136 LARVAL OCELLI AND THE PARIETAL EYE. 



follow the old paths, that is, the outer surface of the eye tube. When the nerve 

 fibers separate from it, the tube is left as functionless epithelium, which may, in 

 whole or in part, disappear. 



The dilated proximal end of the eye tube, from which the nerve roots have 

 separated, remains for a long time (up to three to four inches long), adhering to 

 the anterior surface of the hemispheres, beneath the thick neurilemma sheath. 

 In the adult all traces of it have disappeared. The distal end of the tube like- 

 wise disappears, so that finally the three ocelli are united to the brain by a single 

 solid nerve with four terminal branches and two pairs of roots, each of the four 

 roots ending in a distinct ganglion. 



The parietal eye of Limulus differs from that of the scorpion in the great 

 length of the eye tube, in the presence of the endo-parietal eye, and in the location 

 of the ganglia on the haemal surface of the brain, instead of the neural. These 

 differences, although sufficiently striking, are not fundamental, but due merely 

 to differences in the relative rate of growth of the adjacent organs in the two 

 animals. One cause of the difference was the closing of the anterior neuropore 

 in front of the hemispheres in Limulus, and behind them, in the scorpion. More- 

 over, as the parietal eye in the scorpion lies (morphologically) behind the hemis- 

 pheres, and over the neural surface, the ocellar ganglia are drawn upward, 

 toward the median neural side, as near to the eye as possible. In Limulus, the 

 parietal eye has migrated forward, and then backward on the haemal surface, 

 drawing the nerves and ganglia forward and haemally. (Fig. 47, A and B.) 



The Parietal Eye of Branchipus. 



The early stages in the development of the median ocellus of phyllopods 

 and other Crustacea are imperfectly known. But its structure in the adult in- 

 dicates very clearly that it is the same kind of an eye as the median one in Limulus 

 and other arachnids, and probably develops in a similar manner. That is, it 

 consists of two pairs of ocelli enclosed in a median sac that opens to the exterior 

 for a time at least by a short, median duct, or epiphysis. (Fig. 102, A.) 



The parietal eye of Branchipus is probably typical of many Crustacea. 

 Its characteristic features appear in the nauplius at a very early period. In 

 Branchipus it is a tri-lobed vesicle consisting of two communicating sacs. 

 (Figs. 95 and 96.) The larger, outer one, or ecto-parietal eye, has thick, lateral 

 walls representing two ocellar placodes or retinas. The distal ends of the retinal 

 cells are directed inward and are capped with minute lateral rods, or plates. The 

 cavity of the sac is coated with a layer of dense black pigment, apparently the 

 product of two large cells whose nuclei are seen in the posterior lateral part, pg.c. 



The inner sac, or endo-parietal eye en.p.e. is conical and with a minute cen- 

 tral canal or crevice toward which the inner ends of the retinal cells converge from 



