7 6 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 palseostracan group. What, then, is the 

 nature of the median eyes in the vertebrate ? 



The Median Eyes of Ammoccetes. 



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. 



Anyone 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 w T hiteness of its pigment. Upon opening the brain- case 

 the appearance as in Fig. 20 is seen, and the mass of the right ganglion 

 habenulce {G.H.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 habenulce is present, though much smaller than on 



