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Two sets of nerves are given off from the spinal cord, 

 namely, dorsal sensory nerves to the skin, and ventral 

 motor nerves to the myotomes ; the former have single 

 roots, the latter are multiple-rooted. The nerves of the 

 first two dorsal pairs ("cranial nerves") arise opposite 

 each other ; and there are no ventral nerves corresponding 

 with these. All the other dorsal and ventral pairs of nerves 

 show a segmental arrangement corresponding with the 

 myomeres or myotomes ; consequently the nerves of each 

 pair are not opposite on each side, but arise alternately. 



, \ 2. There is no heart, but the principal vessels are con- 

 * / tractile. Blood from the alimentary canal is collected in 

 an intestinal plexus (i.e., a network of connected vessels), 

 which forms the sub -intestinal vein ; it passes forward to 

 the " liver " where it is spread out as a plexus, and reformed 

 (as the hepatic vein). It is continued forward, below the 

 pharynx, as the ventral aorta, which is contractile and 

 gives off vertical branchial vessels which ascend the septa 

 between the gill-slits on each side (where the blood is 

 purified). Tne branchial vessels are connected, by short 

 cross vessels (in the synapticulae), with parallel vessels 

 (of the tongue-bars). These double branchial vessels pass 

 upward to the dorsal surface of the pharynx, and there 

 form at each side a longitudinal aorta. The two aortae are 

 continued forwards (the right one supplying the anterior 

 region with blood) ; and behind they unite (near the end 

 of the pharynx) to form a single dorsal aorta, which passes 

 backwards along the dorsal surface of the intestine. 



Give a General Account of the Body Cavity (Coelome) of 

 Amphioxus. 



The body cavity or coelome is well developed in the 

 posterior third of the body, around the sides and ventral 

 surface of the hind portion of the intestine. In front of this, 

 owing to the extension of the atrium, it is displaced and 

 greatly reduced. A forward portion of it surrounds the 

 " liver " ; and it further extends along each side of the 

 dorsal region of the pharynx, above the atrium, as two 

 dorsal coelomic canals (dorsal coelome), and along the 

 ventral region of the pharynx as a ventral coelomic canal 

 (endostylar coelome). The dorsal canals and the ventral 

 canal are connected by portions of coelome in the septa 

 between the gill-slits, which portions collectively form 

 the branchial coelomic canals of the two sides of the pharynx, 

 The gonad sacs arc also coelomic. 



