MUSEUM OF COMPARATIVE ZOOLOGY. 203 
the phylogenetic antecedent of the median eyes, a condition has been 
implied which agrees with the essential features of the lateral eyes. Of 
all the eyes in spiders and scorpions, the lateral eyes in scorpions are un- 
doubtedly the least complicated, and they may be looked upon as deviat- 
ing least from the probable ancestral type. 
10. 
Summary of Results. 
Nos. 2-11 refer to the median eyes ; Nos. 12-17 refer to the lateral eyes. 
. The retinas of the median and lateral eyes are strictly hypodermal in 
their origin. 
The median eye is triplostichous, and is formed by an inyolution of 
hypodermis accompanied with an wversion of the middle layer, 
which forms the retina proper. 
The first layer or lentigen, is modified hypodermis immediately ex- 
ternal to the pocket of involution, and, in addition to secreting 
the lens, serves a purpose which gave to it its earlier name of 
** vitreous.” 
The lens is the specialized cuticula produced by the lentigen. It 
differs from ordinary cuticula in containing zo pore-canals, and, 
excepting the external hyaline layer, in being stainable through- 
out. : 
The Jdentigen can produce cuticula independently of the general 
hypodermis. 
The second layer, or retina, is inverted, and consists of two kinds of 
cells, — retinal (nerve-end) cells and pigment cells. It contains 
phaospheres. 
The retinal (nerve-end) cells contain pigment ; their walls are thick- 
ened into prenuclear rhabdomeres, and a nerve fibre emerges from 
their deep ends. They are so arranged in groups of five, that five 
rhabdomeres are united to form one rhabdome. 
Each of the pigment cells is reduced to two sacs, connected by a stiff 
fibre. The external sac contains pigment; the internal, the 
nucleus and pigment. 
The third or post-retinal layer is the “sclera matrix” of Graber. 
It becomes intimately fused with the retina. 
The fibres of the optic nerve in the embryo emerge from the external 
ends of the inverted retinal cells; in the adult, from the opposite 
ends. 
