. The biology of marine animals. Marine animals; Physiology, Comparative. 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 rai


. The biology of marine animals. Marine animals; Physiology, Comparative. 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 - - - hi £/ v „ _ rV /"" N _ w / v/ / / /£ / / J> //& // & // „\> J® III, 10 80 60 40 20 0 100 80 60 40 10 0 Concentration of sea water m medium (%) Fig. 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, the marine variety of Actinophrys sol which has been acclimatized to fresh water (64, 66). T


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