HAEMOGLOBIN 107 



intermediary; but in most animals it passes tlirounh the 

 membrane of the gill or lung into a fluid. This fluid -in most 

 cases blood — has a red substance in it which readily unites 

 with the oxygen in the breathing organ and readily parts with 

 it to a tissue or cell which is oxygen-hungry. The eonunoncst 

 of these substances, haemoglobin, always contains iron. It is 

 found throughout the vertebrates and in some of the in- 

 vertebrates, especially those that are at a disadvantage in the 

 competition for oxygen. 



As a rule haemoglobin is confined to certain cells called 

 blood-corpuscles, which are in most vertebrates nucleated Init 

 in the Mammalia have no nucleus. 



The distribution of haemoglobin is very peculiar. It occurs 

 with one or two exceptions in special red corpuscles in the 

 blood of all the Vertebrata, and it is found in corpuscles in 

 the body fluid of certain bristle- worms, Chaetopoda. It also 

 occurs in a species of Solen and also in Area, its presence in 

 the former being associated with the mollusc's very active 

 life as a borer. And again it is not uncommonly found where 

 strong muscular action is required: thus it occurs in the 

 muscles of the odontophore of many Gastropods, snails, etc. 

 Wherever increased facilities for respiration are required, 

 there haemoglobin may turn up to assist the organs to carry 

 on their work. Many other invertebrates have haemoglobin 

 dissolved in the fluid of the blood, the pla,wia, and here it is 

 not associated with special corpuscles. This is true of the 

 earthworm and the medicinal leech and one other genus of 

 leech. The same also is true of the larvae of a certain gnat, 

 Chironomus, which lives in foul water, and in that of the gut 

 of a common horse-fly, which lives in the alimentary canal of 

 the horse, of certain primitive Crustacea including Apus and 

 the ordinary water-flea, Daphnia, and of a water-snail, 

 Planorhis. 



Haemoglobin is diffused in the musciflar tissues of manunals 

 and of birds, and it is usually most a])parent in very active 

 animals like the hare, or in very active muscles like those 

 used in flight by the black-cock. In both cases it gives a dark 

 colour to the flesh. In reptiles it is confined to certain muscles, 

 probably to those that are most active. The flickering dorsal 



