4.04. Cc. O. WHITMAN. 
mented layer (“ chorioidea ”). This conception of the eye was 
rendered all the more plausible by the supposed central per- 
foration of the epidermal cap, which remained permanently 
open, while the rest of the lumen of the sac was filled by the 
axial fibres. It was thus that Leydig maintained that the 
large clear cells were modified epidermic cells, an opinion in 
which Ranke fully concurs. According to the view I have 
presented, the eye is a solid ingrowth of dermal elements, the 
epidermis being represented by an axial cord of sensory cells 
continuous at the base of the eye with the optic nerve, the 
gland-cells of the skin by the large clear cells forming the bulk 
of the eye, and the sub-epidermal connective tissue by the 
pigment layer. I have not been able to satisfy myself from 
my sections that a distinct membrane-like layer (Leydig’s scle- 
rotica) separates the pigment investment from the large clear 
cells. It remains to be seen how far this view represents the 
actual developmental history of the eye. 
Structurally considered, we are able to distinguish at least 
three different classes of sense-organs in the Leech. The first 
class embrace the segmental sense-organs of the body and 
head, and the non-segmental sense-bulbs scattered over the 
upper surface of the head; the second is represented by deri- 
vatives from segmental sense-organs, the eyes; and the third 
includes the goblet-shaped organs of the margin of the lip. 
In the first and third classes a bulb-like thickening of the 
epidermis forms the larger part of the organ, and the chief dis- 
tinction between them lies in the presence or wbsence of large 
clear cells around the bulb. The distinguishing feature of the 
second class is the massive development of the large clear 
cells. 
With respect to arrangement all these organs may be 
grouped in two divisions, one of which is strictly segmental, 
the other non-segmental or accessory. All agree in represent- 
ing primarily more or less specialised portions of a common 
morphological basis, but the bilateral and metameric symmetry 
of one set of organs must be regarded as a distinctive feature 
of considerable significance. For while the source of origin is 
