98 Comparative Animal Physiology 



fluids with the environment, whereas others regulate in varying degrees, the 

 concentrations of different elements changing in different proportions. 



The ratios of ions in the body fluids in an individual animal, then, are 

 determined in two ways: (1) by its genetic potentialities of ionic regulation, 

 and (2) by its immediate environment. Macallum's proposal that the ionic 

 ratios of an animal correspond with those of the ocean at the time of the closing 

 of its circulatory s^'stem is untenable, although adequate tests with vertebrates 

 have not been made. The principal mechanisms of ionic regulation are selec- 

 tive permeability, protein binding as in the Donnan equilibrium, active ionic 

 absorption from dilute media, and selective excretion. 



The availability of particular elements is rarely a limiting factor in animal 

 distribution. Calcium may determine abundance; fresh-water molluscs, cray- 

 fish, etc., are more abundant in limestone regions than in areas of low calcium. 

 Animals require sodium chloride, whereas higher plants are said to grow 

 without this salt, but sodium chloride is sufficiently widespread not to limit 

 terrestrial animals. Needham ^'"^ points out that the eggs of many marine 

 invertebrates are dependent on their medium for minerals. The only groups 

 which can colonize fresh water have the capacity of providing enough ash 

 within the egg. Dependence on the environment for salts may, then, be more 

 critical in embryonic than in adult life. 



Ecological limitation by trace elements has hardly been investigated. Recent- 

 ly certain sporadic diseases of domestic animals have been correlated with lack 

 of specific elements such as iodine, fluorine, manganese, and cobalt.^-" Con- 

 versely, toxic levels of certain elements may be locally important. It is likely 

 that deficiencies or excesses may operate naturally in limiting fresh-water and 

 terrestrial animals. 



REFERENCES 



1. Adolph, E. F., /. Cell. & Comp. Physiol. 9:117-135 (1937). Osmotic relations 

 in Phascolosoma. 



2. Amberson, W. R., et al., Am. ]. Physiol. 122:224-235 (1938). Tissue and 

 plasma chloride. 



3. Babkin, B. p., et al., Contr. Canad. Biol. & Fish. N. S. 8:209-219 (1933). 

 Saline for Raja. 



4. Bahl, K. N., Biol. Rev. 22: 109-147 (1947). Ionic composition, body fluids, 

 earthworr.is. 



5. Baldwin, E., and Moyle, V., ). Exper. Biol. 23:277-291 (1947). Physiological 

 saline lor Ascaris. 



6. Baku, P., MacLeod's Physiology in Modern Medicine. Ch. 82. (1941) p. 1093- 

 1198, the Kidney by M. I. Gregerson. 



7. Beadle, L. C, J. Exper. BioL 16:346-362(1939). Chloride regulation, mosquito 

 larvae. 



8. Beadle, L. C, and Cragg, J. B., /. Exper. Biol. 17:153-163 (1940). Ionic 

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9. Bear, R. S., and Schmitt, F. O., /. Cell. & Comp. Physiol. 14:205-215 (1939). 

 Electrolytes in blood and nerve of squid. 



10. Berger, E., Pflug. Arch. gcs. Physiol. 228:790-807 (1931). Blood salts of fresh- 

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11. Berger, E., and Bethe, A., Pfliig. Arch. ges. Physiol. 228:769-789 (1931). 

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12. Bernard, A., and Bonnet, V., C. R. Soc. Biol. Paris 103:1119 (1930). Sahne 

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