334 Comparative Animal Physiology 



gills of some polychaetes, where it may facilitate transfer of COg;^*^ most 

 molluscs show little in blood, although there is much in the gills of the squid 

 and in gills and mantle of pelecypods.^-^ 



Among arthropods the gills of Limulus contain large amounts; those of 

 Homarus and Lihinia contain less, and the gills of Palinurus have none. 

 Similarly, in several fishes, much carbonic anhydrase is found in the gills. In 

 general it appears that carbonic anhydrase is most abundant in aquatic animals 

 in gills, and in higher terrestrial forms in blood cells. Carbonic anhydrase is 

 present in lower concentrations in other tissues. 



PIGMENTS AS BLOOD PROTEINS 



It was stated above that among some invertebrates function of the blood 

 pigments in oxygen transport is doubtful. A universal function of these pig- 

 ments, however, appears to be in buffering; they are particularly effective since 

 each protein molecule may have several acid-binding groups. Another function 

 is to provide blood colloid.^'^ In animals with a heart, and particularly in those 

 with both heart and exoskeleton, the hemolymph (blood) is under hydrostatic 

 pressure. These animals would tend to lose fluid, even though at osmotic 

 equilibrium with the sea, were it not for the proteins of their bloods. Their 

 protein concentrations are considerable— often up to 5 per cent. Experiments 

 correlating hydrostatic pressure with blood proteins in aquatic animals are 

 needed. 



CONCLUSIONS 



Certain generalizations can be made regarding the blood pigments. Phylo- 

 genetically blood pigments represent a labile set of characters. Chromogens 

 composed of a protein with a prosthetic group containing a metal have appeared 

 many times. Hemochromogens, being universally distributed in aerobic cells, 

 have been used independently by several groups of animals in blood pigments. 

 The hemoglobins may show slight variations in their porphyrins and great 

 variations in their proteins from species to species and at different stages in one 

 species. The molecules of blood pigments diff^er in size, large ones being free 

 in solution, smaller ones contained in corpuscles. They also differ in function. 



The functions of blood pigments in oxygen transport in the vertebrates are 

 clear; among invertebrates, only in the cephalopod molluscs is the pigment, 

 hemocyanin, as important in oxygen transport. In those others where at 

 normal tensions the pigment carries oxygen, the pigment (chlorocruorin in 

 the sabellids; hemoglobin in the earthworm. Nereis, and Tnhifex; hemocyanin 

 in Busycon^ carries only a fraction of the oxygen required. In others the pig- 

 ment (hemoglobin in Planorhis, Arenicola, Chironomus, Urechis, and Ascaris; 

 hemerythrin in Sipiincidus} functions to supply oxygen primarily at times of 

 physiological stress due to hypoxia. In still others the pigment (hemocyanin in 

 Limulus and crustaceans) seems to function principally as a buffer and pos- 

 sibly in maintenance of colloid osmotic pressure. 



Ecologically, many animals are limited in their range by their blood pig- 

 ments. Correlations between oxygen-combining power and amount of pigment 

 occur, as in mammals at high altitudes. Striking adaptations with respect to 

 the effects of COo and temperature on O2 dissociation are found among fishes 

 and in embryos. The decrease of oxygen affinity with added COo is consider- 



