50 ZOOLOGY SECT. 



From it are given off, on each side, lateral branches, the afferent 

 branchial arteries (Fig. 754, af. br. a. ; Fig. 753, k), with small con- 

 tractile dilatations at their bases, which pass up the primary 

 branchial lamellae and communicate by cross-branches with similar 

 vessels (af. br. a'.) in the secondary or tongue-lamellae . The blood 

 is exposed, while traversing these vessels, to the aerating influence 

 of the respiratory current, and leaves the branchial lamellae dorsally 

 by efferent branchial arteries (ef. br. a.} which open on each side into 

 paired longitudinal vessels, the right and left dorsal aortce (d. ao.), 

 lying one on either side of the epipharyngeal groove. Anteriorly 

 both dorsal aortee are continued forwards to the region of the snout, 

 the right being much dilated ; posteriorly they unite with one 

 another, behind the level of the pharynx, into an unpaired dorsal 

 aorta (d. ao' .}, which extends backwards in the middle line, imme- 

 diately below the notochord and above the intestine. 



The unpaired dorsal aorta sends off branches to the intestine, in 

 the walls of which they break up to form a network of microscopic 

 vessels or capillaries (cp.). From these the blood is collected and 

 poured into a median longitudinal vessel, the sub-intestinal vein 

 (Figs. 751, B, and 754, s. int. v.), lying beneath the intestine : in 

 this trunk the blood flows forwards, and, at the origin of the liver, 

 passes insensibly into a hepatic portal vein (hep. port, v.), which 

 extends along the ventral side of the liver and breaks up into 

 capillaries in that organ. From the liver the blood makes its way 

 into a hepatic vein (hep. v.), which extends along the dorsal aspect 

 of the digestive gland, and, turning downwards and forwards, joins 

 the posterior end of the ventral aorta. 



It will be seen that the vascular system of Amphioxus consists 

 essentially of (a) a dorsal vessel represented by the paired and un- 

 paired dorsal aortae, (6) a ventral vessel represented by the sub- 

 intestinal vein and the ventral aorta, and (c) commissural vessels 

 represented by the afferent and efferent branchial arteries and 

 the intestinal capillaries. So far the resemblance to the vascular 

 system of Annulata is tolerably close ; but two important differences 

 are to be noted. The blood in the ventral vessel travels forwards, 

 that in the dorsal vessel backwards the precise opposite of what 

 occurs in Worms, and the ventral vessel is broken up, as it were, 

 into two parts, by the interposition in its course of the capillaries 

 of the liver, so that all the blood from the intestine has to pass 

 through that organ before reaching the ventral aorta. This passage 

 of the intestinal blood through the vessels of the liver constitutes 

 what is called the hepatic portal system, and is eminently character- 

 istic of Vertebrata. 



The blood is almost colourless with a few red corpuscles, and 

 appears to contain no leucocytes. It is not confined to the true 

 blood-vessels just described, but occurs also in certain cavities or 

 lymph-spaces, the most important of which are the cavities in the 



