SURVEY OF INVERTEBRATES 97 



breathes air through its lung when the oxygen tension 

 of the water drops below a certain level. 



During the winter, escaping to a new habitat is ob- 

 viously impossible if the water is covered with ice, as 

 was noted by AlSterberg (1930). But, according to this 

 investigator's experiments on Limnaea stagnalis, the 

 aquatic snails support anaerobic conditions much better 

 at low temperatures. At 20° C none of the organisms 

 survived longer than 21/2 days, while they all were still 

 alive after seven days when kept anaerobically at 8 to 

 10° and 0°C. 



Worms and clams living in the tidal zone have been 

 shown to owe their survival to different mechanisms. 

 Worms, on the one hand, with the help of their respira- 

 tory pigments, store some oxygen in their blood and are 

 also able to extract oxygen from the water surrounding 

 them even if it contains only very little of this gas. 

 Clams, on the other hand, seem to switch over more 

 readily to a partially anaerobic metabolism. 



Of the numerous snails which live in the regions cov- 

 ered by the tide only Littorina neritoides and L. rudis 

 seem to have been investigated. Fischer, Duval and 

 Raffy (1933) state that these animals consume oxygen 

 when exposed to air, although they consume 5 or 6 times 

 less than when they are submerged in water. Tidal ani- 

 mals, in general, reduce muscular movements to a min- 

 imum when exposed to air and this may largely explain 

 the decreased oxygen consumption observed in Littorina. 

 We have, at the present stage of our knowledge, no rea- 

 son to assume a transfer to partial anaerobic life during 

 the periodic exposure to air that occurs in the life of 

 these animals. 



The less well developed anaerobic functions of snails 

 as compared to those of clams may also become apparent 

 in the reaction of certain organs to the lack of oxygen. 

 Yung (1888) found that the frequency of the heart beat 

 of Helix is reduced under anoxic conditions while Koch 



