THE EVIDENCE OF THE ORGANS OF VISION 117 
SUMMARY. 
The most important discovery of recent years which gives a direct clue to 
the ancestry of the vertebrates is undoubtedly the discovery that the pineal gland 
is all that remains of a pair of median eyes which must have been functional in 
the immediate ancestor of the vertebrate, seeing how perfect one of them 
still is in Ammoceetes. The vertebrate ancestor, then, possessed two pairs of 
eyes, one pair situated laterally, the other median. In striking confirmation of 
the origin of the vertebrate from Paleostracans it is universally admitted that 
all the Eurypterids and such-like forms resembled Limulus in the possession of 
a pair of median eyes, as well as of a pair of lateral eyes. Moreover, the ancient 
mailed fishes the Ostracodermata, which are the earliest fishes known, are all said 
to show the presence of a pair of median eyes as well as of a pair of lateral eyes. 
This evidence directly suggests that the structure of both the median and 
lateral vertebrate eyes ought to be very similar to that of the median and lateral 
arthropod eyes. Such is, indeed, found to be the case. 
The retina of the simplest form of eye is formed from a group of the superficial 
epidermal cells, and the rods or rhabdites are formed from the cuticular covering 
of these cells; the optic nerve passes from these cells to the deeper-lying brain. 
This kind of retina may be called 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 surface, 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 invagina- 
tion, the cuticular surface 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. 
Such inverted retinas are found in the vertebrate lateral eyes and in the 
lateral eyes of the arachnids, but not of the crustaceans. 
The evidence shows that all the invertebrate median eyes possess a simple 
upright retina, and in structure are remarkably like the right median or pineal 
eye of Ammoccetes; while the lateral eyes possess, as in the crustaceans, an 
upright compound retina, or, as in many of the arachnids, a simple inverted 
retina. The lateral eyes of the vertebrates alone possess a compound inverted 
retina. 
This retina, however, is extraordinarily similar in its structure to the 
compound crustacean retina, and these similarities are more accentuated in the 
retina of the lateral eye of Petromyzon than that of the higher vertebrates. 
The evidence afforded by the lateral eye of the vertebrate points unmistakably 
to the conclusion that the ancestor of the vertebrate possessed both crustacean 
and arachnid characters—belonged, therefore, to a group of animals which gave 
rise to both the crustacean and arachnid groups. This is precisely the position 
of the Paleostracan group, which is regarded as the ancestor of both the 
crustaceans and arachnids. 
