STRUCTURE OF NEPHRIDIA OF THE MEDICINAL LEECH. 299 
sufficient to point out that the investing brown-coloured net- 
Avork of fibres consists of what Professor Lankester has 
termed vaso-fibrous tissue/’ which is found throughout the 
body of the Leech, more or less pigmented, and forms the 
pigment fibrils which interpenetrate the superficial epi- 
thelium of the body-surface. 
In the transverse section (fig. 13) this tissue is seen as 
dark granular matter {f) surrounding the lobes of the gland, 
and passing in and out amongst the muscular fibres {g) and 
the unicellular glands (e), all of which are more or less 
closely invested by it. 
The peculiarly close relationship of this tissue to the 
capillary blood-vessels themselves explains how it is that no 
distinct connective tissue penetrates the cell-mass of the 
nephridium hut only fine blood-vessels , which form, in fact, 
an intercellular connective tissue, which is at the same time 
a capillary system. 
The large blood-vessels found in connection with the 
nephridium have been described with great care and ac- 
curacy by Gratiolet. They are as shown in fig. 1. They 
consist of vessels with delicate membranous walls which are 
given off by the great lateral muscular vessel passing near 
the nephridium. A connection is further established between 
the capillary plexus of the nephridium and the dorsal longi- 
tudinal vessel (which runs along the dorsal surface of the 
alimentary canal) by means of the superior vein.” The 
arrangement and distribution of the vessels will be best 
understood from an examination of the diagram fig. 1. The 
smaller vessels which are arranged so as to form a plexus or 
mesh work with a single nephridial cell in each hole of the 
meshwork average in size — o^o-o inch, but are capable 
of being greatly distended with blood, and do not always or 
in all parts of the nephridium appear to be fully developed. 
This, however, is not due, I believe, to any structural 
irregularity, but to a functional state. When the nephri- 
dial cells or their ducts are swollen, the intercellular vascular 
network is compressed, and many of its channels are tem- 
porarily obliterated. But, on the other hand, it may happen 
that we find all the blood- capillaries well injected with 
their natural red-coloured fluid, as shown in figs. 6 and 7. 
These are drawn from such natural injections ” which ex- 
hibit themselves in the most beautiful way when sections 
are cut from specimens of the Leech taken in full nutrition 
(that is, within a week or so of a good meal), and hardened 
in dilute chromic acid. 
The wall of the finer blood-vessels running between the 
