126 LARVAL OCELLI AND THE PARIETAL EYE. 



that form a median, tri-oculate group. They arise during the metamorphosis, or 

 at any rate after the embryonic period, and are quite independent of the primitive 

 ocelli. They are never involved in a palial fold or in a common vesicle, and 

 the retinal cells are, apparently, always upright. They are functional eyes only 

 in adult insects, or in the late larval stages. 



In the arachnids and Crustacea (phyllopods, entomostraca), the frontal ocelli 

 are present in a highly modified form, as two sets of frontal organs two paired and 

 one unpaired. In Limulus, they become the olfactory organs. In spiders and 

 scorpions, they are apparently absent. Their nerve roots arise from the median 

 anterior surface of the forebrain, or from the anterior surface of the optic ganglia 

 and hemispheres (Limulus). 



The lateral or compound eyes are found in adult insects, Crustacea, and 

 arachnids, including the trilobites and merostomes. Like the stemmata, their 

 relation to the primary head segments cannot be easily determined, because at the 

 time the cephalic lobes are most clearly segmented, as in the embryonic stages of 

 Acilius and the scorpion, the lateral eyes are absent, and they do not appear, if at 

 all, till near the close of larval life. In Limulus they belong to the cheliceral 

 segment; in insects, they appear to belong to the antennal segment. 



The development of the lateral eyes is essentially the same in all arthropods. 

 They are derived from large crescentic placodes lying near the posterior lateral 

 margin of the cephalic lobes close to the edge of the infolding for the optic gang- 

 lion; but they never lie inside the fold, and the visual cells are never inverted. 

 The entire visual layer is formed from a single layer of primitive ectoderm. 



The placodes are frequently divided, or may be entirely separated, into 

 two distinct parts, which differ in their histological characters, and in function 

 (hymenoptera, neuroptera, coleoptera). One part may be especially developed 

 in males (ephemeridae), or one may serve for vision under water, and the other for 

 vision in air. 



Cerebral Eyes of Vertebrates. 



In vertebrates we recognize as belonging to the forebrain, the median or 

 parietal eyes, the lateral eyes, and the olfactory organs. At an early embryonic 

 period they lie on the outer margins of the open neural plate, in similar positions 

 to the ones they occupy in arthropods. 



The Parietal Eye. There are probably two pairs of ocellar placodes that 

 for a short time occupy this marginal position. Later, they are caught in the 

 palial overgrowth and carried on the inner limb of the closing neural crests to 

 the median line. There they form a group of one, or two, or three placodes lying 

 in the membranous roof of the brain. During or after the closing of the cerebral 

 vesicle, the brain roof is evaginated at the place where the ocelli are located, thus 

 forming a sac or tube in the blind end of which the ocellar placodes lie. 



The extraordinary way in which the vertebrate parietal eye develops is, 



