3o6 THE LANCELET. PHYLUM CHORDATA 



and an intracellular duct into which hangs a single long flagellum. 

 It is in close contact with special blood vessels, and as the flagel- 

 lum seems to drive a current outwards to the atrium, it may be 

 presumed to pass out excretory matter. The brown tubes, which 



are two dorsal diverticula of the 

 f©) ^ posterior part of the atrium, 



may also be excretory. 



VASCULAR SYSTEM 



The blood is colourless and 

 contains white corpuscles. The 

 vascular system is closed. There 

 is no heart, but the larger vessels 

 are contractile and set up a 

 circulation in which the blood 

 flows forward ventrally and 

 backwards dorsally (Fig. 233). 

 A ventral or branchial aorta in 

 the subendostylar coelom gives 

 off to each primary gill bar a 

 vessel which divides to take part 

 in the formation of a rather 

 complicated branchial system in 

 the primary and secondary bars 

 and synapticulae. On each side 

 the branches of this system are 

 gathered up into a series of 

 vessels, which open into 

 a longitudinal suprabranchial 

 artery (or lateral dorsal aorta) 

 beside the epibranchial groove, 

 some of the blood passing 

 through nephridial plexuses on 



Fig. 232. — Solenocytes of Branchio- 

 stoma, showing the nuclei, long 

 flagella, and the openings into the 

 main excretory canal which leads to 

 the atrium. — From Young, after 

 Goodrich. Young, The Life of Verte- 

 brates, 1950. Clarendon Press, Oxford. 



the way. Behind the pharynx 

 the suprabranchial arteries unite to form a dorsal aorta, which 

 runs back under the notochord ; from the dorsal aortae blood 

 passes to lacunae in the tissues, which take the place of capillaries. 

 From the gut and body-wall, blood is collected by a subintestinal 

 vein. This is for much of its course a plexus, but in front becomes 

 a single vessel which runs to the liver and there breaks up agam 



