X 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 (i.) 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 (iii.) are the aortic arches 

 (Fig. 154), paired vessels running round and supplying the proven- 

 triculus and oesophagus. These unite ventral ly 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. 203 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 it* 

 branches encircle the rectum, and uniting open into the svperior 

 abdomiiial 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-cardiac canal returns the blood 

 from each gill to the cavity of the pericardium, and so through 

 the ostia to the heart. Eight veno-'pericardiac 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- 



