THE EVIDENCE OF THE ORGANS OF VISION 75 



in the best-preserved head-shields of the most ancient fishes, 

 especially well seen in the Osteostraci, in Cephalaspis, Treniataspis, 

 Auchenaspis, Keraspis, a pair of large, prominent lateral eyes existed, 

 between which, in the mid-line, are seen a pair of small, insignificant 

 median eyes. 



The evidence of the rocks, therefore, proves that the pair of 

 median eyes which were originally the principal eyes (Hauptaugen), 

 had already, in the dominant arthropod group been supplanted by 

 a pair of lateral eyes, and had, in consequence, become small and 

 insignificant, at the time when vertebrates first appeared. This dwind- 

 ling process thus initiated in the arthropod itself has steadily continued 

 ever since through the whole development of the vertebrates, with the 

 result that, in the highest vertebrates, these median or pineal eyes 

 have become converted into the pineal gland with its ' brain-sand.' 



In the earliest vertebrate these median eyes may have been 

 functional ; they certainly were more conspicuous than in later forms. 

 Alone among living vertebrates the right median eye of Ammoccetes 

 is so perfect and the skin covering it so transparent that I have 

 always felt doubtful whether it may not be of use to the animal, 

 especially when one takes into consideration the undeveloped state 

 of the lateral eyes in this animal, hidden as they are under the skin. 

 Thus the one living vertebrate which is comparable with these 

 extinct fishes is the one in which one of the pineal eyes is most well 

 defined, most nearly functional. 



Before passiDg to the consideration of the structure of the 

 median eyes of Ammoccetes, it is advisable to see whether these 

 median eves in other animals, such as arachnids and crustaceans, 

 belong to any particular type of eyes, for then assuredly the median 

 eyes of Arnmoccetes ought to belong to the same type if they are 

 derived from them. 



In the specialized crustacean, as in the specialized vertebrate, the 

 median eyes have disappeared, at all events in the adult, but still 

 exist in the primitive forms, such as Branchipus, which resemble the 

 trilobites in some respects. On the other hand, the median eyes 

 have persisted, and are well developed in the arachnids, both 

 scorpions and spiders possessing a well- developed pair. The cha- 

 racteristics of the median eyes must then be especially sought for in 

 the arachnid group. 



Both scorpions and spiders possess many eyes, of which two are 



