the outer distal cells. A similar condition would apply to 

 the ocelli of Agrion, and, in fact, Hesse describes such (35), 

 but adds, " Ich kenne nirgends eine iihnliche Pjinrichtung." 

 In the Heteropod eye there also appears to be a device for 

 the reception of rays from objects at different distances from 

 the eye. There is, however, only one series of cells, but the 

 free ends bearing the comb- like margins are turned so that 

 they are at right angles to the plane of the retina, and some 

 are nearer the lens than others. 



The development of the Pecten eye still remains incom- 

 pletely known, and Patten's observations need confirmation. 

 The derivation of the various layers will certainly throw 

 much light on the structure of the adult eye and the inversion 

 of the retina. Unfortunately the material for such a research 

 is somewhat difficult to acquire as all the elements are formed 

 in extremely young specimens, and I have been unable there- 

 fore, so far, to follow out this line of inquiry. 



It will be perhaps useful if the most interesting features in 

 the general structure of the Pecten eye are summarised here 

 and a few comparisons made with other eyes, which may bear 

 some resemblance to the former. The eye is a closed vesicle ; 

 there is a cellular cornea continuous with the surface epithe- 

 lium, and below this a cellular lens. The retina is made up 

 of two series of recipient cells innervated by two branches of 

 an optic nerve. The cells of the distal layer have each a 

 comb-like margin, and the proximal visual cells bear rods 



