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VASCULAR SYSTEM—GILLS 269 
pericardial membrane. Posteriorly the pericardial chamber 
receives five paired veins on each side coming from the gills 
and returning the purified blood to the heart. 
Eleven arteries arise from the heart. These are Gi.) a median 
frontal artery which, passing forward, divides into a right and 
left marginal artery. These run round the edge of the carapace 
to its posterior angle, where each receives a branch of the collateral 
artery mentioned below. (ii.) and (ii1.) are the aortic arches 
(Fig. 154), paired vessels running round and supplying the proven- 
triculus and oesophagus. ‘hese unite ventrally in a vascular ring 
which encloses the nerve-ring, and is continued along the ventral 
nerve-cord as the ventral artery and along some of the chief nerves. 
This vascular ring supplies the lateral eyes and all the append- 
ages mentioned on p. 263 up to and including the genital oper- 
culum. The ventral artery supplies the respiratory appendages, 
and gives branches to the rectum, caudal spine, etc. Two of its 
branches encircle the rectum, and uniting open into the superior 
abdominal artery.  iv.-xi. are paired lateral arteries which leave 
the heart beneath the anterior four ostia, and soon enter a longi- 
tudinal pair of collateral arteries which unite behind in the just 
mentioned superior abdominal artery ; they also give off branches 
to the muscles and to the intestine, and a stout branch mentioned 
above which passes into the marginal artery posteriorly. The 
venous system is lacunar, and the blood is collected from the 
irregular spaces between the various organs into a pair of longi- 
tudinal sinuses, whence it passes into the operculum and the five 
pairs of gills. A large branchio-cardiae canal returns the blood 
from each gill to the cavity of the pericardium, and so through 
the ostia to the heart. Eight veno-pericardiae muscles run from 
the under surface of the pericardium to be inserted into the 
upper surface of the longitudinal sinus; they occur opposite the 
ostia, and play an important part in the mechanism of the 
circulation. The blood is coloured blue by haemocyanin ; 
amoeboid corpuscles float in the plasma. 
The respiratory organs are external gills borne on the 
posterior face of the exopodite of the lamella-like posterior five 
mesosomatic limbs. Each gill consists of a series of leaves like 
the leaves of a book, and some 150-200 in number. Within 
the substance of each leaf the blood flows, while without the 
oxygen-carrying water circulates between the leaves. These gill- 
