142 LARVAL OCELLI AND THE PARIETAL EYE. 



The amount of pigment, and its distribution, varies greatly in different 

 individuals and at different stages. In many cases, the cells of the outer walls 

 are colorless, and the inner wall, and especially the two layers of rods, are densely 

 crowded with pigment, a condition similar to that seen in Apus and Branchipus. 



The inner sac en.p.e. resembles the outer one, except that its retinal cells are 

 less highly specialized, and its outer wall consists of a thin layer of indifferent, 

 columnar cells. 



The groove on the floor of the outer sac is hardly recognizable anteriorly, 

 but it gradually deepens toward the posterior margin, where it leads into the en- 

 larged, distal end of the epithelial eye tube or epiphysis. This part of the tube 

 persists in the adult as the conical "atrium" of Studniaka. The proximal part 

 of the tube likewise persists as a small solid cord, extending over the outer surface 

 of the ganglion habenula. A trace of its original opening may be seen as a conical 

 recess, in front of the superior commissure. (Figs. loo and loi, 141.) 



A similar groove, leading into a short blind tube, is seen in the floor of the 

 inner sac, et''. This tube leads toward the base of the "atrium," but at this stage 

 does not unite with it. It undoubtedly represents the remnants of the connection, 

 existing during the early stages, between the inner sac and the main eye tube. 



After the metamorphosis the parietal eye loses the clear cut histological 

 details seen in the early stages, and is then undoubtedly of less importance 

 functionally. 



The Parietal Eye Ganglia, or Ganglia Habenula, consist of a main right and 

 left ganglion, each consisting of an anterior and a posterior lobe. We may there- 

 fore, recognize four lobes, or two pairs of ganglia, for the parietal eye, a condition 

 in complete harmony with the presence of two pairs of retinal placodes in 

 the eye. 



The left ganglion is smaller than the right and differs from it in minor, 

 histological details. It gradually moves forward and mesially, till the anterior lobe 

 lies close to the posterior, inner wall of the inner sac, with which it is connected 

 by a large bundle of nerve fibers. This nerve divides into two, one passing on 

 either side of the median groove. (Fig. 100, n^.) 



The larger, outer eye is said to be connected by nerve fibers with the larger, 

 or right ganglion. I have not been able to satisfy myself that this was the case. 

 In fact the nerves to the outer sac are small and very difficult either to identify, 

 or to follow to their terminals. 



The right and left ganglia are connected by at least two commissures that 

 originate in two large cores of neuropile. (Fig. loi.) From the latter, two 

 pairs of nerve tracts arise, the anterior pair, a.lr., passing downward and forward 

 to the median face of the olfactory lobes; the posterior pair, p.lr., downward and 

 backward to the floor of the midbrain. 



It is a surprising fact that the two anterior bundles are of approximately 

 equal dimensions, while of the posterior pair, the right is very much larger than the 

 left. 



