76 THE ORIGIN OF VERTEBRATES 
always separate and median in position, while the others form lateral 
groups ; all these eyes possess a simple retina and a simple corneal 
lens. Grenacher was the first to point out that in the spiders two 
very distinct types of eye are found. In the one the retina is up- 
‘right; in the other the retina is inverted, and the eye possesses a 
tapetal layer. The distribution of these two types is most suggestive, 
for the inverted retina is always found in the lateral eyes, never in 
the two median eyes; these always possess a simple upright retina. 
In the crustaceans, the lateral eyes differ also from the. median 
eyes, but not in the same way as in the arachnids; for here both 
types of eye possess an upright retina, but the retina of the lateral 
eyes is compound, while that of the median eyes is simple. In other 
words, the median eyes are in all cases eyes with a simple upright 
retina and a simple cuticular lens, while the retina of the lateral 
eyes is compound or may be inverted, according as the animal 
in question possesses crustacean or arachnid affinities. The lateral 
eye of the vertebrate, possessing, as it does, an inverted compound 
retina, indicates that the vertebrate arose from a stock which was 
neither arachnid nor crustacean, but gave rise to both groups—in fact, 
was a member of the great paleostracan group. What, then, is the 
nature of the median eyes in the vertebrate ? 
Tue MeEpIAN EYES OF AMMOCGETES, 
The evidence of Ammoccetes is so conclusive that I, for one, can- 
not conceive how it is possible for any zoologist to doubt whether 
the parietal organ, as they insist on calling it, had ever been an eye, 
or rather a pair of eyes. 
Any one who examines the head of the larval lamprey will see 
on the dorsal side, in the median line, first, a somewhat circular orifice 
—the unpaired nasal opening ; and then, tailwards to this, a well- 
marked circular spot, where the skin is distinctly more transparent 
than elsewhere This spot coincides in position with the underlying 
dorsal pineal eye, which shines out conspicuously owing to the 
glistening whiteness of its pigment. Upon opening the brain-case 
the appearance as in Fig. 20 is seen, and the mass of the right ganglion 
habenule (G.A.R.), as it has been called, stands out conspicuously as 
well as the right or dorsal pineal eye (Pn.). Both eye and ganglion 
appear at first sight to be one-sided, but further examination shows 
that a left ganglion habenule is present, though much smaller than on 
