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 lens 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, U). Under this is a 

 second layer of abou.t equal thickness with the first, but usually of a deep 

 yellow color (PI. I. fig. 2, II'). These two layers together form about 

 one fourth the whole thickness of the cuticula. The third layer (PI, I. 

 fig. 2, II"), embracing the remaining three fourths, is distinctly laminated. 

 The deepest lamella of this third layer readily takes up borax-carmine. 

 The remaining lamellse 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 

 hickness 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 lamellse. 



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. 



