XIII 



PHYLUM CHORDATA 



51 



A, CO., Fig. 744, sc.) lying one on either side of the dorsal region of 

 the pharynx above the atrium, and connected by narrow canals in 

 the primary branchial lamellae (Fig. 744, right side) with a 

 median longitudinal space below the endostyle (ec). In the 

 intestinal region it is much reduced on the right side, being 

 displaced by the backward extension of the atrium (Fig. 742, B, 

 atr., Fig. 743, atr'.), but is well developed on the left side a 

 forward extension of it surrounds the liver (Fig. 742, A, I.). The 

 whole series of spaces is lined by coelomic epithelium. 



Blood-System — The blood-vessels of Amphioxus are all of one 

 kind, but, owing to certain undoubted homologies with the more 

 complex vessels of the Craniata (see below), some of them receive 

 the name of arteries, others of veins. 



Lying in the ventral wall of the pharynx, below the endostyle, 

 is a- median longitudinal vessel, the ventral aorta (Fig. 745, v, ao, 

 Fig. 699, si.); it is contractile, and drives the blood forwards. 



ef.br'.a. 



ef.hra ' 

 brcl 



Pig. 745. — Diagram of the vascular systeTn of Ampbioxus. af. br. a. afferent branchial 

 arteries ; cp, intestinal capillaries ; d. ao. paired dorsal aortas ; d. ao.' median dorsal aorta ; 

 ef. br. a. efferent branchial arteries ; hep. port. v. hepatic portal vein ; hep. v. hepatic vein ; 

 int. intestine ; Iv. liver ; ph. pharynx ; s. int. v. sub-intestinal vein. 



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

 hranchial arteries (Fig. 745, af. hr. a. ; Fig. 744, h), which pass 

 up the primary branchial lamellas and communicate by cross- 

 branches with similar vessels {af. h: a'.) in the secondary or 

 tongue-lamellEe. The blood is exposed, while traversing these 

 vessels, to the aerating influence of the respiratory current, 

 and leaves the branchial lamellae dorsally by efferent hranchial 

 arteries (ef. hr. 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 aortae 

 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 pharjmx, into an impaired dorsal aorta {d. ao'.), which 

 extends backwards in the middle line, immediately 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 



