82 transactions liverpool biological society. 



Vascular System. 



Delate (1881) has described the vascular system of 

 Ligia oceanica in his Memoir on the circulation of the 

 Edriophthalmia, and, except in a few details, my results 

 confirm his account. 



The heart (PI. III., fig. 1, lit) 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 (ost.), 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. II., fig. 16, px.) 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 III., fig- 1, rned. 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 



