PHYLUM CHORDATA 



327 



glossus, is bolh a respiratory and a food current, the animal feeding 

 passively on the minute organisms in the surrounding water. 



There is a system of blood-vessels, but no heart. A contractile 

 median ventral vessel, the ventral aorta, runs forward in the ventral 

 wall of the pharynx, and gives off lateral branches, the afferent bran- 

 chial vessels, which pass upwards in the branchial lamelke. Efferent 

 branchial vessels receive the blood from the wall of the pharynx and 

 open dorsally into a pair of longitudinal vessels, the dorsal aorta. The 



Fig 20s — AmphlOXUS lanceolatUS. A, transverse section of the pharyngeal re- 

 gion a, dorsal aorta; b, atrium: c, notochord; co, coelom; e, endostyle; g, 

 eonad; kb, branchial lamella;; id, pharynx; /, liver; my, myomere; «, neph- 

 ndium- r, neuron or dorsal nerve tube; sn, spinal nerves; sp, gill-slits B, 

 transverse section of the intestinal region; atr, atrium; coel, coelom; d. ao, 

 dorsal aorta: int. intestine; myom, myomere: nek, notochord: niu, neuron; 

 s. int. v, subintestinal vein. (A, from Hertwig, after Lankester and Boven; B, 

 partly after Rolph.) 



latter join to form a median dorsal aorta, which runs backwards imme- 

 diately below the notochord and above the intestine. 



The principal organs of excretion are about ninety pairs of peculiarly 

 modified nephridia (Fig. 204, npK) situated above the pharynx and in 

 relation with the main ccelomic cavities. 



An excretory function has also been assigned to a single pair of 

 organs called the brown funnels (Fig. 204, br.f), also situated on the 

 dorsal aspect of the pharynx at its posterior end. 



