306 
PROFESSOR E. RAY LANKESTER. 
A similar perforation of superficial epidermic cells for the 
transit of ducts of subjacent glands is seen in the skin of the 
common frog. 
Intra-epithelial Blood-capillaries , — In sections from any 
region of the Leech’s body the fine vessels of the haemal 
system naturally injected with their own haemoglobin- 
coloured fluid may be traced up to the sub-epidermal con- 
nective tissue, and thence finer branches may be here and 
there followed into the spaces between the handles ” of the 
mallet-like epithelial cells. Three examples of this dispo- 
sition of structures are given in Plate XXVI. Fig. 2 is 
drawn from a very thin section, which allows the observer to 
trace a horizontal vessel threading its way among the columns 
of the epithelium. In fig. S a section is shown which has 
fortunately cut one of the^intra-epithelial vessels transversely, 
'whilst in fig. 4 a number of vessels are seen supplying the 
intra-epithelial plexus, the section being too thick to show 
well the individual form of the epithelial cells. 
The sections shown in these three figures are merely 
samples of a perfectly general arrangement which occurs 
over the whole surface of the Leech’s body. The intra- 
epithelial vessels form a continuous plexus, giving to the 
whole of the epithelial layer of th^ skin a vascular cha- 
racter. 
General Remarks , — As a histological fact, the vascularity 
of this epithelium is a novelty. The only vascular epithe- 
lium, so far as I know, which has been hitherto described, is 
that of the clitellum of the Earthworm ; but in that case the 
vessels do not penetrate between the cells of the most super- 
ficial layer. 
The penetration into the epithelium of the processes of 
the pigmented vaso-fibrous tissue is a phenomenon which 
might be expected to accompany the penetration of the ves- 
sels, since the vaso-fibrous tissue and the capillary blood- 
vessels are virtually one and the same tissue. 
Physiologically the significance of the arrangement I have 
described is obvious enough. The true respiratory organ of 
the Leech is clearly this vascular epiderm, and amongst 
respiratory organs it stands alone in the nearness with which 
the absorbent blood-vessels succeed in bringing themselves 
through all structural obstacles into direct contact with the 
oxygenating medium. 
