xni PHYLUM CHORDATA 49 



The Atrium. The gill-clefts lead into a wide chamber occupying 

 most of the space between the body- wall and the pharynx, and 

 called the atrium (Figs. 751, $, and 752, atr.). It is crescentic in 

 section, surrounding the ventral and lateral regions of the pharynx, 

 but not its dorsal portion. It ends blindly in front , opens 

 externally, behind the level of the pharynx, by the atriopore 

 (atrp'.) ; and is continued backwards by a blind, pouch-like exten- 

 sion (atr.) lying to the right of the intestine (Fig. 751, B, atr.). 

 The whole cavity is lined by an atrial epithelium of ectodermal 

 origin. As in Ascidia, the cilia lining the gill-clefts produce a 

 current setting in at the mouth, entering the pharynx, passing 

 thence by the gill-slits into the atrium, and out at the atriopore. 

 The current, as in Tunicata and Balanoglossus, is both a respiratory 

 and a food current, the animal feeding passively on the minute 

 organisms in the surrounding water. 



Ccelome. Owing to the immense size of the atrium the body- 

 cavity, which is a true coelome, is much reduced. It is represented, 



ft 0.0 ^ 



BiB-M=iMa- ^a. 



int 



ffbra 



FIG. 754. Diagram of the vascular system of Amphioxus. /. br. a. afferent branchial 

 arteries ; br. cl. branchial cleft ; cp. intestinal capillaries ; d. ao. paired dorsal aortae ; d. ao'. 

 median dorsal aorta ; ef. br. a. efferent branchial arteries ; hep. port, f . hepatic portal vein ; 

 hep. v. hepatic vein ; int. intestine ; Ir. liver ; ph. pharynx ; s. int. r. sub-intestinal vein ; 

 i'. ao. ventral aorta. 



in the pharyngeal region, by paired cavities (Fig. 752, ccel., Fig. 751, 

 A, co., Fig. 753, 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. 753, right side) with a median 

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

 region it entirely surrounds the intestine, but is much reduced on 

 the right side, being displaced by the backward extension of the 

 atrium (Fig. 751, B, atr., Fig. 752, atr'.) ; on the left side a forward 

 extension of it surrounds the liver (Fig. 751, A, I.). Separate 

 cavities lie in the metapleures. 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. 754, v. ao, 

 Fig. 753, si.) ; it is contractile, and drives the blood forwards. 



VOL. II E 



