OF ARTS AND SCIENCES. 79 



ever this fluid may be, it is not stained by any of the dyes in common 

 use. The protoplasm of these cells forms a thick peripheral layer, 

 with a rounded thickenini^ at one point, which projects into the vacu- 

 olar space. The very small nucleus is usually located near the base 

 of the internal i)rotuberance. 



The axial portion, consisting of glassy cells, is enveloped by a thick 

 layer of pigment on all sides except the external end. The epidermal 

 cap covering these cells is convex and entirely free from pigment, 

 forming thus a window-like opening into the black pigment-cup which 

 holds the large clear cells. An optic nerve enters the eye near its 

 deeper end, and runs along the axis for a 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 jiapillte, we find all the elements of the 

 eye, except the pigment. There is a branch of the lateral nerves that 

 runs to each ; and from four to six or more of those 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 doubtfu whether the papillo3 can be regarded as visual or- 

 gans; but it does not, to my mind, weaken the evidence of their serial 

 homology with the eyes. 



It is generally found that the posterior eyes, especially the fifth 

 pair, are smaller than those preceding them ; and I have noticed cases 

 in which only a mere trace of pigment could be seen iu one or both of 

 the last pair of eyes. While it appears doubtful what the special func- 

 tion of the papilhe is, still the presence of large clear cells, precisely 

 like those in the eye, situated just below the window-like opening in 

 the surface pigment, and their obvious serial equivalence with the eyes, 

 makes it not improbable that they represent incipient organs of vision. 



Although the evidence appears to me conclusive that the eyes and 

 the segmental papillae were, originally, morphological as well as physio- 

 logical equivalents, it does not, of course, follow necessarily that both 

 or<rans now have the same functional siirnificance. The oriciiial 

 papil];e may have represented sense-organs of a more or less indiffer- 

 ent order, among which, in tlie course of the historical development of 

 the leech, a division of labor was introduced, a few at the anterior 

 end becoming specialized as light-perceiving organs, the rest either 

 remaining in their early indifferent condition, or becoming specialized 

 in some other direction. 



The discovery that these papillfc are sense-organs might lead us to 

 speculate on affinities of a distant and somewhat uncertain nature, 



