XIX] CIRCULATORY SYSTEM 433 



mass of liver-tubes projecting into this vein has broken it up into a 

 network of capillary channels called the hepatic portal system. 

 In front of this, where the vein enters the sinus venosus, it 'is known 

 as the hepatic vein; behind, branches from the walls of the intestine 

 so overshadow the original ventral trunk that this, embedded between 

 the limbs of the spiral valve, appears as merely a small branch of 

 the composite trunk or portal vein. 



The blood from the muscles and kidneys, in a word, from the 

 dorsal and outer parts of the coelom, collects into two longitudinal 

 channels called the cardinal veins. These empty into the sinus 

 venosus by transverse trunks called ductus Cuvieri. These trans- 

 verse trunks divide the veins into anterior cardinals returning 

 blood from the head, and posterior cardinals returning it from 

 the rest of the body. In the tail the two posterior cardinals are 

 represented by the median caudal vein, which further forward splits 

 into two. Just as the course of the original sub-intestinal vein has 

 been obstructed by the growth of the liver, so that of the posterior 

 cardinal has been choked by the growth of the kidney tubes. The 

 blood from the tail and hind limbs is forced to filter amongst these in 

 a series of narrow channels called the renal-portal system. The part 

 of the vein in front of the kidney retains the name posterior cardinal : 

 the hinder part is called the renal-portal vein. A vessel running 

 along the inner side of each kidney and collecting the blood which 

 percolates through it from the renal-portal .vein is termed the sub- 

 cardinal vein. The vessels of the renal-portal system apparently 

 do not give off capillaries to the kidney tubules ; these latter receive 

 all their blood supply from branches of the dorsal aorta. 



The blood of Craniata has in addition to the ordinary amoebo- 

 cytes a much larger number of oval or round cells impregnated 

 with haemoglobin, called red blood-corpuscles. Haemoglobin 

 has been mentioned when describing the earthworm Lumbricus, in 

 which it is found diffused in the blood fluid. The great character- 

 istic of haemoglobin is its power of forming a bright red, unstable 

 compound with oxygen. This compound is formed in the respira- 

 tory organ and carried by the circulation to all parts of the 

 body. In the capillaries it is broken up and the oxygen absorbed 

 by the tissues. The haemoglobin having lost its oxygen changes in 

 colour, and the impure blood which leaves the tissues is in conse- 

 quence bluish. From the tissues the blood takes up carbon dioxide 

 which, like the oxygen, is conveyed in loose chemical combina- 

 tion, though with the sodium of the blood instead of with the 





