96 SURyEY OF INVERTEBRATES 



during rainy periods, their oxygen supply may be con- 

 siderably depleted. It should be emphasized, however, 

 that normally hibernating terrestrial snails do have ac- 

 cess to oxygen and it has been shown that they are able 

 to consume it. Even in such species as Helix, which 

 close their shells with a rather heavy calcareous epi- 

 phragm, the persistence of an oxygen consumption has 

 been demonstrated. The epiphragm itself, and even — 

 though to a much lesser degree — the shell proper are per- 

 meable to gases (Fischer, 1931). Both Schurmans-Stek- 

 hoven (1920) and Fischer (1931) found that the oxygen 

 consumption of hibernating snails is increased when the 

 epiphragm is removed. Of course, this does not neces- 

 sarily indicate the replacement of anaerobic by aerobic 

 respiration ; the increase in evaporation, or the small mus- 

 cular movements that may take place under these circum- 

 stances may increase the need for energy. 



It is not uncommon for aquatic snails, on the contrary, 

 to find themselves in surroundings very poor in oxygen 

 or even entirely deprived of oxygen. An important dif- 

 ference between snails and lamellibranchs is the fact that 

 the former are able to move rather rapidly from place 

 to place. When oxygen disappears during the summer 

 stagnation period, the snails simply wander to places 

 that are still aerated, a thing which clams cannot usually 

 do. From the standpoint of the survival of the species, 

 therefore, it matters little that the resistance of aquatic 

 gastropods to the complete lack of oxygen is not so well 

 developed. 



Furthermore, some aquatic snails have a sufficiently 

 powerful respiratory pigment in their blood to enable 

 them to secure significant amounts of oxygen even at low 

 tensions. This question has been investigated thorough- 

 ly only in the case of Planorhis (Leitch, 1916; Borden, 

 1931; Wolvekamp, 1932) whose respiratory pigment is 

 haemoglobin. This snail, it is true, is not purely aquatic 

 in its respiratory habits. It comes to the surface and 



