146 



LARVAL OCELLI AND THE PARIETAL EYE. 



quent absence of a lens and vitreous body, in the otherwise well developed parietal 

 eye of arthropods, is remarkable, since it does not occur in the other types of 

 arthropod ocelli. The fact is all the more significant when we recall that in 

 vertebrates true lenses to the parietal eye are never present. In place of them, we 

 find a transparent spot, or tubercle, or a thin place in the overlying tissues. The 

 thickening of the outer wall of the eye vesicle, which may possibly serve, in ex- 

 ceptional cases, as a lens (reptiles), is probably the remnant of a retinal placode. 



Location of the Placodes. — The location of the retinal placodes in the 

 parietal eye vesicle varies greatly. In the arthropods, they may lie in the side 

 walls (Branchipus), or in the outer wall (scorpion and Limulus), or in the inner 

 wall or floor, as in Apus. The prevailing position in arachnids is in the outer wall. 



pa, eg. 



A B 



Fig. 105. — A, Section through the posterior margin of the parietal eye in stage i?, showing the approaching 

 union of the two retinas, and the palial folds; B, section through the parietal eye of a newly bom scorpion, showing 

 the parietal eye vesicles, and the ventricle, V , formed by the optic ganglia, the palial folds, and the forebrain 

 neuromeres. The ventricle extends forward and downward, into the cavity of the olfactory lobes, ol.v. 



thus inverting the cells, and turning the rod bearing end toward the cavity of 

 the vesicle. But the retinal cells have a remarkable method of readjustment, 

 so that in the later stages, they appear to be standing in an upright position. 



In vertebrates, the placodes in general appear to occupy the floor of the 

 vesicle, but they may develop on both walls, as in Petromyzon. 



Minute Structure. — In the arthropods, there is nothing constant in the 

 histological structure of the parietal eye retinas. The principal elements are 

 columnar, sensory cells arranged either in a continuous layer, with terminal 

 rods projecting into the eye chamber (Apus, Branchipus), or they may be 

 arranged in definite groups, or ommatidia, consisting of from two to five or 

 more cells with plate-like rods attached to the side walls of each cell (Limulus, 

 scorpion, Phalangium, Lycosa). 



Where the eye, to all appearances, has become functionless, i.e.. endo-parietal 

 eye of adult Limulus, the cells form a confused mass, without any definite arrange- 

 ment in layers, or in respect to the source of whatever light may reach them. 

 The black pigment is then absent and the cells are filled with a dense mass of 

 ghstening white granules. Even in this degenerate condition, the visual rods 

 may be retained as irregular plates, singly or in groups, attached to the side walls 

 of the retinal cells. 



