752 COMPARA TIVE PHYSIOLOG Y. 



proteid ; the pigment contains iron in its molecule. In the higher 

 Vertebrates, haemoglobin is during life continually undergoing decom- 

 position. The iron is mostly retained within the body, and is 

 probably re-utilised in metabolism ; the proteid is probably also 

 utilised, while the iron -free hsematin undergoes chemical changes, 

 and is excreted as the pigments of bile and urine. In pathological 

 conditions haematin may be deposited in the tissues in different forms. 

 This deposition of pigments derived from hamratin, which only occurs 

 in disease in Vertebrates, is said to occur normally in certain In- 

 vertebrates, in the shells of some Gasteropods, the skin of star-fishes, 

 etc., apparently in some cases in forms in which haematin itself does 

 not occur. With regard to the distribution of haemoglobin, we must 

 note that the occurrence of the same pigment in widely separated forms 

 indicates similar physiological processes, but not necessarily a similar 

 function. Thus haemoglobin is said to occur in considerable quantity 

 in the perivisceral fluid of Holothurians, where we can hardly suppose 

 that its respiratory importance is very well marked. In fact, the wide 

 and irregular distribution of haemoglobin among Invertebrata forbids 

 the supposition that it can there possess the supreme importance which 

 it has in higher Vertebrates. 



The efficiency of haemoglobin is due to its power of forming a 

 loose combination with oxygen ; it is, however, also capable of uniting 

 with other gases, as CO and C0 2 . 



Hcemocyanin is found in many Crustacea, also in other Arthropods, 

 and in Molluscs. In the reduced state it is a colourless substance, but 

 turns blue when oxidised. It is absent in the few Crustaceans 

 (Dapknia, etc.) which contain haemoglobin, and is a true respiratory 

 pigment. It consists of a proteid united to copper, but in a few cases 

 it is said that the copper is replaced by iron. There is said to be more 

 difficulty in reducing haemocyanin than there is with haemoglobin. 

 The question as to the fate of the copper of hsemocyanin is one of 

 considerable interest, for it must be noted that, while iron is of supreme 

 importance in cell-life, apart altogether from haemoglobin, there is no 

 evidence that copper has similar importance. In an interesting paper 

 on poisonous green oysters, Professors Herdman and Boyce put forward 

 evidence to show that normally copper is continually being eliminated 

 from the body, and that its retention gives rise to pathological 

 conditions, and incidentally to a green coloration ; but there appear to 

 be many kinds of green oysters. 



Hcemerythrin occurs in the blood of Gephyreans ; it undergoes a 

 colour-change dependent on processes of oxidation and reduction. 



The number of pigments which we can definitely classify as respira- 

 tory, or as waste products resulting from the decomposition of such, is 

 veiy small ; in the great majority of cases we can say nothing as to 

 function. In some cases, however, we can point to the physical or 

 chemical conditions which favour the development of pigments. Thus 

 in some animals the pigments indicate the normal reaction of the 

 tissues. For example, those sea-anemones which contain peptic 

 ferments are red, those which contain tryptic, yellow or brown. Again, 

 light and absence of oxygen are necessary for the development of certain 

 of the black pigments ; the black pigment in a frog's skin disappears in 



