22 



lyin<^- underneath the endostyle (PI. III., fio-. 10, hr. ao.) 

 to the waHs of the branchial sac, where, in passing along 

 the transverse and interstigmatic vessels, it is purified, 

 and receives a supply of oxygen from the water passing 

 through the stigmata. It is then conveyed, by the great 

 dorsal vessel and the branchio- visceral vessel and its 

 branches, to different parts of the viscera and body-wall, 

 so that all the organs may receive food and oxygen, and 

 have their waste materials carried away. Some of these 

 branchio-visceral vessels from the branchial sac lead to 

 the walls of the stomach and intestine (see PI. III., fig. 10), 

 and thus bring us back to the point from which we started. 



The great dorsal and ventral vessels of the branchial 

 sac are connected (PI. III., fig. 10), not only by the trans- 

 verse vessels, which run like hoops round the walls, but 

 also at their anterior extremities, by a circular vessel 

 which surrounds the front of the branchial sac, underneath 

 the peripharyngeal bands. A short branch runs from near 

 the front of the dorsal vessel to the sinuses which sur- 

 round the nerve ganglion. 



From each end of the heart "vessels" are also given 

 off to supply the body- wall and test. Moreover, " connec- 

 tives " run from the transverse vessels of the branchial 

 sac directly outwards, on each side, to the body-wall and 

 viscera. On the left side there are three especially large 

 blood tubes amongst the connectives, which cross to the 

 alimentary canal and ovary, and branch through these 

 viscera. 



But the course of this circulation of the blood is not 

 always the same ; sometimes it is exactly reversed, the 

 blood flowing from the branchial sac to the heart, and 

 from that organ to the viscera, and then back to the 

 branchial sac again. This curious state of affairs is caused 

 by the remarkable manner in which the Ascidian heart 



