108 RESPIRATION 



fin of the sea-horse, Hippocampus, of our coast is tinged 

 with haemoglobin, and so are the muscles of that active 

 organ in the vertebrates, the heart. In many molhiscs whose 

 jaws are constantly working haemoglobin is found in the 

 muscles of the pharynx, and curiously enough it is diffused 

 throughout the nervous sy^stem of a certain bristle- worm, 

 the sea-mouse Aphrodite, and in a certain Nemertine worm 

 whose nervous system stands out brilliantly crimson in 

 colour. 



Haemoglobin, like other respiratory substances, combines 

 loosely with oxygen, which it takes up from the air or the 

 water and readily parts with to the tissues of the body. In 

 the blood of man the red corpuscles exist in numbers which 

 no one but a German Chancellor of the Exchequer could 

 possibly cope with. There are 5,000,000 corpuscles in each 

 cubic millimetre of blood, and there are 5,000,000 cubic 

 millimetres of blood. So altogether we have inside us some 

 25,000,000,000,000 of the non-nucleated cells in the blood. 

 They are circular and bi-concave, with a diameter of seven 

 or eight //,. They vary slightly in size amongst the mammals, 

 being smallest in the deer and largest in the elephant. In 

 the camel they are bi-convex and oval — why, nobody has 

 ever been able to explain. In no mammals do the red 

 corpuscles have nuclei; but in all other vertebrates, fishes, 

 amphibia, reptiles and birds, they are oval, bi-convex, and 

 nucleated. They are in all these cases larger than those of 

 mammals, and the biggest of all are found in certain tailed 

 amphibia. 



In some of the marine worms the respiratory pigment is 

 known as chlorocruorine. It is green and gives a character- 

 istic colour to its owners. But it is chemically altered by 

 alcohol as is also haemoglobin. In the Crustacea haemoglobin 

 is replaced by a fluid containing coj^per instead of iron. This 

 is known as haemocyanin, and is colourless during life, but 

 acquires a plumbago-like tint when exposed to the air. Any- 

 one who has eaten a lobster will have seen this faint bluish 

 colour in the larger vessels. The same oxygen-fixing agent in 

 which copper largely takes the place of iron is common in 

 many mollusca. 



