RESPIRATION 173 



lower chordates. Reviews dealing with the comparative physiology of 

 respiratory pigments have been prepared by Redfield (129, 130), Florkin 

 (53), Prosser (127) and Eliassen (43). 



Occurrence and Chemical Characteristics of Respiratory Pigments in 

 Marine Animals 



The distribution of respiratory pigments in marine animals is shown in 

 Table 4.8. The principal categories are haemoglobin, chlorocruorin, 

 haemocyanin and haemerythrin. Several additional pigments, the func- 

 tional roles of which remain obscure, are also known. 



Haemoglobin is a reddish pigment containing an iron-porphyrin complex, 

 widely but sporadically distributed through the animal kingdom. It is 

 found in the blood of all vertebrates, with the exception of certain pelagic 

 transparent fish larvae (Leptocephalus) and certain antarctic teleosts of the 

 family Chaenichthyidae. It is the dominant respiratory pigment in anne- 

 lids (polychaetes, oligochaetes and echiuroids). Elsewhere it is distributed 

 in isolated instances among hemichordates, holothurians, phoronids, 

 arthropods, lamellibranchs and nemertines (137). 



In animals with closed circulatory systems haemoglobin occurs in 

 corpuscles (erythrocytes) or dissolved in the plasma. Where haemoglobin is 

 found in the coelomic fluid or haemolymph it is always enclosed in cor- 

 puscles. In certain terebellids (Terebella and Travisia) both conditions 

 coexist: the blood contains haemoglobin in solution, and the coelomic 

 fluid is provided with erythrocytes. Among invertebrates with closed 

 circulatory systems there is a general tendency for haemoglobin to be 

 dissolved in the plasma. Haemoglobin has also been recognized in other 

 tissues. As muscle haemoglobin (myoglobin) it occurs in heart muscle of 

 vertebrates; striated muscle of fish {Hippocampus) and homoiotherms; 

 pharyngeal and radular muscles of gastropods (Busycon); locomotory 

 muscles of Arem'cola, Potamilla and Urechis. It is also found in the nervous 

 system of worms {Urechis, Aphrodite, nemertines) and lamellibranchs 

 (Tivela). 



Haemoglobins are complexes made up of a protein globin in combina- 

 tion with a prosthetic haem group. The latter is a metalloporphyrin con- 

 taining ferrous iron combined with protoporphyrin (p. 477). The haem 

 component of haemoglobin is identical in all species but the protein 

 (globin) fraction shows specific differences. The oxygen-transporting 

 function of haemoglobin is due to haem, which combines loosely with 

 oxygen in the proportion 1 molecule of 2 to 1 atom of Fe (oxygenation). 

 More than one haem unit is combined with globin, the exact number vary- 

 ing with different species. The iron content of haemoglobins will differ 

 accordingly. When present inside corpuscles, haemoglobin has its own 

 chemical environment which may be of functional significance. Haemo- 

 globins dissolved in plasma have high molecular weights. The large mol- 

 ecular size will tend to retain the haemoglobin in the vessels but increases 

 the viscosity of the blood. The haemoglobin and erythrocyte contents of 



