RELATION OF MEDIAN TO LATERAL EYES 363 



the ocelli never form unpaired eyes enclosed in a common chamber or 

 vesicle." 



2. " The parietal eye (Entomostracan eye) (Fig. 250). In the Crustacea 

 and Arachnida two pairs of ocelli unite to form an unpaired ocellar vesicle 

 or parietal eye. The ocellar placodes remain more or less distinct and 

 form the side walls of the dilated anterior or distal end of the vesicle. The 

 proximal or posterior end is generally tubular, and may open on the 

 outer surface of the head or it may merge with the pallial folds and open 

 into the forebrain vesicle. The parietal eye usually persists through life, 

 and it may be the largest and most important one functionally." 



3. " The frontal eyes or stemmata (Figs. 241 and 242, Chap. 23, pp. 343, 

 345) of insects consist of two pairs of placodes that form a median triocular 

 group. They arise during the metamorphosis, or at any rate after the 

 embryonic period, and are quite independent of the primary ocelli. 

 They are never involved in a pallial fold or 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 eyes 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 hemisphere." 



4. " The lateral or compound eyes are found in adult Insecta, Crustacea 

 and Arachnida, including the trilobites and merostomes. Like the 

 stemmata, their relation to the primary head segments cannot be easily 

 determined, because at the time that 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, until 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 develop- 

 ment of the lateral eyes is essentially the same in all arthropods. They 

 are developed from large, crescentic placodes lying near the postero- 

 lateral margin of the cephalic lobes, close to the infolding for the optic 

 ganglion, 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 primary 

 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 well developed in males (Ephemeridse), or one 

 may serve for vision under water and the other for vision in air." 



Now Patten in using the term " parietal eye " to denote the entomo- 



