PIGMENTS. 747 



classes, usually in isolated members of groups. It consists 

 of a pigment, Haematin, united to a 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 Haematin under- 

 goes 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 Haematin, which only 

 occurs in disease in Vertebrates, seems to occur normally 

 in certain Invertebrates, in the shells of some Gasteropods, 

 the skin of star fishes, &c., 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 dis- 

 tribution 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 of course due to 

 its power of forming a loose combination with oxygen ; 

 it is, however, also capable of uniting with other gases, 

 as CO and CO 2 . 



Hamocyamn is found in many Crustacea, and in Molluscs. 

 In the oxidised state it is a colourless substance, but turns 

 blue when reduced. It is absent in the few Crustaceans 

 (Daphnia, &c.), 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. 



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 



