92 Comparative Animal Physiology 



tioned in Chapter 2. He found that a wide variety of aquatic animals actively 

 absorb certain ions from their medium. In order that electrical equilibrium be 

 maintained the electrolytes must be absorbed either ^s neutral molecules or, if 

 as ions, by exchange for another ion of the same sign. CI", for example, may 

 be exchanged for HCOs". Krogh assumed that ammonia and bicarbonate 

 are not taken up by any active processes, and he measured anion absorption 

 from NH4+ salts, and cation absorption from HCOs" solutions. A fresh- 

 water bivalve (Unio pictorum) can concentrate CI" as NaCl or can selectively 

 absorb CI" from CaClo. A frog also absorbs CI" with Na+ from NaCl against 

 a concentration gradient of 1:1000. The frog can selectively take up CI" 

 from NH4CI, CaCl2, and KCl; it also absorbs Br" but takes up I", NO3-, 

 and CNS' only by diffusion; Na+ is actively absorbed, but not K+, NH4+, 

 or Ca+ + . The crab, Eriocheir, actively absorbs CI", Br", and CNS", but not 

 NO3-, I-, or S04=; it absorbs Na+ and K+, bur not Ca+ + . In Crustacea 

 and fish the gills are permeable to salts in both directions; the skin is less so. ^^ 

 Very little is known of the membrane mechanism for selective absorption 

 and exclusion of ions, which is a problem in cellular permeability. Important 

 factors are ion mobility, membrane characteristics, and exchange products of 

 metabolism. An increasing amount of evidence is accumulating, particularly 

 from the use of radioactive tracers, that there occurs continually a two-way 

 flow of salts and water across cell membranes. Measurements of differences 

 in total concentration on the two sides of a membrane, therefore, give no 

 picture of the dynamic exchange. Continuous flow of ions into and out of 

 plant cells occurs; similar two-way passage across the mammalian intestinal wall 

 of Na+, Cl~, and water, with net balances favoring the blood, has been 

 demonstrated.^-"* Apparently salts cannot move without some accompanying 

 water movement. It is likely that the use of tracers will demonstrate much more 

 lively movement of salts into and out of aquatic animals than has been sus- 

 pected from chemical analyses of blood, urine, and medium. Active uptake 

 can be most easily demonstrated in fresh-water animals, but active absorption 

 by marine animals occurs (e.g. Carcinus^'-^^^ . In growing animals the net 

 effect of ionic exchange must favor accumulation. 



Undoubtedly salt replacement from food is important in most species. Krogh 

 was unable to demonstrate active absorption of ions by the eel; it must rely 

 entirely on food for its salts. 



Excretion. Active selective absorption of ions is necessary in adult animals 

 only because of continuous salt excretion. Conversely, a mechanism for the 

 baling out of unwanted ions is necessary because of their continuous penetra- 

 tion. In the preceding chapter it was shown that in fresh-water animals the 

 kidneys eliminate osmotic water. Yet kidneys certainly evolved in marine 

 animals, which are isotonic with their medium. The coelenterates and echino- 

 derms lack special excretory organs, and they show least ionic regulation. In 

 general, the most successful ion regulators are animals with excretory organs. 

 It is reasonable to assume that in annelids, molluscs, and arthropods, kidneys 

 arose and still function in ionic regulation; on migration to fresh water the 

 kidneys took over water regulation secondarily. In the fishes, on the other 

 hand, kidneys arose in connection with water regulation, and on migration to 

 the sea the kidneys failed to take over ionic regulation; hence extrarenal routes 

 were required. Whether salt is excreted extrarenally by invertebrate animals 



