.142 AN ELEMENTARY TEXT-BOOK OF BIOLOGY. 



current. In this case the epibranchial groove, judging from its 

 contents, serves as a channel along which food passes back to the 

 gullet. 



The ciliated lining of the buccal cavity and pharynx sets up 

 currents which not only bring in food but also the oxygen 

 required in respiration. These currents continually stream into 

 the pharynx, through the gill-slits into the atrial cavity, and 

 out at the atriopore. The blood contained in the vessels of 

 the gill-arches is thus oxygenated and at the same time gets rid 

 of its carbon dioxide. The current emerging from the atriopore 

 .also serves to carry the sperms or ova, as the case may be, out of 

 ,the body. 



5. Circulatory Organs. These are in a degenerate condition. 

 There is no heart, but this is compensated for by the contractile 

 nature of some of the blood-vessels. A distinction can be drawn 

 between blood and lymph-systems, but these appear to com- 

 municate with one another and consequently contain the same 

 .circulatory fluid, which may be termed blood or lymph indiffer- 

 ently. It is colourless, and chiefly consists of coagulable plasma 

 in which a few amoeboid corpuscles are suspended. 



( 1 ) Blood System. A cardiac aorta runs along the floor of the 

 pharynx, giving off branches, the aortic arches, which traverse 

 the primary gill-arches, uniting above to form a dorsal artery 

 -running along each side of the epibranchial groove. These two 

 arteries are indirectly connected in front, while posteriorly they 

 form by their union a dorsal aorta, which runs below the noto- 

 chord. The aortic arch which is furthest forward on the right 

 side is larger than the rest and supplies the front end of the body. 

 A lateral artery runs along each side of the body, just within the 

 gonads. It is connected by transverse vessels with the correspond- 

 ing dorsal artery. 



The remaining important blood-vessels are the portal and hepatic 

 veins. The first of these run along the under side of the intestine, 

 passing to the liver and breaking up into capillaries from which 

 the hepatic veins arise. These run along the dorsal side of that 

 organ and unite at its origin to form the cardiac aorta. 



Course of the Circulation. This is only imperfectly known, 



but is probably as follows : Impure blood passes from the hepatic 



veins to the cardiac aorta, thence through the aortic arches (and 



connected vessels in the secondary gill-arches), where it is oxygen- 



