378 THE PINEAL ORGAN 



eye of such arthropods as Limulus, scorpions, spiders, phyllopods, cope- 

 pods, trilobites, and merostomes, but not with the frontal stemmata or 

 other ocelli of insects. 



11. "In the arthropods various stages in the evolution of a cerebral 

 eye are shown in detail, from functionless eyes on the outer margin of 

 the cephalic lobes to a median group of ocelli enclosed within a tubular 

 outgrowth of the brain roof. 



12. " The most primitive type of a parietal eye is shown 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. 

 247, p. 350). In the higher arachnids the process of forming an eye vesicle 

 is merged with the process of forming a cerebral vesicle, the external 

 opening of the forebrain vesicle and that of the parietal tube forming a 

 common opening or neuropore. 



Fig. 259. — Anterior and Ventral Aspect of an Embryo Spider, showing 

 the Position of the Median (Parietal) Eyes and the Lateral Eyes. 

 (After Patten.) 



chl. : chelicera. ped. : pedipalp. 



I.e. : lateral eyes. ro. & st. : rostrum and stomodseum. 



m.e. : median eye. 1, 2, 3, 4 : first to fourth legs. 



13. " 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 in activity, but it rarely if ever wholly 

 disappears. 



14. " During the evolution of vertebrates from arachnids there was 

 a considerable period during which the lateral eyes were adjusting them- 

 selves to their new position inside the brain chamber, when they were in 

 functional abeyance. At this period ancestral vertebrates were monoculate, 

 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. 



15. " The parietal eye is the only one now present in tunicates (Fig. 

 260). In the oldest ostracoderms, like Pteraspis, Cyathaspis, Palceaspis, 



