THE SNAIL 



387 



of aquatic pulmonates. not only serves in the capacity of a gill or 

 lung but also may serve as a hydrostatic organ, thus enabling snails 

 to ascend to the surface by flotation. Such movements are prob- 

 ably made possible through contraction of the mantle walls, thus 

 decreasing or increasing the volume of air. Most of the marine 

 species arc gill breathers, and some, such as the sea slugs, have 

 external feather-like gills. 



Fig. 153. — Arrangement of teeth in the radula of a snail. 



-Hermaphroditic Duct 

 '' ^.O^ofesf-is 



,'i5emino/ Pecepfac/Q 

 ///nfestine 

 / 'Albumen G/anc/ 



Vas Defer 



Sac 



CIojtk 

 una 

 ivar^ C/anc/ 



7/}/s 



Anterior 

 /Tenfaclz 



—Pharynx 

 --Mouth 



^^^ "Cerehral Gono/ion 

 ^5afi>'ary Over 



Fig. 154.— Internal anatomy of Helix. Shell removed. 



Circulation 



The blood of the snail consists of a plasma which is usually color- 

 less, but in Helisoma, hemoglobin is dissolved in the plasma, thus 

 giving it a red color, and in Lymnaea and some species of Helix the 



