591.1 



II. 



The Pigments of Animals. 



Part I. 



THE problems connected with the colours of animals have always 

 attracted considerable attention, both on account of their 

 intrinsic interest and, more recently, on account of their relation 

 to general theories. It is, perhaps, an unfortunate consequence of this 

 latter fact that the subject has been chiefly studied from the binomical 

 standpoint. Important as this aspect of the question undoubtedly is, 

 it has been of late so exclusively dwelt upon that it has been thought 

 better in this paper rather to consider recent work from the standpoint 

 of chemical physiology. 



As is well known, the colours of animals are due to one of two 

 causes, either to pigments deposited in the tissues, or to light-effects 

 produced by the structure of the tissues. Of the meaning to the 

 individual of the latter kind of colours we know practically nothing ; 

 but our knowledge of the pigments of animals is slowly but surely 

 increasing. 



We will take up, in the first case, the pigment haemoglobin, not 

 because of its importance in producing coloration, for this is rarely 

 marked, but because, at least in the higher vertebrates, it has been 

 carefully worked out from the physiological side. 



First, as to the distribution of haemoglobin. It occurs in the blood 

 of the craniate vertebrates with the exception of Amphioxus 1 and the 

 little fish Leptocephalus. In the striped muscles of vertebrates it is 

 widely but irregularly distributed, often in a single species being 

 invariably absent in some muscles, and invariably present in others. 

 Of this peculiarity the rabbit is, perhaps, the most familiar example, 

 but the common fowl affords 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 Luvants, 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 perivisceral fluid of some turbellarians, of Glycera, and of 

 Phoronis; in the haemolymph of Lumbricus, Tubifex, 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 nemerteans, and so on. 



1 Its absence in Amphioxus has, however, been denied. 



