332 VII. COMPARATIVE BIOCHEMISTRY OF HEMOGLOBINS 



of the problem have been reviewed elsewhere (119J39,410,1853J 

 3080), but the position is still unsatisfactory. We have to reckon 

 with much greater changes, in both the micro- and macroenvironment 

 of the organism. Both intracellular and extracellular hemoglobin is 

 found in the blood of the annelids. In other phyla, the pigment is 

 found free in the circulation as well as in a variety of other cells, for 

 instance in the tracheal cells of Gastrophilus larvae, in the adductor 

 muscle of the whelk, and in the body wall of Ascaris — to name three. 

 The most important physiologic differences in the macroenviron- 

 ment of the pigment are those related to the different respiratory 

 systems in the lower phyla: as examples we cite the specialized epi- 

 thelium in the gills of fish and certain other aquatic animals, tracheal 

 respiration in insects, and finally, transfer of oxygen and carbon 

 dioxide across unspecialized epithelium. The environment of the 

 whole organism may be aquatic or aerial and in some species may 

 be either. Some parasitic species may spend some portion of their 

 life cycle in the moist, if not aqueous, environment of the host's 

 intestine. Within these environmental ranges, we may find a variety 

 of temperatures and partial pressures of oxygen and carbon dioxide. 

 While in some cases the environment of a single species may remain 

 relatively constant, in others the species must adapt itself to regular 

 or irregular changes in one or other of the above-mentioned factors. 

 W'e deal in detail only with certain aspects of these problems, since 

 detailed discussion is outside the scope of this work. 



11.2. Adaptation to Low Pressure of Oxygen 



Table IX, taken from Carter (^10), gives data for a number of 

 oxygen carriers. The loading tension is taken as that giving 95% 

 saturation and the unloading tension, 50% saturation. It can be 

 seen that adaptation has taken place to the lower partial pressure of 

 oxygen in marine environments, being reflected by the much higher 

 affinity for oxygen found in the carriers. 



The simplest form of adaptation is probably the production of a 

 greater amount of pigment. Among mammals thi^ is a normal physio- 

 logic response (Chapter XIII). Baldwin summarizes the oxygen 

 capacities in the bloods of a number of species (119, p. 79). The 

 values of those invertebrates which contain hematin oxygen carriers 

 range from 1.5 volume per cent for the extracellular erythrocruorin 

 in molluscs to 9.0 for the chlorocruorin of annelids. Fish, amphibia, 

 and reptiles are found to have lower oxygen capacities than birds or 



