88 SURVEY OF INVERTEBRATES 



deficiencies or complete lack of oxygen often occur. But 

 one cannot, of course, conclude that the metabolism is 

 purely anaerobic in all these cases. 



Alsterberg (1930) has sho^^^l that Sphaerium corneum 

 reacts to the micro-stratification of oxygen by extending 

 its siphon, and that the poorer the surroundings become 

 in oxygen the farther out the siphon is projected, in other 

 words, this mollusk behaves, in principle, like Tuhifex. 



Such a behavior, which may be widespread among la- 

 mellibranchs, seems to be correlated with the resistance in 

 experimentally induced anaerobic conditions. Thus, ac- 

 cording to Moore (1931), the clam Syndesmya alba, 

 which normally keeps its siphon in contact with the 

 water overlying the bottom mud, never survived longer 

 than 3^2 days in the absence of oxygen, while Nucula 

 tenuis, which feeds at a depth of several inches below the 

 surface of oxygen-free mud, keeping no direct com- 

 munication with the overlying water, remained alive and 

 active during anaerobic periods of 5 to 17 days; Moore 

 even attributed their death after that time to poisoning 

 of the water by the animals' excreta rather than to as- 

 phyxiation. 



Certain fresh-water clams, especially Pisidium ida- 

 hoense, are not killed by periodic stagnation of the over- 

 lying water masses (Juday, 1908). They also show re- 

 markable endurance under experimental conditions (Ju- 

 day, 1908; Cole, 1921; compare Table 12). They re- 

 duce their energy requirements to a minimum by keep- 

 ing their shells closed and performing no movements. 

 The survival of lamellibranehs over periods of winter 

 anaerobiosis which are often of long duration, is prob- 

 ably due to the fact that in them, as in many other ani- 

 mals, the length of the survival time under anaerobic con- 

 ditions is inversely related to temperature. This is shown 

 by Collip's (1921), Fieri 's (1895) and Dotterweich and 

 Elssner's (1935) data recorded in Table 12. 



