CHAPTER II 



LAND AND FRESH-WATER MOLLUSCA, THEIR HABITS AND 



GENERAL ECONOMY 



The majority of the Land Mollusca are probably more sensi- 

 tive than is usually believed. The humidity of the air must 

 affect the surface of their skin to a considerable extent. Every 

 one has noticed how the snails * come out ' on a damp evening, 

 especially after rain. As a rule, they wait till rain is over, 

 probably objecting to the patter of the drops upon their delicate 

 tentacles. Snails kept in captivity under a bell-glass are acutely 

 sensitive of a damp atmosphere, and will bestir themselves after 

 rain just as if they were in the open air. Certain Helices which 

 are accustomed to live in moist places, will find their way to 

 water, if removed from their usual haunts. A case is recorded^ 

 of a specimen of H. arhiistorum^ kept in a kitchen, which used 

 to find its way directly under the cold water tap, and appeared 

 to enjoy the luxury of a douche. How delicately the conditions 

 of life are balanced in some of these creatures is seen in the case 

 of Omalonyx^ a genus akin to Succinea^ which is found in Brazil 

 and the northern parts of South America. It lives creeping on 

 plants which overhang the margin of water, but perishes equally, 

 if placed in the water itself, or removed to a distance from it for 

 any length of time.^ 



Endurance of Heat and Cold. — The Mollusca are capable, 

 at least as far as some species are concerned, of enduring severe 

 extremes both of cold and heat. The most northern pulmonate 

 yet observed is a fresh-water species, Pliysa (^Ajylecta^ hypnoriim 

 L. This hardy mollusc, whose shell is so fragile as to need most 

 careful handling, has been noticed on the peninsula of Taimyr, 

 North Siberia, in 73° 30' N. lat., a region whose mean annual 

 1 T. Scott, Journ. of Conch, v. p. 230. 2 j. g. Gibbons, ibid. ii. p. 129. 



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