THE PINEAL BODY 237 



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, open- 

 ing by a median pore or tube on the outer surface of the head. In the 

 higher arachnids, the process of forming an embryonic eye vesicle 

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

 opening of the forebrain vesicle and that of the parietal eye tube, form- 

 ing a common opening or anterior neuropore. 



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. 



During the revolution 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, 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 Iarva3, while the lateral eyes are deeply 

 buried in the tissues of the head, and useless. During the transfor- 

 mation, 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. 



All the theories advanced concerning the significance of the 

 parietal eye as an index to the process of evolution from the 

 invertebrates to the vertebrates have their great value in the 

 suggestions which they offer. To accept any of them without 

 further evidence seems unwise at the present time. It is pos- 

 sible to conceive of the median eye of invertebrates as analogous 

 to the parietal eye o vertebrates. It is, however, a long step 

 for the most part without the intervening support of evidence to 

 maintain that these structures are homologous. In fact it 

 seems out of the question to establish any such basis of compari- 

 son until this subject of homology in the invertebrate and verte- 

 brate brain is on much firmer ground than it is to-day. It is 

 evident that nothing short of the definite establishment of an 



