No. 2.] EVES OF ARTHROPODS. 261 
eyes, in which the sclera has attained no excessive growth, very 
often show these small lenses along the upper margin, and they 
would, therefore, appear to be developed there; (2) it has 
already been shown that after the average normal mature growth 
of the animal has been reached, the number of the lenses 
becomes less with advancing senility. This fact must be 
explained either by the gradual envelopment of the lenses of 
the upper margin by the sclera and palpebrum, and their entire 
concealment within the substance of the latter, unless it is pos- 
sible that atrophy of the ommatidial nerve branches and con- 
comitant reabsorption of the lenses takes place with advancing 
old age. 
From the examination of eyes limited to ezg/¢ rows of lenses, 
it appears that with this number of rows there may be consid- 
erable variation in the number of lenses, as seen on the plate 
(Fig. 9, thirty-one lenses; Fig. 10, forty lenses). These figures 
also indicate the fact, that with a constant diminution in the 
number of lenses from the upper and lower extremities of the 
rows, the eight diagonal rows would ultimately be reduced to a 
single or double longitudinal row parallel to the margins of the 
visual surface. Hence, without overmuch hypothesis, the pri- 
mary lenses probably appeared in a single or double row, a 
visual line parallel to the margins of the orbital node. 
DEVELOPMENT OF THE LENS. 
There is sufficient evidence at hand for the statement that 
that portion of the ommatidial cavity which penetrates the 
test arises from an evagination from the internal surface of the 
test accompanied by a corresponding but very shallow invagi- 
nation from the upper surface. Natural casts of the internal sur- 
face of the visual area not infrequently show minute lensar cav- 
ities at the ends of the rows of lenses, which appear not to have 
penetrated to the upper surface, and bear at their summits no 
impression of a corneal surface or corneal cavity, as do the other 
lenses in the same eye. It would be inferentially true that the 
cornea is developed from the attenuated integument (cuticular 
epithelium), and is the specialized film of the test left between 
the depressions from its lower and upper surfaces, eventually 
becoming discrete. 
