138 LARVAL OCELLI AND THE PARIETAL EYE. 



The Parietal Eye of Apus. 



In Apus, the conditions are a little more complicated. Here, as in many other 

 phyllopods, there is a remarkable skin fold directed forward, forming a broad, 

 shallow chamber over both the compound eyes and the ocelli. (Fig. 102, B.} It 

 opens to the exterior by a narrow pore plugged w r ith chiten. (Fig. 98, O.) This 

 opening should not be confused with the epiphyseal pore of scorpion and Limulus. 

 The parietal eye forms a closed chamber with a retinal placode on each side 

 wall, and two unpaired placodes, one on its posterior, the other on its inner wall. 

 (Figs. 97-99.) 



Each placode consists of a single row of large, colorless, columnar cells. 

 Their distal ends are buried in a dense mass of dark brown, or black pigment; 

 their proximal ends are colorless. 



o at 



c.e v. 



I 



a rt 



FIG. 97. Sagittal section of the parietal eye vesicle of an adult Apus. m, Fold covering the lateral eyes; O, 

 opening of the lateral eye vesicles, c.c.v; o.at., remnants of canal leading into parietal eye vesicle; a.t.. cavity of 

 the same; p.rt., posterior retina; a.rt., anterior ratina. 



As in Branchipus, there are two large cells which appear to give rise to the 

 greater part of the pigment that fills the cavity of the vesicle. (Fig. 98, p.g.c.) 



When the pigment is partially dissolved, it is seen that each retinal cell is 

 capped with a large brush-like mass of fine fibers (retinidium) , apparently the 

 free ends of nerve fibers passing through the interior of the cells, or over their 

 outer surfaces. They are comparable with the nervous network described by 

 me in the visual rods of Pecten, Acilius, Lycosa, etc., except that they are not reg- 

 ularly arranged, and are not imbedded in a dense, transparent matrix, which 

 usually forms the most conspicuous part of a visual rod. 



The parietal eye sac of Apus probably contains the retinas of four distinct 

 ocelli, which during development migrated from the sides of the head toward 

 the median line. There they became enclosed in a common sac, that opened to 



