THE EYE OP PECTEN. 107 



The distal layer of sense-cells lie touching the lens, almost 

 like the outer cells of Pecten touch the septum. 



A striking difference from the Pecten retina is, howevei-, 

 present which lends at the same time support to the view of 

 Leydig, upheld by Lankester in 1883, namely that the com- 

 pound ej'e is formed by the segregation of the elements of a 

 simple eye, and this is the segregation of the retinal cells. 

 The visual cells do not remain, as in the Pecten eye, alto- 

 gether independent with their recipient ends directed towards 

 or away from the lens, but bear a comb-like mai^gin of 

 neurofibril endings laterally aud are collected in groups 

 of threes, each group being a retinula. Thus we have a 

 monomeniscous eye with a retinulate retina, the whole being- 

 very different from the Pecten retina except in the one point 

 — the presence of visual cells arranged in two layers. 



The central eyes of the Scorpions may finally be mentioned 

 here. These are also monomeniscous and present a far 

 greater resemblance to the Pecten eye than appears at first 

 sight. They are vesicular, though the cavity of the vesicle 

 has disappeared and the retina is inverted, though, owing to 

 a secondary reversion during development, this is not 

 obvious. 



The eyes are developed from an involution of the hypo- 

 dermis or ectoderm, which, however, does not lie vertical to 

 the surface. The outer wall becomes thickened and forms 

 the retina; the inner wall remains thin and represents the 

 post-retinal layer of ectoderm cells in the adult. This is 

 strikingly like the process in the Pecten eye where the 

 inner wall becomes the pigment layer. The retinal cells 

 are of course inverted. The nerve-fibres are attached to 

 the outer ends of these cells in the embryo, but, owing to 

 reversion in the course of development, become connected 

 to the inuer ends in the adult eye. In the course of these 

 changes the optic nerve must penetrate the post-retinal 

 layer, and this has been shown by Ray Lankester and 

 Bourne (46) to be the condition actually prevailing in the adult. 

 Beyond this remarkable similarity in development the eyes 



