46 Comparative Animal Physiology 



is entirely possible, then, that sufficient solute could be secreted to permit the 

 osmotic filling of the c. vacuoles. Similar calculations for marine and brackish 

 protozoans are reasonable if it is assumed that the vacuolar fluid contains salt 

 equivalent in concentration to the medium. 



Efficiency of Contractile Vacuoles. The efficiency of c. vacuoles in volume 

 and concentration regulation is not perfect. In cells such as marine eggs the 

 product of the osmotically active volume times the concentration is a constant, 

 K. In Amoeba mira the volume increases initially in dilute medium but in an 

 hour and a half adjustment has occurred so that the equilibrium volume is 

 constant in all dilutions. In Amoeba mira the product of the vacuolar output 

 (yu, ^/sec.) at equilibrium times the external concentration is exceedingly con- 

 stant; this indicates that the osmotic concentration of A. mira and its water 

 permeability are similar in sea water and in fresh water. ^"^^ 



When marine peritrichous ciliates are transferred to dilute sea water the 

 vacuolar output is initially high, then declines to a little above the output in 

 100 per cent sea water while the body volume remains elevated (Fig. 19). 

 When vacuolar output is multiplied by the osmolar concentration, K increases 

 by several times in going from 100 per cent to 20 per cent sea water; K also 

 increases, although by a smaller amount, when the body volume increase is 

 considered. ^^^' ^"^^ The vacuolar output increases out of proportion to the 

 decrease in external concentration. Other data ^^^ agree in showing an 

 increasing K with decreasing concentration down to approximately 30 per 

 cent sea water, but below this K decreased, probably because equilibrium was 

 not established. Also in the fresh-water Rhahdostyla hrevipes^^'^- ^^^ K is 

 greater in dilute than in concentrated media. In these ciliates, the relative 

 increase in water output in dilute solutions indicates an increase in rate of 

 entrance of water. In the ciliates volume changes persist and the relation 

 between excretion and concentration fails to be linear, whereas in Avioeba 

 m^ira volume regulation occurs and the relation between excretion and tonicity 

 is regular. 



The species differences in water and salt permeability, the striking differ- 

 ences in adaptability among the three species of Paramecium mentioned above, 

 the irregularity of pulsations of vacuoles of many marine forms in dilute media 

 and the differences in reactions in different solutes indicate that contractile 

 vacuoles are complex mechanisms. The process of filling may not be the same 

 in all Protozoa in all media. Many questions will be answered when direct 

 osmotic measurements of the vacuolar contents and of the cytoplasm are 

 possible. 



AN INVASION OF THE OCEANS FROM FRESH WATER; FISHES 



The history of fishes represents a migration from fresh to salt water. Fishes 

 as a group seem to have arisen in fresh water. During the Silurian and Devon- 

 ian periods numerous groups of fish— ostracoderms, elasmobranchs, dipnoans, 

 and ganoids— inhabited fresh water. Fossil records show that in the late Devon- 

 ian and early Carboniferous periods there were marine elasmobranchs but 

 these died out and new migrations occurred in the Jurassic and Cretaceous 

 periods. Bony fish have been in the ocean at most since the Cretaceous period. 

 Some modern fish (teleosts) have reinvaded fresh water, whereas others 

 (holosteans) have lived continuously in fresh water. Numerous migrations 



