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 proventriculus and oesophagus. These 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 appendages mentioned on p. 263 up to and including the genital operculum. 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 longitudinal 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 longitudinal 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 gillbearing appendages can be flapped to and fro, and they seem to be at times held apart by the _flabellum_, a spatulate process which Patten and Redenbaugh regard as a development of the median sensory knob on the outer side of the coxopodite of the last pair of walking limbs. [Illustration: