30 
CHARLES BADHAM. 
fig. 11, S.m.f). In somite 19 the dorsal vessel enters into 
intimate’connection with the caeca of the thick-walled intestine. 
This remarkable arrangement was first described by Johansson 
for the Ichthyobdellids inCallobdella, and in many respects 
the relations of the blood-vessel with the gut-walls, as described 
by him, hold good also for Austrobdella. 
Text-fig. 3 shows the relations of this vessel with the 
epithelium and muscular walls of the gut. At the beginning 
of the thick-walled intestine, the dorsal vessel is seen connected 
with the muscular layer of the gut, and almost immediately 
it opens on either side into the intestinal sinus, and ceases to 
be distinguishable from the walls of the sinus. 
The intestinal sinus is formed by the separation of the 
•epithelial and muscular walls of the gut. This separation is 
not complete in Austrobdella, for here and there the 
normal relations of the epithelial layer and the muscle layer 
are seen (Text-fig. 3, M. g. ep.), but save at these places 
of attachment, which are usually small, the blood-stream 
surrounds the epithelial layer of the thick-walled intestine. 
These relations are such as described for Callobdella. In 
the intermediate portions of the thick-walled intestine, which 
-connect the paired pouches, the dorsal blood-vessel separates 
from the intestinal sinus and lies in the dorsal side of these 
regions. Also, in the region of the fourth rudimentary pair 
of caeca, the sinus developed from the dorsal vessel is very 
small, and only for a short distance does the dorsal vessel 
cease to be defined : following this part the dorsal vessel is 
clearly defined and remains single until above the ganglion 
of the 24th somite ; here it divides in two. The two branches 
then run laterally to the gut closely connected with its 
muscular wall. In the 27th somite these two vessels diverge 
aud run round the periphery of the posterior sucker on either 
.side and finally unite (PI. 1, fig. 2). The valves in the 
•dorsal vessel are found from just before the giving off of the 
.second pair of lateral vessels to the beginning of the intestinal 
.sinus (PI. 1, fig. 2, VI.). They are placed somewhat irregu- 
larly, one or two in each somite. They are generally shaped 
