116 How Animals Changed 



tains a dynamic steady state during which its internal fluids differ 

 in osmotic pressure and composition from those outside must con- 

 tinually do work (Adolph, 1925; Pantin, 1931; Hill, 1931); the 

 regulation is not merely diffusion but an active process carried on 

 by living tissues. 



Schlieper (1930) classifies animals as homoiosmotic, in which 

 the body fluids differ from the surrounding medium, and poikilos- 

 motic, in which they are essentially the same. Oxygen requirements 

 and Oo consumption increase in euryhaline invertebrates as the salt 

 concentration of the external medium decreases. "It is assumed 

 that this increased respiration is required for the work done against 

 an osmotic intake of water from the outside." In Australia Ed- 

 monds (1935) tested species of crabs from diverse habitats. "Five 

 species . . . have been found to be homoiosmotic in diluted sea 

 water." One species was poikilosmotic. Black (1948) studied the 

 adjustment of Fundulus to fresh water and found that it involved 

 changes in gas in the swim-bladder, chloride content of body fluids, 

 weight, and density. The gain in weight was temporary, due to 

 water absorption. After 10 days in fresh water 10^ ^ of CI was lost. 

 In fresh water the fish sank and regained buoyancy by secreting 

 gases into the swim-bladder. All these changes took place in 24 

 hours, but when a fish was replaced in sea water it regained its 

 normal density in 6 hours. 



Beadle (1934) has studied a flatworm (Gunda ulvae) which 

 lives where it is alternately subjected to water from streams and 

 from the ocean. This worm can live permanently in any concen- 

 tration of sea water down to 5%. It is able to maintain a relatively 

 constant internal state against a rapidly changing external en- 

 vironment. When it is transferred to dilute sea water there is an 

 initial flow of water through the integument into the parenchyma, 

 which causes swelling and a temporary lowering of activity. The 

 water is soon taken up by the gut, and forms intracellular vacuoles. 

 This process requires an expenditure of energy, as it is inhibited by 



