THE EVIDENCE OF THE ORGANS OF VISION 



83 



lamprey, and correspond, therefore, more nearly to the l'etromyzon 

 than to the Ammoccetes stage of the European lamprey. 



The Dioptric Apparatus. 



Besides the retina, all eyes possess a dioptric apparatus. What 

 is the evidence as to its nature in these vertebrate median eyes ? 

 Lankester and Bourne have divided the eyes of scorpions and 



I 



Til 



ret 



Fig. 35. — Eye of Acilius Lakv^e. (After Patten.) 



I., chitinous lens ; c, corneagen; pr., pre-retinal layer ; rlu, rhabdites ; ret., retinal 



end-cells. 



Limulus into two kinds, monostichous and diplostichous. In the 

 first the retinal cells are supposed to give rise to not only rhabdites 

 but also the cuticular chitinous lens, so that the eye is one-layered ; 

 in the second the lens is formed by a well-marked hypodermal layer, 

 in front of the retina, composed of elongated cells, so that these eyes 

 are two-layered or diplostichous. The lateral eyes, according to 

 them, are all monostichous, but the median eyes are diplostichous. 

 This distinction is not considered valid by other observers. Thus, 



