30 COLOUR IN NATURE chap. 



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, hzemoglobin 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 Pkoronis ; 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 



