THE SNAIL 



381 



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

 condition of torpidit}- the body of the snail may be well protected 

 by one or several thin parchmentlike membranes called epiphragms 

 which are stretched across the shell aperture. When warm weather 

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



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

 available. Naturally their movements are slow^ed down in the winter 

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

 ments of Lymiiaea, Physa, or Helisoma 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 

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

 tion in retarding water loss. 



hesodermal 

 band 



npical or^an 

 Zye 



E^opha§ 



Head kidney. 

 Qtoc/st 



he^enchyme 



Stomach 



Pre oral 



ciliated 

 rin§ 



Blastocoef'z 



Anal vehicle 



Apical or^an 



Endoderm 

 Embryonic muf'CiC 



Prototrocb 



f'l&soderm 



Telatroch 



Fig. 147. — A, Trochophore larva of Eupomatus (a polychaete annelid), side 

 view. (After Shearer.) B, Veliger larva of Patella (a marine snail) frontal sec- 

 tion. (After Patten.) (Drawn by Joanne Moore.) 



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

 Helix aspersa, H. po7iiatia, and Polygyra roemeri have all been ob- 

 served to occupy as ''home" a definite place and go out from this 

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

 morning. 



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

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

 years 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 sqiimnosiim and the crustacean Gebia stellata. 



