METAZOA 133 



this gas, however, would be quite insufficient for it to maintain the 

 metabolism of an active animal if the gas were carried in mere solution. 

 This deficiency is met, when necessary, by the presence in the blood 

 of respiratory pigments. These bodies are compounds of a protein 

 with a nitrogenous pigment which contains a metal. They are re- 

 lated to one another, to chlorophyll, and to the colourless substance 

 cytochrome which is very widely distributed in the protoplasm of 

 animals and plants, where it plays a part in the regulation of oxida- 

 tions. They form very labile addition compounds with oxygen, which 

 they can thus take up in the organs of respiration and carry to the 

 tissues, where they yield it up by dissociating under the lower oxygen 

 tension, undergoing at the same time a change in colour. The most 

 important of them are haemoglobin, which contains iron and is red, 

 chlorocruorin, also containing iron, which is green, and haemocyanin, 

 containing copper, which is blue when oxygenated. Haemoglobin is 

 present in Vertebrata, where it is carried in the "red corpuscles", 

 and sporadically in many invertebrates, as in the earthworm, where 

 it is in solution in the plasma. Chlorocruorin is found in solution in 

 the blood of various polychaete worms, haemocyanin in solution in 

 the blood of the higher Crustacea, the king-crab (Limulus), and various 

 molluscs. Both haemoglobin and haemocyanin are slightly different 

 compounds in different animals, and with these differences are as- 

 sociated differences in the pressure at which they take up or yield 

 oxygen. Broadly speaking, the blood pigments of animals which live 

 under conditions of low oxygen pressure take up the gas at a lower 

 pressure than those which live under high oxygen pressure. On the 

 other hand they do not maintain so high a pressure in the tissues. 

 Independently of such differences, the haemocyanins are less efficient 

 oxygen carriers than the haemoglobins. In tracheate arthropods, 

 where air is brought direct to the tissues by a system of tubes, there 

 are no blood pigments. 



The blood of the higher invertebrates contains in solution a con- 

 siderable amount of protein, of which the respiratory pigment, if 

 present, is only a part. This protein is comparable with the organic 

 ground substance of a skeletal tissue. It is not a food for the tissues 

 but by maintaining the osmotic pressure of the blood it is of im- 

 portance in regulating the distribution of water between that fluid 

 and the tissues, and, since proteins combine with both acids and 

 alkalies, it helps to neutralize excess of either of these. In vertebrates 

 some of this protein provides the material for clotting, by which loss 

 of blood or injury is prevented; but invertebrates, when they form a 

 clot, do so from material furnished by corpuscles. 



(b) The secondary body cavity or coelom is from the first completely 

 surrounded and separated from the blastocoele by the mesothelium, 



