40 



THE BIOLOGY OF MARINE ANIMALS 



It appears, therefore, that contractile vacuoles are not indispensable to 

 marine Protozoa, since they are absent in most rhizopods and some 

 ciliates, and they function at a much lower rate in marine peritrichs than 

 in comparable freshwater species. It may be that in marine forms they 

 serve to remove excess water taken in with the food and metabolic water, 

 and counteract the osmotic pressure of cellular colloids; the possibility of 

 ionic regulation by contractile vacuoles has also been raised. The rate of 

 vacuolar output in Podophrya (freshwater suctorian) increases tenfold dur- 



3-0 - 



\15 - 



§ 





TO 1-5 - 



10 

 80 60 40 20 100 80 60 40 10 



Concentration of sea water m medium (%) 



Fig. 2.8. Osmoregulation in Marine Peritrichs 



{Left) relation of rate of vacuolar output to concentration of sea water mZoothamnium 

 marinum (solid circle) and Cothurnia curvula (open circle). (Right) relation of body 

 volume to concentration of the medium in the presence and absence of cyanide in 

 C. curvula. (From Kitching, 1936, 1938.) 



ing feeding. This increased vacuolar activity reduces hydration, resulting 

 from ingestion of food, and reduces body volume (65, 67). 



In dilute media, such as that encountered by estuarine ciliates, vacuolar 

 output is increased. The cell membrane is considered to be semipermeable 

 and to resist the passage of salts. Consequently, when the organism is in a 

 hypotonic environment, water is drawn into the cell by osmosis and the 

 increased activity of the contractile vacuole serves to bale out water as it 

 pours into the cell. There are instances, however, of Protozoa lacking hard 

 coverings and contractile vacuoles, which tolerate transfer to fresh water 

 for some time, e.g. the marine variety of Actinophrys sol which has been 

 acclimatized to fresh water (64, 66). 



There are probably species differences in salt permeability and volume 

 regulation among different protozoans. In marine ciliates studied by 

 Kitching and others, swelling of the cytosome persists in dilute media, and 



