38 Comparative Animal Physiology 



The salts which are lost in the hypotonic urine of worms in water must be 

 replaced. Some salts come from ingested food. Earthworms actively absorb 

 chloride from very dilute salt solutions. ^^'- Leeches which have lost some of 

 their body salts by living in distilled water for about two weeks absorb from 

 0.01 Ringer solution as much as 0.48 mM salt per gm. body weight during 

 the first half hour of immersion. ^^^ 



ADAPTATION TO MARINE, FRESH-WATER, AND 

 PARASITIC LIFE (PROTOZOA) 



Protozoa are a diverse group of animals which are so small that direct 

 measurement of the osmotic concentration of their cytoplasm and their excre- 

 tion is difficult. Indirect evidence indicates, however, that many marine and 

 parasitic Protozoa have no osmotic problem, that is, they are isotonic with their 

 medium. All fresh-water Protozoa have a distinct osmotic problem in that 

 they are hypertonic to their medium. Some species can readily withstand 

 transfer to extreme concentrations of the medium,, from distilled water to 

 concentrated sea water; others are extremely limited in their osmotic tolerance. 

 It has been variously suggested that the contractile vacuoles of Protozoa func- 

 tion in excretion of water, nitrogenous wastes, CO2, and salts. 



Structure of Contractile Vacuoles. Some Protozoa have one contractile 

 vacuole, some have two, and others have several. In some marine species con- 

 tractile vacuoles are absent in sea water but they appear when the animals 

 are transferred to fresh water. Usually a contractile vacuole is formed when 

 numerous small vacuoles or vesicles fuse; these may arise in a given area or 

 in various parts of the cell. In some species, particularly among ciliates, there 

 are canals of various shapes which fill and empty into the contractile vacuole. 

 During the period of filling, or diastole, the viscosity of the cytoplasm in the 

 vicinity of the contractile vacuole is low, while at the time of emptying, or 

 systole, viscosity is high. 



The contractile vacuole membrane has many of the properties of cell mem- 

 branes. Several investigators^^''' ^^^ have succeeded in keeping vacuoles 

 intact after removal from the cell. The membrane resembles the plasma mem- 

 brane in holding back solutes such as some dyes. The vacuole wall stains with 

 neutral red injected into an Amoeba and when first punctured the vacuole 

 wall shows a wrinkled surface. ^^' Morita and Chambers ^^^ stained Ainoel7a 

 duhia yellow with methyl red; injected hydrochloric acid then caused the 

 cytoplasm to turn red while the vacuole remained yellow. 



The emptying of contractile vacuoles is apparently a gelation-solation 

 phenornenon. A gelated region is needed around the vacuole before discharge 

 occurs in Amoeba dubia. ^^^ In Euplotes local solation seems to occur at the 

 point of discharge, although behind this point there may be gel. -"' 



Volume Regulation. Volume regulation has been examined in only a few 

 species and these show extreme differences. The volumes of Amoeba pro- 

 teus ^"^^ and of Pelomyxa carolinensis '^'•' have been measured by putting the 

 specimens into special capillary tubes. The volumes of Amoeba mira and of 

 Amoeba lacerta have been calculated from their diameters, since they assume 

 a spherical form when agitated.'"^' ''^•"' Volumes of three species of peri- 

 trichous ciliates, Rhabdostyla brevipes, Zoothamniinn marinum, and Cothiir- 

 nia curvxda, have been calculated from measurements of several dimen- 



