98 SURVEY OF INVERTEBRATES 



(1917) observed no marked change in the case of Ano- 

 doufa. There seems to be some doubt, however, whether 

 the slowing of the heart beat of Ildix is directly due to 

 lack of oxygen. Ysseling (1930), in experiments of much 

 shorter duration than those of Yung,- found that the 

 frequency of the heart beat depends only indirectly on 

 the oxygen tension. The decisive point, according to him, 

 is whether the lung orifice is kept open or closed. If it is 

 open, the heart beat is almost normal in a nitrogen at- 

 mosphere containing 2 per cent oxygen; if it is closed, 

 the frequency of the heart pulsations decreases and the 

 organ may cease to beat entirely for varying lengths of 

 time. If Helix is transferred from air to an atmosphere 

 poor in oxygen (below about 9 per cent), its respiratory 

 movements become gasping, i.e., the lung orifice is alter- 

 nately closed and opened in rapid sequence (Ysseling, 

 Lc). 



Earlier observations on the respiratory movements of 

 snails in oxygen-poor or oxygen-free surroundings were 

 reported by Schurmans-Stekhoven (1920). He found a 

 tendency on the part of the animals under these condi- 

 tions to keep the lung orifice closed. This tendency, ac- 

 cording to him, leads to a preservation of the oxygen 

 that was originally present in the lungs, since it then 

 does not become diluted with the inert gas as it would 

 be if an open communication existed between the lungs 

 and the surroundings. 



3. CEPHALOPODS 



Cephalopods have very poorly developed anaerobic 

 functions and it is one of their characteristic ecological 

 features that they are never found in environments which 

 are really poor in oxygen. 



Redfield and Goodkind (1929) introduced squids {Loli- 

 go pealii) into aquaria of 5 to 8 liters capacity filled with 

 water rich in oxygen and overlayed with V^ inch of par- 



