No. 3.] SOME NEW FACTS ABOUT THE HIRUDINEA. 589 
ment investment. In view of these facts, and others yet to be 
noticed, we can no longer regard pigment as an essential ele- 
ment of the leech eye. It will not do to fall back on the 
hypothesis of degeneration, and assume that these eyes sup- 
plied with little or no pigment are functionless rudiments. The 
visual cells are here as perfectly developed as in the pigmented 
eyes, and the same is true of the optic nerves. 
g. The test of a leech eye, then, is the presence of visual cells. 
Now what are the visual cells, and how are they to be recog- 
nized? After spending a good deal of time in the study of the 
structure and development of the sense-organs of the leech, 
I feel quite confident of having reached a point in the inves- 
tigation where I can safely answer these questions. The visual 
cells are the “/arge clear cells” of Leydig, the so-called “ Glas- 
korper.’ The proof of this lies in a variety of facts, only a few 
of which can here be presented in a summary way. The leading 
points are as follows: 1. These cells always make up the bulk 
of the eye, and in the Hirudo pattern they are the only cells 
supplied by the optic nerve. 2. The main axis of these cells — 
that passing through the centre of the cell and the eccentric 
nucleus — is generally, though not invariably, parallel with the 
axis of the eye. This is most clearly seen in some species of 
Clepsine, and is very evident in Branchelliopsis. 3. In these 
genera, the nucleus lies on the side exposed to the light, the 
clear rod-like part of the cell being directed towards the pig- 
ment. The cells are practically inverted, the nerve-fibres enter- 
ing at the nucleated pole. 4. A comparison of the different 
patterns of eye represented in Hzrudo, Nephelis, Clepsine, and 
Branchetliopsis, with the typical segmental sense-organ, shows 
that the chief distinction between the two classes of organs 
lies in the relative abundance of the clear cells. 
10. The segmental sense-organs are double organs, both in 
structure and in function. There is an axial cluster of elon- 
gated cells, terminating at the surface in minute hairs, and 
representing most likely a tactile organ. Around and beneath 
the factile cells, are the large, clear visual cells, so characteristic 
of the eye. Thus we have a visual and a tactile organ combined, 
both derived from a common mass of indifferent epidermal cells, 
and both supplied by fibres from a common nerve branch. 
11. Incredible as the double nature of these organs may at 
