AN ICHTHYOBDELLID PARASITIC ON SAND WHITING. 27 
relating to this system in Piscicola. In 1902 Oka published 
a paper, in which he summarised his investigations concerning 
the blood- vascular system in the various families of the 
Hirudinea. In a lucid manner he showed that only in the 
Grlossiphonidee and Ichthyobdellidas was a true blood- 
vascular system present and that it had no communication 
with the lacuna system. Again, in 1904, Oka described in 
some detail the vascular system in Ozobranchus. My 
investigations of Austrobdella have shown that a closed 
blood-vascular system is also present here. 
In general this system in Austrobdella resembles that 
described in Ozobranchus and differs from the Piscicola 
and Callobdella type. 
There are, however, several important differences from 
Ozobranchus. The lateral paired branches in the anterior 
part of the body are three as compared with the four pairs 
found in Clepsine and the Ichthyobdellid leeches so far 
described. It is the second pair which are wanting. There 
is also a ring vessel in the posterior sucker with which the 
loops from the dorsal and ventral vessels connect; this is a 
very different arrangement from any so far described. 
Lastly, the division of the dorsal vessel into two parts takes 
place in the 24th somite, which is much higher up than in 
Clepsine and Piscicola. 
I have been favoured in these observations on the blood- 
vascular system by the transparent nature of the leech. The 
diagram of the blood-vascular system (PI. 1, fig. 2), is a 
careful representation of the course and relations of the 
blood-vessels in the neck and anterior sucker. 
The course of the blood-vessels is as follows : 
The dorsal blood-vessel gives off, in the anterior part of 
the body, three pairs of lateral branches, and an unpaired 
proboscis branch. The first of these (PI. 1, fig. 2, L. v. 1.), is 
formed by the forking of the dorsal vessel in the oral sucker. 
The two branches given off run round the eye-spots and unite 
in the region of the subcesophageal ganglion to form the 
ventral blood-vessel. The course of this first pair of lateral 
