x VASCULAR SYSTOI GILLS 269 



pericardial membrane. Posteriorly the pericarclial 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. 15-4), paired vessels running round and supplying the pro veil - 

 triculus and oesophagus. These unite ventral ly in a vascular rinu 

 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 central artery supplies the respiratory appenda-.-.-. 

 and gives branches to the rectum, caudal spine, etc. -Two of its 

 branches encircle the rectum, and uniting open into the scjn //</ 

 alxlnnii nal 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 Im- 

 pairs of gills. A large branehio-cardiac canal returns the blond 

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

 the ostia to the heart. Kight veno-pericardiac muscles run I'mm 

 the under surface of the pericardium to lie 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 haeinocyanin ; 



\j \J 



amoeboid corpuscles lloat in the plasma. 



The respiratory organs are external gills borne mi the 

 posterior face of the exojiodite of the lamella-like posterior li\e 

 mesosomatic limbs. .Kadi gill consists of a -cries of leaves like 

 the leaves uf a book, and some ir.O-L'OO in number. Within 

 the sub-tance of cadi leaf the Mood llows. while \\ithoiit the 

 oxygeii-earrying water circulates between the leavi These -ill- 



