3° 
COLOUR IN NATURE 
So far we have spoken of haemoglobin as the 
supremely important pigment of Vertebrates, as a 
pigment perpetually justified by its usefulness, but 
we must notice that it has a wide distribution in 
the animal kingdom and occurs under circumstances 
where usefulness is difficult to prove, although this 
has not prevented the assertion being widely made. 
In the striped muscles of Vertebrates, for example, 
it is widely but irregularly distributed, often in 
a single species being invariably absent in some 
muscles, and invariably present in others. Of a 
well-marked distinction between red and white flesh 
the rabbit is a familiar example ; the common fowl 
is another as well known to the physiologist as to the 
epicure. Other notable cases are those of the fish 
Hippocampus , where only the muscles of the dorsal 
fin are red, and of the rare fish Luvarus , where the 
difference between red and pale muscles is very well 
marked ; but it would be easy to multiply examples 
almost indefinitely. Among the unstriped muscles 
of Vertebrates, haemoglobin is said to be found only 
in the wall of the rectum. 
Among Invertebrates, haemoglobin shows the 
same peculiarities of distribution as in the muscles 
of the Vertebrates. Thus it is present in the peri¬ 
visceral fluid of some Turbellarians, of Glycera , and 
of Phoronis ; in the haemolymph of Lumbricus , Tubi- 
fex, and other Annelids ; in the muscles of the 
pharynx in Buccinum undatum , Littorina , and other 
Gasteropods ; in the sheath of the nerve-cord in 
Aphrodite aculeata; in the cephalic slits of Nemer- 
teans ; and so on. 
It is well known that in the case of the red 
