22 KT. II.— A. 1ZUKA : OBSERVATION ON THE 



and the thick epidermis. The nuclei of the retinal cells are 

 situated close to the outer end of the cells, forming a zone. The 

 dark blue pigment is especially densely developed at the inner 

 end, along the line demarcating the rods from the cell-bodies 

 proper. The peripheral end of each cell tapers out into a process 

 continuous with a nerve fibre, which can be traced uninterruptedly 

 for some distance into the opic nerve. The clear refractive rods 

 form, as indicated, an inner lining to the pigmented retina ; on 

 the internal aspect they are indistinctly marked off from a clear 

 central mass, the lens. They are longest in the fundus, and 

 become shorter towards the rim, of the retinal cup. The lens 

 not only fills up the entire cavity of the cup, but even projects 

 slightly out of it at the pupil, thus coming "here in direct contact 

 with the epidermis. The name of cornea is given to that portion 

 of the epidermal layer which intervenes between the lens and the 

 cuticula. The latter is directly eontiunous with that over the 

 entire head and is in no way modified over the eyes. 



In adult worms the eyes have grown considerably in size ; the 

 retinal cup is now about twice as large in diameter as in immature 

 worms, while the diameter of the pupil has increased about four 

 times. The lens now projects outwards to a much greater degree 

 than before, pressing against and reducing the thickness of both 

 the cornea and the cuticula. The former may sometimes become 

 even entirely pressed aside, thus becoming perforate in the center 

 and bringing the lens into direct contact with the cuticula. The 

 result of these changes is that the eyes are now very conspicuously 

 visible in surface view (PI. IL, fig. 18), in both the living and 

 the preserved state of specimens. They are especially distinct in 

 specimens killed with picrosulphulic acid, in which they appear 

 as whitish spots surrounded by a black ring, the margin of the 



