LIMN^lDiE. 77 



when the tide has retired. One species of Melampus is 

 found in brackish, but never in fresh, water. 



In this country, about three-fourths of the Pulmono- 

 branchs are terrestrial; the remaining fourth live in 

 fresh and shallow water, occasionally rising to the sur- 

 face or crawling out of their native element to renew 

 the supply of air. They are nearly all herbivorous ; but 

 the Testacella, or Shell- Slug, feeds exclusively on the 

 earthworm, and the Slugs and many kinds of Snail (both 

 terrestrial and aquatic) eat animal as well as vegetable 

 substances, and are occasionally cannibals. It has been 

 proposed to call the tentacles of the aquatic kinds by 

 another name (viz. vibracles), in consequence of their 

 being contractile, instead of retractile like the tentacles 

 or horns of land-snails, and also because they do not 

 bear the eyes on their tips or extremities, as in the last- 

 mentioned kinds. This distinction seems, however, to be 

 unnecessary ; and much confusion might result from such 

 an innovation, as the word ' tentacles ^ has been so long 

 and universally applied to the feelers of all the Gastero- 

 poda or univalve MoUusca. 



They may be conveniently divided according to their 

 different habitats ; and the freshwater kinds will be de- 

 scribed first, so as to complete this branch of the subject. 

 These are aU covered or protected by a shell, and are 

 comprised in the undermentioned family. 



Family LIMN^ID^. 



Body generally long and spirally coiled, but in one genus 

 short and hood-shaped : mantle covering the upper part in front: 

 head short : tentacles 2, contractile : eyes placed on the inner 

 base of the tentacles, a little towards the front : foot oval, used 

 for crawling or floating. 



Shell spiral, or hood-shaped. 



