LIMN^IDiE (-WATER-SNAILS). 267 



sometimes present, having at its base the ear-sacs plus the " ear- 

 stones" or otoliths. The"radula" is always well developed: 

 this ribbon- like mass of teeth is composed of chitin (fig. 

 137). Some families breathe air (Pulmonata) ; the majority are 

 aquatic, chiefly marine. In those that breathe water we find 

 three types — (i) Those that have no special respiratory organs, 

 the blood simply being bathed by the water in the mantle 

 cavity; (ii) another type is seen in the "nudibranch. gastero- 

 pods," which have gills on their back ; and (iii) there are some 

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

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

 Snails {Helicidce) and Slugs (LimacidcB), &c., are hermaphrodite. 

 The young when first developed have always an embryonic 

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

 larva being " nautiloid " 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 Helicidce (Snails) and Limacidce (Slugs), both ter- 

 restrial in habits. A third family of economic importance are 

 the Limnceidce or Water-snails. 



LiMN^iD^, OR Water-snails. 



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

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



