98 NATURAL SCIENCE. Feb.. 



MacMunn claims to have found haematin in the tissues of many- 

 invertebrates, such as gasteropods, starfishes, and others. We 

 unfortunately know almost nothing of the history of haemoglobin in 

 the invertebrates in which it occurs ; but according to MacMunn 

 the pigment of the skin in Uraster rubens is due to haematoporphyrin, 

 probably derived from the haematin of the tissues. If pigments 

 derived from haemoglobin were found in invertebrates to any consider- 

 able extent, we should expect to find that they were less rapidly 

 eliminated than in most vertebrates, and so played a more important 

 part in coloration ; but as to this there is no direct evidence. 



As to haemoglobin, then, its primary physiological significance is 

 unknown. It is an unstable substance, by the retrogressive changes 

 of which certain pigments are produced. Pigments, therefore, 

 may arise as products of the katabolic change of a substance of 

 direct importance in the physiology of the individual. In this case 

 thay have no primary importance, but may acquire secondary 

 importance as colouring agents, e.g., in the case of birds' eggs. 



Another method by which animal coloration may arise, is the 

 direct introduction of pigment into the body by means of the food. 

 This is less uncommon than might have been expected. 



What might be called the simplest case is that of " green oysters," 

 fully described some years ago by Professor Ray Lankester (9), and 

 more recently studied by A. Chatin (1). As is well-known, the very 

 marked pigmentation in this case is produced by a green pigment 

 derived from a diatom taken in as food. The pigment is extremely 

 stable, and apparently undergoes no change in the alimentary canal, 

 and for some unknown reason is not eliminated with the excreta but 

 absorbed into the tissues and there deposited. The great interest of 

 this case in connection with theories of animal coloration is that no 

 one has suggested that the colour of the green oyster is of any use to 

 it. The coloration is a consequence of the nature of the food, and of 

 the incapacity of the oyster to completely eliminate or to digest the 

 pigment. Similarly, it is said that the colour of " red mullet " is due 

 to a pigment derived from the crustaceans of its food. In his book 

 on " Animal Coloration," Mr. Beddard gives some other instances. 



As to this subject of extrinsic pigments, the most detailed 

 information which we have is due to the observations of Mr. Poulton 

 (n, 12), who has recently made some careful experiments on the subject. 

 Mr. Poulton has chiefly worked at the colouring of caterpillars, and 

 has found that in some cases the coloration is due to two sets of 

 pigments : first, true or cuticular pigment, which is found in the 

 cuticle ; and secondly, adventitious pigment, found in the connective 

 tissues, and forming a background to the other pigment. Some years 

 ago, Mr. Poulton gave his reasons for supposing that this second 

 kind of pigment was derived from the chlorophyll of the food ; more 

 recently he has endeavoured to prove this experimentally. 



For the purpose of this experiment, Mr. Poulton obtained a 



