336 ; Cc. O. WHITMAN. 
the course of the historical development of the Leech, a divi- 
sion of labour was introduced, a few at the anterior extremity 
becoming specialised as organs of vision, the rest either 
remaining in their early indifferent condition or becoming 
specialised in some other direction. 
It seems more probable, however, that the segmental papille 
are incipient eye-spots—visual organs in statu nascendi— 
and that the eyes are organs of the same nature, only struc- 
turally improved and functionally exalted. 
The Structure of the Eyes and Segmental Papille. 
—If any such relationship exists between the eyes and the 
segmental papillz as is indicated by their correspondence in 
position, we should expect to find some important resem- 
blances in their structure and composition. As this subject 
will be considered in detail in a later paper provided with 
illustrations, I shall here call attention only to the more 
important points. 
The eye of the Land Leech, like that of the aquatic Leech, is 
formed of large clear ceils (“ eigenartige helle Zellenkorper,” 
of Leydig), which are usually regarded as a corpus vitreum, 
surrounded by a thick layer of pigmented cells. The epi- 
dermal layer covering the eye is composed of closely packed 
columnar cells, which are not perpendicular to, but inclined 
towards the centre of, the convex outer surface of the eye. 
This epidermal cap is further distinguished from the epidermis 
elsewhere in being entirely free from pigment. The cell 
nature of the large clear bodies forming the central portion of 
the eye has been denied by Ranke, on the ground that no 
nuclei had been discovered in them by himself or other 
authors. My sections, however, demonstrate the existence of 
nuclei in these bodies. The nuclei are extremely small and 
usually situated very near the outer side of the cells, close to 
the pigmented layer, and are therefore easily overlooked. 
Within each of these clear cells there is found a very peculiar 
white corpuscle which never stains. Leydig represents this 
body in a few cases as a complete ring, but in most cells as an 
imperfect ring opening towards the base of the eye. In the 
