THE EVIDENCE OF THE ORGANS OF VISION 83 
lamprey, and correspond, therefore, more nearly to the Petromyzon 
than to the Ammoccetes stage of the European lamprey. 
THE Dioprric 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 
Fic. 35.—Eysz or Aciuius Larva. (After Parrun.) 
1, chitinous lens; c., corneagen; pr., pre-retinal layer; rh., 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, 
