xxv HAEMOGLOBIN 283 



ends blindly. The anterior parts of the commissural vessels 

 lie in the peristomium and have an oblique direction, one on 

 each side of the gullet. The whole of these vessels form a 

 single, closed vascular system, there being no communication 

 between them and any of the remaining cavities of the 

 body. 



The vascular system contains a fluid, the blood, which 

 varies in colour in the different species of Polygordius, being 

 either colourless, red, green, or yellow. In one species cor- 

 puscles (? leucocytes) have been found in it. 



The function of the blood has not been actually proved 

 in Polygordius, but is well known in other worms. In the 



j o * 



common earthworm, for instance, the blood is red, the colour 

 being due to the same pigment, haemoglobin, which occurs 

 in our own blood and in that of other vertebrate animals. 



Haemoglobin is a nitrogenous compound, containing, in 

 addition to carbon, hydrogen, nitrogen, oxygen, and sulphur, 

 a minute quantity of iron. It can be obtained pure in the 

 form of crystals which are soluble in water. Its most 

 striking and physiologically its most important property is 

 its power of entering into a loose chemical combination with 

 oxygen. If a solution of haemoglobin is brought into contact 

 with oxygen it acquires a bright scarlet colour, and the solu- 

 tion is then found to have a characteristic spectrum distin- 

 guished by two absorption-bands, one in the yellow, another 

 in the green. Loss of oxygen changes the colour from scarlet 

 to purple, and the spectrum then presents a single broad 

 absorption-band intermediate in position between the two of 

 the oxygenated solution. 



This property is of use in the following way. All parts 

 of the organism are constantly undergoing destructive meta- 

 bolism and giving off carbon dioxide : this gas is absorbed 

 by the blood, and at the same time the haemoglobin gives up 



