364 THE PINEAL ORGAN 



stracan type of median eye, definitely assumed not only that this type 

 of invertebrate eye resembled in its structure and mode of development 

 the parietal eye of vertebrates but that this correspondence of the parietal 

 eyes was merely one of many structural and developmental resemblances 

 between the entomostracan and vertebrate phyla. He, further, pre- 

 sumed that there was a long period of functional activity of the median 

 eye or eyes of vertebrates, during which the lateral eyes passed through a 

 corresponding period of inactivity while they were evolving and before, 

 as he states, " they again became functional." During this period he 

 believed that the parietal eyes were the only functional visual organs. 

 This latter concept is, we believe, for many reasons untenable, of which 

 we may mention : (1) that the general tendency is for inactive or non- 

 functioning organs to regress rather than evolve ; and (2) that the oldest 

 fossil fishes which are known, such as the Anaspidee and Cephalaspidae, 

 lived during the same period side by side with the giant forms of marine 

 Merostomata, e.g. Eurypterus and Pterygotus. In both the ostracoderm 

 fishes and the Merostomata many of the principal distinguishing characters 

 of the two phyla were already developed, including those relating to the 

 cranial skeleton, the brain and cranial nerves, the sense-organs, branchial 

 and vascular systems (Figs. 238, 245), and we may therefore conclude 

 that although the parietal eyes of living Cyclostomes and Entomostraca 

 resemble each other in certain respects and their special characters appear 

 to have persisted for an immensely long period without undergoing any 

 essential changes in structure, there has during this period been a 

 simultaneous and gradual evolution with differentiation of the lateral 

 eyes of both invertebrates and vertebrates. As the differentiation of each 

 type became more pronounced, namely, the invertebrate and vertebrate 

 types, so they diverged more and more from the simple form of eye, 

 from which they both originated, and from each other ; whereas the 

 median or parietal eyes tended to regress rather than evolve, except in 

 certain genera among Arthropoda, Monorhina, Amphibia, and Reptilea. 

 These evolutionary changes have resulted in two widely different types 

 of eye, one of which is upright and faceted, while the other has an inverted 

 retina and single lens protected by a smooth transparent cornea. Thus 

 at the present day it is difficult to believe that these two types of eye could 

 have arisen from a common ancestral form. Intermediate stages are 

 found between simple ocelli and the compound faceted eyes of inverte- 

 brates, but the special characters of the vertebrate eye seem to have been 

 already evolved in the earliest fishes of which we have any geological 

 record. It is true that the geological evidence is circumstantial rather 

 than direct, but the close agreement in the structural details of the cranial 

 skeletons of many of the extinct Palaeozoic fishes with their living repre- 



