42 



AMPHIOXOS. 



forwards along the floor of the pharynx : from the cardiac aorta 

 the blood passes along a series of vessels- in the gill-arches, becom - 

 ing aerated on the way, to the dorsal aortae, a pair of longitudinal 

 vessels (Figs. 12 and 13, R), lying just beneath the notochord : 



FIG. 13. Amphioxus lanceolatus. Transverse section through the hinder 

 part of the pharynx of an adult female, passing through the liver and the 

 ovaries. The boundary of the atrial cavity is indicated by a thick black 

 line. The section is taken at about the level of the reference line K in 

 Fig. 11. (From Marshall and Hurst.) 



A, skeleton of dorsal fin. B. spinal cord. C, rtotocliord. D, connective-tissue 

 sheath of notochord. E, cavity of pharynx. J 1 , epibranHiial groove. G, endostyle. 

 TT, atrial cavity. L, liver. M, dorsal ccelomic canal. M", branchial coelomic canal. 

 O, ccelomic space surrounding liver. OV, ovary, P, metapleural canal. R, left 

 dorsal aorta. S, cardiac aorta. T, hepatic veins. 



these unite behind the pharynx to form a single dorsal aorta, 

 from which branches supply the various parts of the body. 



The nervous system consists of a tube of nervous matter, 

 the spinal cord, which lies immediately above the notochord, 

 and extends almost the entire length of the body. It tapers 

 slightly at its anterior end, and more markedly behind. The 



