THE CONQUEST OF THE DRY LAND 203 
Molluscs, that have no hemoglobin, but some 
other blood-pigment (e.g. hemocyanin) not 
quite so good. Yet we may be sure that the se- 
cret of making hemoglobin was never lost. It 
was too good to lose. If hemoglobin was not 
always continued along the main line, where 
hemocyanin 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 larve of certain Harlequin-flies (Chiro- 
nomus) ; they are called ‘ blood-worms,” be- 
cause they are so red; the redness is due to 
hemoglobin, which few insects have; the 
hemoglobin is present in “ blood-worms,” be- 
cause they live in situations where oxygen is 
very scarce, where hemocyanin 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 larve to live in very foul 
water. 
The land animals’ new way of breathing, 
notably by means of internal surfaces, like the 
