34 



Guide to the Mollusca. 



tory siphon. They arc marine, and are found on rocks between 

 tide-marks, chiefly in tropical countries. 



The LimncH idae arc found only in fresh water. Most of them 

 occasionally rise to the surface to breathe, where they glide along 

 foot uppermost, at times suspending themselves by a glutinous 

 thread, after t ho. fashion of a spider. Limnaea stagnates is the 

 common Pond-Snail of Britain. 



The freshwater Limpets (Ancylidae) live attached to stones and 

 Leaves of plants. Like the Limnaeidae they feed on freshwater 

 algae, confervae, and decayed vegetable matter. 



Sub-order 2. Stylommatophora. 



With the exception of the cosmopolitan marine family 

 Onchidiidae these Pulmonates are terrestrial. 



Fig. 2.".. 



British Land-Snail (Helix pornaha). 

 a. eye-bearing tentacles ; b. lower or smaller tentacles. 



are 



True Snails (Helicidae, Bulimulidae, Stenogyridae, &c.) 

 nearly all protected by a spiral shell. They are almost exclusively 

 vegetable feeders, subsisting chiefly on leaves. Many of the 

 species are beautiful objects on account of the brilliancy of their 

 coloration, and some are remarkable, for the variation they 

 exhibit in this respect. Species of Helicidae are found in nearly 

 every part of the world and in all situations, from sea-level to 

 an altitude of 12,000 feet. They are fond of moisture, and in hot 

 and dry weather retire within their shells, remaining torpid until 

 the return of dew and rain. Helix pomatia (Case 1 1 9) is commonly 

 eaten in Austria, France, and Belgium. 



The eggs of Land-Snails vary in texture and size; they are 

 usually white, but in some instances yellow and pale green. 

 Those of some of the large South-American forms are as hard as 

 that of a hen, and more than an inch in length (Case E). 



Slugs (Case 106) are anatomically very like Snails, but possess 

 no external shell ; most of them, however, have a small internal 

 shelly plate, or a few calcareous granules hidden beneath the 



