34 The Field N aturalist' s Quarterly Feb. 



portions of the pond. It is said that the almost amphibious 

 Succinea elegans hibernates in the mud as well as under 

 stones and amongst rubbish on land. Whether any of the 

 species of LimncEa deliberately leave the water to hibernate 

 I do not know, but the sub-terrestrial L. truncatula I have 

 found in winter alive in the soil some distance above the 

 level of the water. After all, it should be remembered that 

 aquatic species, though exposed for long periods to quite a 

 low temperature, are not subjected to those intense frosts 

 which the terrestrial forms have to combat. 



A large number of our fresh-water forms usually obtain the 

 oxygen necessary for their existence by coming to the surface 

 and taking in atmospheric air. A good deal of question has 

 been raised at one time or another as to how such species 

 survive when the free surface of the water is covered, it may 

 be for weeks together, with a thick and impermeable coating 

 of ice. But there are two considerations which seem to 

 remove most of the difficulties in the matter. In the first 

 place, the degree of metabolism, and consequently the 

 amount of oxygen required, is very much reduced with a 

 reduction of external temperature. It has been shown, 

 for example, that growth does not take place in L. stag}ialis 

 unless the water be almost warm. It is also known that 

 the number and duration of the excursions which the flat 

 pond-snail, Planorbis, takes to the surface to fill its pul- 

 monary sac, vary directly with the temperature. As a 

 reverse experiment, it is easy to demonstrate that in hot 

 summer weather L. peregra will not live more than four 

 or five days if kept in a small body of water and prevented 

 from reaching a free surface. In the second place, there is 

 no doubt that pulmonate aquatic mollusca can under some 

 circumstances obtain an efficient supply of oxygen from the 

 gas dissolved in the water through the walls of their air- 

 sacs, and also most probably through their general cutaneous 

 surface. Such a mode of respiration seems to be the normal 

 habit of life with the abyssmal Limncea from the deep Swiss 

 lakes ; and it would indeed be surprising if we found that 

 the pulmonary sac in these snails can alone conduct a 

 proper exchange of gases, when animals as high in the 

 vertebrate scale as Amphibia (and with a lung far more 



