124 CEPHALOCHORDATA CHAP. 
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 ina median tube, the endostylar coelom (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, 7). 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.ado) 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 
bars open dorsally by efferent branchial vessels into the paired 
dorsal aortae (Fig. 77, d.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-shits, 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 
