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MUSEUM OF COMPARATIVE ZOOLOGY. 183 
edge of the lens, and, as Graber has indicated (79, p. 62), forms in the 
adult a complete circle, —the iris. In the iris proper the whole of each 
cell contains pigment granules, while in the adjoining hypodermis the 
granules are scattered in small groups through the cells, and are especially 
abundant at their outer ends. 
The dens owes its origin exclusively to the activity of the lentigen. As 
is well known, it consists of a thickening of the external cuticula. The 
lentigen bears the same relation to the lens as the hypodermis does to 
the indifferent cuticula. 
In Centrurus the cuticula at most points on the body consists of three 
layers. The outermost, first recognized as distinct by Graber (’79, p. 59), 
is a thin, homogeneous, colorless layer (Pl. I. fig. 2, //). Under this is a 
second layer of about equal thickness with the first, but usually of a deep 
yellow color (Pl. I. fig. 2, d/’). These two layers together form about 
one fourth the whoie thickness of the cuticula. The third layer (Pl. I. 
fig. 2, id”), embracing the remaining three fourths, is distinctly laminated. 
The deepest lamella of this third layer readily takes up borax-carmine. 
The remaining lamelle are distinguishable from the second layer chiefly 
by their want of color. The cuticula is very commonly penetrated by two 
sets of pore-canals (Pl. I. fig. 2, can. po. and can. po.’), fine and coarse. 
As the indifferent cuticula passes into the region of the lens, the fol- 
lowing conditions are noticeable. The external hyaline layer passes un- 
changed either in thickness or texture over the front of the lens. The 
second or colored layer becomes perfectly colorless, and by its increased 
thickness adds to the convexity of the lens. The bulk of the lens, how- 
ever, is produced by a thickening of the third layer. 
Whereas in the indifferent cuticula only its deepest lamella is colored 
with borax-carmine, in the lens all parts below the outer homogeneous 
layer readily take up this dye. A similar condition has been observed 
in several other local thickenings in the general cuticula, especially on 
the ventral side of the animal. The conclusion to be drawn from these 
observations is, that the lens in its composition is more closely related to 
the last-formed cuticular lamella than it is to the older lamelle. 
The coarse pore-canals never occur in the lens. Grenacher (’79, p. 90) 
was unable to find fine pore-canals in the lens of Phalangium, although 
Leydig had previously claimed them to be found in such lenses. Graber 
stated (’79, p. 60) that all arthropod lenses which he had examined con- 
tained fine pore-canals. In Centrurus, notwithstanding that many sec- 
tions of lenses have been examined, fine pores have never been visible, 
although in the adjoining cuticula they are plainly evident. 
