PHYLUM MOLLUSCA 



239 



Water snails are active all four seasons, provided open water is 

 available. Naturally their movements are slowed down in the winter 

 due to cold, but when the pond or stream is frozen over, the move- 

 ments of Lymnaea, Physa, or Relisoma may be observed through the 

 ice. During periods of dry weather when ponds and creeks dry up, 

 snails embed themselves in moss and mud, and in this manner are 

 able sometimes to survive long periods of drouth. During this con- 

 dition epiphragms may be formed in certain species {Lymnaea 

 palustris), the same as in land snails; these structures probably func- 

 tion in retarding water loss. 



At least a few species of land snails possess a homing instinct. 

 Helix aspersa, H. pomatia, and Polygyra roemeri have all been ob- 

 serv^ed to occupy as "home" a definite place and go out from this 

 ' ' home ' ' on nocturnal foraging trips, then return by sunrise the next 



mornmg. 



humboldbiono 

 chisosensis 



Polyqyra 

 roemen 



bulimulus 

 dealbatus 

 liquabilis 



Rurnina 

 decoUata 



Fig. 131. — Common terrestrial snails. 



The life span seems to vary considerably in snails; some of the 

 aquatic genera, such as Lymnaea and Helisoma may live two to four 

 j^ears whereas some species of Helix may live to be six or eight 

 years old. 



Parasitism and commensalism are both exemplified by certain 

 species of snails. A commensalistic relationship exists between the 

 rare mollusk Lepton squamosum and the crustacean Gehia stellata. 

 The former feeds on secretions produced by the latter. A few species 

 of sponges, echinoderms, annelids, and mollusks are parasitized by 

 various species of mollusks. 



External Anatomy 



Shell. — The shell of the snail may be in the form of a low, broad, 

 or flattened spiral (Humholdtiana chisosensis and Polygyra roemeri), 

 or a long, tapering spire (Lymnaea stagnalis) ; on the other hand, 



