148 LARVAL OCELLI AND THE PARIETAL EYE. 



and an inner posterior one, or endo-parietal eye, containing the remaining two 

 placodes, now completely united into one organ, and with greatly reduced struc- 

 tural details. 



7. The membranous tube, or epiphysis may disappear in whole or in part, 

 leaving the terminal eye sacs either isolated, or united by distinct nerves with the 

 parietal eye ganglia, or the ganglia habenulse. 



8. The parietal eye of vertebrates is homologous with the parietal eye of such 

 arthropods as Limulus, scorpion, spiders, phyllopods, copepods, trilobites, and 

 merostomes, but not with the frontal stemmata or other ocelli of insects. 



9. In the arthropods, various stages in the evolution of a cerebral eye are 

 shown in detail, from functional eyes on the outer margin of the cephalic lobes, 

 to a median group of ocelli enclosed within a tubular outgrowth of the brain roof. 



The most primitive type of a parietal eye is seen in the nauplii of phyllopods 

 and entomostraca, where the eye is a pear-shaped sac, opening by a median pore 

 or tube on the outer surface of the head. (Fig. 272, 308.) In the higher arachnids, 

 the process of forming an embryonic eye vesicle merged with the process of form- 

 ing a cerebral vesicle, the external opening of the forebrain vesicle and that of the 

 parietal eye tube, forming a common opening or anterior neuropore. 



10. The parietal eye of arthropods is an important visual organ until the 

 lateral eyes, which represent a later product, are fully developed. It may then 

 diminish in size and activity, but it rarely, if ever, wholly disappears. 



11. During the evolution of vertebrates from arachnids, there was a consider- 

 able period during which the lateral eyes were adjusting themselves to their new 

 position inside the brain chamber, and when they were in functional abeyance. 

 At this period, ancestral vertebrates were mon-oculate, that is they were dependent 

 solely on the parietal eye, which had come to them from their arachnid ancestors 

 as an efficient and completely formed organ. 



When the lateral eyes again became functional, the parietal eye began to 

 decrease in size and effectiveness. 



The parietal eye is the only one now present in tunicates. In the oldest 

 ostracoderms, like Pteraspis, Cyathaspis, Palaeaspis, the lateral eyes are absent, or 

 at least do not reach the surface of the head, the only functional one being the 

 parietal eye, which is of unusual size. 



In the lampreys we see the same conditions, the parietal eye being very well 

 developed in the larvae, while the lateral eyes are deeply buried in the tissues of 

 the head, and useless. During the transformation, the lateral eyes again become 

 functional, and the parietal begins to atrophy, finally losing many of its structural 

 details and its function, although still retaining very nearly its original form. 



