398 



COLLEGE ZOOLOGY 



but is lined with an ectodermal epithelium and is really external 

 to the body, as has been proved by the study of its development. 

 Water which is carried into the pharynx by way of the mouth 

 passes through the gill-slits into the atrium and out of the atrio- 

 pore (Fig. 341, atp ; Fig. 343, atrp). The gill-slits, sometimes 

 as many as one hundred and eighty, are separated by gill-bars 

 (Fig. 342, p.b.); these are ciliated and supported by chitinous 

 rods. Respiration takes place as the water, driven by the cilia, 

 flows through the gill-slits. 



CIRCULATION. Amphioxus does not possess a heart. The 

 position of the principal blood-vessels and the direction of the 



cf.OLO 



FIG. 344. Diagram of the vascular system of Amphioxus. af.br. a, afferent 

 branchial arteries ; cp, intestinal capillaries ; d.ao, paired dorsal aortae ; 

 d.ao, 1 median dorsal aorta; ef.br.a, efferent branchial arteries; hep.port.v., 

 hepatic portal vein; hep.v, hepatic vein; int, intestine; Ir, liver; ph, pharynx; 

 s.int.v, subintestinal vein. (From Parker and Haswell.) 



blood flow are shown in Figure 344. The subintestinal vein 

 (s.int.v) collects blood loaded with nutriment from the intes- 

 tine (int) and carries it forward into the hepatic portal vein (hep. 

 port.v), and thence to the liver (Ir). The hepatic vein (hep.v) 

 leads from the liver to the ventral aorta (v.ao). Blood is forced 

 by the rhythmical contractions of the ventral aorta into the af- 

 ferent branchial arteries (af.br. a), which are situated in the gill- 

 bars, and then through the efferent branchial arteries (ef.br.a) 

 into the paired dorsal aortae (d.ao). It passes back into the 

 median dorsal aorta (d.ao 1 ) and finally byway of intestinal capil- 

 laries (cp) into the subintestinal vein (s.int.v). The blood is 



