LIMNiBID^ (AVATER-SNAILS). 297 



is sometimes present, having at its base the ear-sacs containing 

 the " ear-stones " or otoliths. The " radula " is always well 

 developed : this ribbon-like mass of teeth is composed of chitin 

 (fig. 170). Some families breathe air {Pulmonata) ; the majority 

 are aquatic, chiefly marine. In those that live in water we find 

 three types of respiration — (i) Where there are no special res- 

 piratory organs, the blood simply being bathed by the water in 

 the mantle cavity ; (ii) another type is seen in the " nudibranoh 

 gasteropods,'' which have gills on their back ; and (iii) those 

 which have gills in the mantle cavity {Haliotis or Ear-shells, 

 &c.) The sexes are for the most part distinct (dioecious) ; but 

 Snails [Hdicidce) and Slugs (Limacidce), &c., are hermaphrodite. 

 The young when first developed have always an embryonic 

 shell. Many marine species develop by a metamorphosis, — the 

 larva being " nautUoid " in form, with a ciliated expansion 

 called the " velum." Terrestrial forms have no meta- 

 morphosis, but develop direct. A young snail is much like a 

 fully developed snail, save for its smaller size and more trans- 

 parent shell. 



The Gasteropods that we are interested in all belong to the 

 group Pulmonata, in which a shell is generally present and 

 respiration is aerial, the deflected mantle forming a pulmonary 

 chamber. 



MoUusca injurious to our crops belong to the two families 

 known as Helicidoe (Snails) and Limacidce (Slugs), both ter- 

 restrial in habits. A third family of economic importance are 

 the LimnceidcB or Water-snails. 



LiMNiEIDiE, OR WaTBR-SNAILB. 



These aquatic ou semi-aquatic molluscs are the hosts of the 

 liver-flukes in various parts of the world. They are found at 

 all elevations, on mountain-sides as well as in damp fenny 

 countries. Water-snails are hermaphrodites, and frequent shal- 

 low and still waters and damp meadows ; many may often be 



