THE PARIETAL EYE OF BRANCHIPUS. 



137 



all sides. It is colorless, and doubtless represents one completely fused pair of 

 ocelli, corresponding with the inner, colorless eye sac of Limulus. 



In sagittal sections, the pigmented floor of the outer sac, in younger specimens, 

 appears to be continuous with the outer ectoderm, leaving a narrow pore or crevice 



,. Oo 



Fig. 95. — Parietal eye vesicle of a young Branchipus, with the adjacent frontal organs (or lateral olfactory organs) . 



Composite frontal section. Camera outline. 



by which the cavity of the eye sac communicates with the exterior. (Fig. 102, A.) 

 This opening is doubtless comparable with the eye tube of scorpion and Limulus. 

 As the tube itself is very short the opening leads directly into the common eye 

 chamber containing the three ocellar placodes. 



There is no lens for any part of the eye, light having free access to the shallow 

 retina of the outer sac from either side, and to the inner sac from all sides. 



Fig. 96. — Sagittal section of the parietal eye of a young Branchipus. Camera outlines. 



The ecto-parietal eye has two distinct nerves distributed over the outer 

 surface of the retinas, n.ec.e. They arise from ganglionic enlargements of the 

 anterior median portion of the brain. The endo-parietal eye has a single nerve, 

 n.en.e. 



