34 



CHORDATE ORGANIZATION 



II. 8- 



ant. card- 



d. cuv. 



>m 



d ao: 



post, card' 



d.ao. 



from which spring vessels carrying blood up the branchial arches. 

 At the base of each primary bar there is a little bulb, functioning as 



a branchial heart. From the gill bars 

 blood is collected into paired dorsal 

 aortae, which join behind the pha- 

 rynx. From the paired and median 

 aortae blood is carried to the system 

 of lacunae that supplies the tissues. 

 There are no true capillaries. From 

 the lacunae blood is collected into 

 veins, the most important of which 

 are the caudals, cardinals, and a 

 plexus on the gut. The cardinals are 

 a pair of vessels in the dorsal wall of 

 the coelom, and they collect blood 

 from the muscles and body wall. 

 They lead to the sinus venosus by 

 a pair of vessels, ductus Cuvieri, 

 which pass ventrally and across the 

 coelom to join the sinus venosus on 

 the floor of the gut. The caudal 

 veins join the plexus on the gut, 

 from which blood is collected by 

 a large subintestinal vein running 

 on to the liver; from here another 

 plexus leads to the sinus venosus. 



Contractions arise independently 

 in the sinus venosus, branchial 

 bulbs, subintestinal vein, and else- 

 where. The rhythms are very slow 

 (once in two minutes), irregular, 

 and apparently not coordinated by 

 any control system. 



The blood is colourless and is not 

 known to contain any respiratory 

 pigment. It contains no cells. Pre- 

 sumably the tension of dissolved 

 oxygen acquired by simple solution 

 is sufficient for the small energy needs of the animal, w r hich spends 

 most of its life at rest. It is by no means certain that any oxygenation 

 of the blood takes place in the gills. Orton has suggested that since 



'subuit 



^ 



Fig. 



i i. Diagram of the circulation of 

 amphioxus. 



aff.d. afferent vessel of diverticulum; ant. 

 card, anterior cardinal vein; br.a. bran- 

 chial arch; d.ao. dorsal aorta; d.cuv. ductus 

 Cuvieri; eff.d. efferent vessel of diver- 

 ticulum; post. card, posterior cardinal 

 vein; sin. sinus venosus; subint. subin- 

 testinal vein; v. a. ventral aorta. (After 

 Grobben and Zarnik.) 



