124 CEPHALOCHORDATA 



seen in Fig. 74, B, and in the region of the pharynx in Fig. 74, 

 A. Fig. 72 shows the distribution of the spaces more in detail 

 (see also Fig. 71). Beginning anteriorly, along the dorsal sur- 

 face of the pharynx and beneath the notochord run a pair of 

 dorsal coelomic canals, one at each side of the epipharyngeal 

 groove ; these give off ventral diverticula which pass down 

 the primary branchial bars of the pharyngeal wall and unite 

 ventrally in a median tube, the endostylar coeloni (see Fig. 72, ec]. 

 At the posterior end of the pharynx these dorsal and ventral 

 canals unite in a narrow coelomic space encircling the stomach, 

 inside the wall of the atrium, and sending an extension forwards 

 around the liver (Fig. 74, A, /). In the region of the intestine, 

 behind the atriopore, the coeiom is allowed to expand to its 

 primitive condition on the left-hand side (Fig. 74, B), but is 

 still reduced on the right side, where there is a prolongation of 

 the atrial cavity reaching nearly to the anus. All these coelomic 

 spaces are lined by a coelomic epithelium. 



The Blood System of Amphioxus, although as simple as that 

 of a Chaetopod worm, is undoubtedly laid down on the Vertebrate 

 plan even though there is no distinct heart and the vessels are 

 few and of simple structure. Capillary networks are formed in 

 some places, but the colourless blood also extends into many 

 lacunae or lymph spaces, such as those around the fin-rays and 

 in the metapleura. As in a typical lower Vertebrate, there is a 

 contractile ventral vessel (the ventral or branchial aorta, Fig. 77, 

 v.ao} running forwards under the alimentary canal to the pharynx, 

 and giving off on each side afferent branchial vessels, which pass 

 up the primary branchial bars and give off branches joining the 

 vessels in the secondary bars. These latter do not communicate 

 directly with the ventral aorta, but the vessels in all the branchial 

 bjtrs open dorsally by efferent branchial vessels into the paired 

 dorsal aortae (Fig. 77, cl.ao), which run backwards along the top 

 of the pharynx, one at each side of the epipharyngeal groove. 

 In the vessels of the branchial bars and their connectives the 

 blood is aerated by the current of water passing through the 

 gill-slits, and so reaches the dorsal aortae in a purified condition. 

 The right-hand dorsal aorta is continued forward further into 

 the snout than its fellow of the other side, and is dilated at 

 its extremity (Fig. 77). At the posterior end of the pharynx 

 the paired dorsal aortae unite to form the median dorsal aorta 



