1106 The Segmental Sense-Organs of the Leech. (Novena, 
these cells has a vacuole-like space occupying a central position 
which is probably filled with some kind of fluid. Whatever this 
fluid may be, it is not colored by any of the dyes in common we 
The protoplasm of these cells forms a thick peripheral envelope 
with a rounded thickening on one side which projects into the 
vacuolar space. The very small nucleus is usually located near 
the base of this internal protuberance. This axial portion consist- 
ing of clear cells is enveloped by a thick layer of pigment onal 
sides except the external end. The epidermal cap covering thes 
cells is convex, and entirely free from pigment, forming thusa 
window-like opening into the black pigment-cup which holds the 
large transparent cells. An optic nerve enters the eye near is 
deeper end, and runs along the axis fora larger portion of its 
length. It is probable that branches of the nerve connect with 
the clear cells, but precisely how has not been ascertained. 
In sections of the segmental papillae we find all the elements of 
the eye except the pigment. There is a branch of the 
nerves that runs to each; and from four to six or more of those 
peculiar large glassy cells are found a little below the epidermal 
cap, which is convex and free from pigment. 
The absence of a pigment-cup holding the glassy cells makes 
it doubtful whether the papilla can be regarded as light peset 
ing organs, but it does not, to my mind, weaken the oa 
their serial homology with the eyes. It is generally found on 
the posterior eyes, especially the fifth pair, are smaller than Ho 
preceding them; and I have noticed cases in which oily a 
trace of pigment could be seen in one or both of the last if 
eyes. While it appears doubtful what the function of the pi pe 
is; still, the presence of large cells precisely like thos E a 
eye, situated just below a window-like opening in the surface ai 
ment, and their obvious serial equivalence with the F 
it not improbable that they represent incipient organs oka 
Although the evidence appears to me conclusive that et 
and the segmental papillæ were, originally, morpholog! a en 
as physiological equivalents, it does not of course follow nece 
sarily that both now have the same functional signi i 
original papillæ may have represented sense organs of a p 
less indifferent order, among which, in the course of the p 
~ Cal development of the leech, a division of labor was "ig 
~ a few at the anterior extremity becoming specialized aS orga : 
f 
