AN ICHTHYOBDELLID PARASITIC ON SAND WHITING. 29' 
Posteriorly the two branches of the dorsal blood-vessel in 
the 26th somite leave the lacuna, formed by the union of the 
intestinal and ventral lacunae, and enter the connective tissue 
to course round the periphery of the posterior sucker. 
The ventral vessel lies in the ventral lacuna from the 7tli 
to the 26th somite. It is formed by the union of the first 
pair of lateral vessels, just above the 7th somite. These 
lateral vessels enter the lacuna opposite the spot where they 
fuse. The ventral vessel lies quite free in the ventral lacuna 
above or at the side of the nerve- cord and always above the 
nerve ganglia. It leaves the ventral lacuna near the same 
spot as the dorsal vessel in the 26th somite. 
The histological features of the blood-vessel walls agree- 
closely with those described for Callobdella by Johansson 
(1896 6) and for Piscicola by Salensky (1906). 
The dorsal vessel, immediately on passing out of the dorsal 
lacuna, develops in the 13th somite a strong muscle layer of 
annular nature internal to a layer of finer muscular fibres- 
This structure continues until the diameter of the dorsal 
vessel becomes smaller after the proboscis branch has been 
given off. The anterior part of the dorsal vessel takes its 
origin in a peculiar way from that part of the dorsal vessel 
which has a much greater diameter. On the side of the- 
dorsal vessel, opposite to the point of origin of the proboscis 
vessel, this anterior part lies laterally to the dorsal vessel 
and opens into it at two places, both of which are guarded 
by valves. The lateral vessels, possessing this well-developed 
muscle layer for but a little distance after they spring from the- 
dorsal vessel, gradually come to resemble the ventral vessel 
in the nature of their walls. The wall of the dorsal blood- 
vessel in the dorsal lacuna possesses only a thin epithelium, 
with scattered nuclei (PI. 2, fig. 10, Nu.epi.), save only at 
those places where the valves are placed. Here there are 
one or two annular muscle-fibres, such as are found in the 
preclitellar region, and which, in contraction, form a sphincter, - 
against which the valve is pressed by the backward pressure 
of the fluid during the contraction of the dorsal vessel (PI. 2,. 
