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TEXTBOOK OF ZOOLOGY 



exposed habitat. Movements of most land snails appear to be co- 

 incident with moisture rather than darkness. Preceding prolonged 

 periods of cold, land snails may move to protected places, such as 

 beneath dead logs, dense mats of humus, crannies in or under 



Lymnoea 



buUmoides 



techelia 



Physa 

 bumerosa 



Tropicorbis 

 liebmanni 



Lymnaea . 

 stagnalis 



(Lymnaeidae) 



Physa 

 anatina 

 ( Fhysidoe) 



Menetus 

 dllatatus 



Lymnaiza 

 palusbris 



Ferrissia 

 excenthca 

 (Ancylidaej 



HelisoiriQ 



tnvolvis 



lentum 



(Planorbidac) 



Fig-. 130. — Some common fresh water pulmonate snails. 



rocks, and there begin their period of hibernation. During this 

 condition of torpidity the body of the snail may be well protected 

 by one or several thin parchmentlike membranes called epiphragmas 

 which are stretched across the shell aperture. When warm weather 

 arrives, the membranes are broken and the snail resumes its activities. 



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