94 RESPIRATORY MECHANISMS 



perhaps with the exception of the Cyclostomata. In Myxine 

 Svedberg found only 2. Among the invertebrates the larvae 

 of Chironomus have haemoglobins of 1 unit, certain polychaete 

 worms of 2, but the rest have very large molecules with from 

 18 to 144 units in each. Each unit can combine with 1 mole- 

 cule of oxygen or of carbon monoxide. 



The occurrence of haemoglobins in the animal kingdom is 

 very puzzling, and it is necessary to assume that the substance 

 has been evolved independently in many different animal 

 forms, a conception which became easier to entertain when it 

 was shown by Keilin (1925) that derivatives of haematin are 

 very widely distributed in animals and present in almost every 

 cell. Pantin (1932) mentions the case of two nearly related 

 species of Holothuria (Cucumaria elongata and saxicola) of which 

 one living in mud possesses haemoglobin corpuscles while the 

 other does not. He stresses the point that Hb must have been 

 evolved in one step to be at all useful. Outside the verte- 

 brates, in which haemoglobin is the sole and universal oxygen 

 carrier (lacking only in Amphioxus and in the planktonic eel- 

 larvae, Leptocephali) , it is found in a number of Annelida and 

 Mollusca, in several insects and in single representatives of 

 other groups. 



In most of the Invertebrata haemoglobin is dissolved in the 

 blood plasma, but in all vertebrates and in some invertebrates 

 it is confined within corpuscles. Haemoglobin in solution can 

 exist only in rather low concentration without causing a high 

 degree of viscosity and, in case of the smaller Hb-molecules, a 

 high colloid osmotic pressure. Inside corpuscles the haemo- 

 globin has its own chemical environment which may be of 

 some functional significance (Barcroft, 1922). 



The respiratory characteristics of blood containing haemo- 

 globin (or any other respiratory pigment) can be expressed 

 by (1) the oxygen capacity, and (2) the dissociation curve. 

 The oxygen capacity is the maximum amount of oxygen in 

 volumes per cent with which the blood will combine. The 

 oxygen capacity of the blood in most invertebrates is quite low, 

 ranging from 1 to 2 vols % in most molluscs, from 3 to 10 in 



