70 COMPARATIVE PHYSIOLOGY 



animal is put under water no bubbles escape and asph}^ia 

 results. 



Thus in insects there has been evolved — and in passing 

 we may note that analogous structures have appeared indepen- 

 dently in at least two other groups of Arthropods (isopoda 

 and arachnida) — a system by which oxygen is brought direct 

 to the tissues by a ramifying system of minute tubules which 

 penetrate even to the individual cells of the lining epithelium 

 of the gut. The evolution of this remarkable arrangement 

 is correlated with a very degenerate condition of the vascular 

 system. The efficiency of the tracheal system, which as we 

 have seen accounts for the greater part of the respiratory 

 exchange of these animals, must be extremely high when it is 

 remembered that insects in muscular activity surpass all other 

 invertebrates and many vertebrates also. It is interesting, how- 

 ever, to note that a limit is set to the efficiency of this device 

 by the size of the organism, since the internal surface of the 

 tracheal system cannot increase proportionately to the body- 

 weight. 



Respiratory Pigments.— The body fluids of many animals 

 are known to contain substances whose affinity for oxygen 

 enables them to take up far more of this gas than is contained 

 by serum or sea- water in physical solution. The most familiar 

 example of a respiratory pigment is provided by the substance, 

 haemoglobin, present in the erythrocytes of all craniata and in 

 the serum of some invertebrates, especially annelida. 



As is well known, haemoglobin on taking up oxygen assumes 

 a diiferent colour ; reduced haemoglobin is of a purple tint, 

 whereas oxy- haemoglobin is bright scarlet. The difference 

 is correlated with characteristic absorption spectra. Oxy- 

 haemoglobin has two absorption bands in the green ; reduced 

 haemoglobin has one which overlaps the space included by the 

 outer edges of the oxy-haemoglobin bands. Haemoglobin also 

 combines very readily with carbon monoxide to form carboxy- 

 haemoglobin, a much more stable compound than the oxygen 

 derivative. This also has two bands in the green ; it is not 

 of such a bright red colour. Oxy-haemoglobin can be reduced 

 to haemoglobin by exposure to a vacuum or neutral gas and 



