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 se- 

 cret of making haemoglobin was never lost. It 

 was too good to lose. If haemoglobin was not 

 always continued along the main line, where 

 haemocyanin often took its place, it was con- 

 tinued on side-lines of descent; and all back- 

 boned animals have red blood. A pretty case, 

 illustrating the value of the red-blood-pigment, 

 is that of the " blood-worms," which are some- 

 times 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 ( Chiro- 

 nomus) ; they are called "blood-worms," be- 

 cause they are so red; the redness is due to 

 haemoglobin, which few insects have; the 

 haemoglobin is present in "blood-worms," be- 

 cause 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 pos- 

 sible for their larvae to live in very foul 

 water. 



The land animals' new way of breathing, 

 notably by means of internal surfaces, like the 



