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, Tremataspis, 
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 passing to the consideration of the structure of the 
median eyes of Ammoccetes, it is advisable to see whether these 
median eyes in other animals, such as arachnids and crustaceans, 
belong to any particular type of eyes, for then assuredly the median 
eyes of Ammoccetes 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 
. 
