THE CONQUEST OF THE DRY LAND 203 



Molluscs, that have no haemoglobin, but some 

 other blood-pigment (e.g. haemocyanin) not quite 

 so good. Yet we may be sure that the secret of 

 making haemoglobin was never lost. It was too 

 good to lose. If haemoglobin was not always 

 continued along the main line, where haemocy- 

 anin often took its place, it was continued on 

 side-lines of descent ; and all backboned animals 

 have red blood. A pretty case, illustrating the 

 value of the red-blood-pigment, is that of the 

 "blood- worms," which are sometimes to be found 

 in rain-water barrels and in stagnant pools, where 

 the oxygen in the water is very scarce. These 

 " blood-worms " are the aquatic larvae of certain 

 Harlequin-flies (Chironomus) ; they are called 

 " blood- worms," because they are so red ; the 

 redness is due to haemoglobin, which few insects 

 have ; the haemoglobin is present in " blood- 

 worms," because they live in situations where 

 oxygen is very scarce, where haemocyanin is 

 hardly good enough. More strictly, perhaps, 

 we should say that Harlequin-flies are insects 

 with red blood, and that this makes it 

 possible for their larvae to live in very foul 

 water. 



The land animals' new way of breathing 

 notably by means of internal surfaces, like the 



