236 FREDERICK TILNEY AND LUTHER F. WARREN 



a simple retina, and characterizes the eyes, both median and lateral, 

 of the scorpion group. 



In other cases a portion of the optic ganglion remains at the sur- 

 face, when the brain sinks inwards, in close contiguity to the epidermal 

 sense-cells which form the retina; a tract of fibres connects this optic 

 ganglion with the under-lying brain, and is known as the optic nerve. 

 Such a retina may be called a compound retina and characterizes the 

 lateral eyes of both, crustaceans and vertebrates. Also, owing to the 

 method of formation of the retina by invagination, the cuticular sur- 

 face of the retinal sense-cells, from which the rods are formed, may be 

 directed towards the source of light or away from it. In the first case 

 the retina may be called upright, in the second inverted. 



The evidence of the optic apparatus of the vertebrate points most 

 remarkably to the derivation of the Vertebrata from the Palseostraca. 



Gaskell, in this argument, seems to have lost sight of his 

 well-known contention that the roof of the brain in vertebrates 

 is to be considered the dorsal wall of the invertebrate stomach. 

 The stress which he laid upon this relation, to which he gave 

 further emphasis by calling attention to the glandular appear- 

 ance of the roof-plate in Ammoccetes, does not coincide well 

 with his idea that the pineal body is primordially a portion of a 

 neural mechanism. He, of course, admits that the pineal eye in 

 vertebrates must be considered as resulting from a neural in- 

 vasion of the roof-plate, yet from his contention this roof-plate 

 is primitively the dorsal wall of the stomach, and neural deriva- 

 tives appearing in it must be due to a secondary neural invasion 

 and, therefore, cannot be considered primordial. 



In a word, by holding the pineal eye to be fundamentally 

 neural in structure he did injury to his own theory concerning 

 the evolution of the vertebrates. 



Patten, 303 in considering the significance of the parietal eye, 

 gives the following conclusions: 



The parietal eye of vertebrates is homologous with the parietal eye 

 of such arthropods as Limulus, scorpion, spiders, phyllopods, cope- 

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

 or other ocelli of insects. 



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. 



