82 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 
VASCULAR SYSTEM. 
Delage (1881) has described the vascular system of 
ligia oceanica 11 his Memoir on the circulation of the 
Edriophthalmia, and, except in a few details, my results 
confirm his account. 
The heart (PI. IIL, fig. 1, ht) is a fairly wide tubular 
structure, extending from the fifth abdominal segment to 
the anterior end of the fourth thoracic segment. This 
posterior position of the heart is correlated with a posterior 
position of the organs of respiration. Its walls are 
muscular, and are perforated by two ostia (os¢.), which are 
oblique slit-like orifices provided with muscles and two 
small inwardly projecting flaps. They are situated in the 
anterior and posterior regions on the right and left sides 
respectively. 
The pericardium (PI. I1., fig. 16, p.c.) extends from the 
anterior end of the heart to beyond the posterior end. 
It receives the efferent vessels from the branchiae, and is 
continuous with the venous lacunae in the anterior 
regions of the body. It is separated from the body cavity 
by a horizontal septum upon which the heart rests. 
The heart is continued anteriorly as the median 
aorta. On each side four arterial thoracic trunks arise. 
The first pair may be termed the lateral arteries; the 
remaining three pairs are the fifth, sixth and seventh 
thoracic arteries, and they arise in the anterior half of the 
heart. 
The median aorta (Pl. IIl1., fig. 1, med. ao.) runs 
forward along the dorsal wall of the gut to the 
cephalic region. In_ the anterior region of the 
second thoracic segment two arteries arise from the 
dorsal side, and run a_ sinuous course in the 
hypodermal tissues towards the epimera. In the first 
thoracic segment a pair of large arteries arise laterally 
