CHAPTER VIII. 



LARVAL OCELLI AND THE PARIETAL EYE. 



I. THE DIFFERENT KINDS OF EYES IN ARTHROPODS AND VERTEBRATES. 



Since the principal facts in their embryonic development became known, 

 it has been generally assumed that the vertebrate eyes originated inside the brain 

 chamber, and that the retina was a highly specialized part of the brain wall. 



There are fundamental objections to this interpretation, namely: a. it reverses 

 the usual order of histological development, for nerve cells are to be regarded as 

 specialized sensory cells, not vice versa; and b. it fails to establish any connection 

 or relation between the eyes of vertebrates and those that are almost universally 

 present, and often highly developed, in the invertebrates. In fact, it neither ex- 

 plains how the eyes got into the brain chamber from without, nor under what 

 conditions they developed "de novo" from within. 



The arthropod theory is not open to these objections, for we shall show that 

 the evolution of a cerebral eye has already taken place in the arachnids, and that 

 the principal steps in the process are recorded there in great detail. 



Eyes of Arthropods. 



In the arthropods, we may recognize four types of eyes, namely: paired lar- 

 val ocelli; parietal eyes; frontal ocelli, or stemmata; and the lateral or compound 

 eyes. 



The larval ocelli, of which there may be six pairs, two for each of the fore- 

 brain segments, are present in the active larvae of most insects, but disappear 

 during the metamorphosis (coleoptera, lepidoptera, neuroptera, hymenoptera). 

 They are cup-like infoldings of the ectoderm, with upright or horizontal retinal 

 cells or rods. In the insects, the retinal cells are never completely inverted, 

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



The Parietal Eye. In the Crustacea and arachnids, 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 palial 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. 



The frontal eyes or stemmata of insects consist of two pairs or placodes 



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