TERRESTRIAL. 



The Slugs and Land-Snails^ which (as I before observed) 

 constitute about three-fourths of the British Pulmono- 

 branch Mollusca, may be conveniently divided into two 

 sections. The first section agrees in all essential particu- 

 lars^ except that of having retractile (instead of contrac- 

 tile) tentacles^ with the aquatic family oi Limnaida, which 

 have been above described. The second corresponds with 

 the Pectinibranch Mollusca in having separate sexes, 

 their eyes at the base of the tentacles, and univalve spiral 

 shells which are furnished with opercula ; and the main 

 point in which it differs from that great Order consists 

 in the organs of respiration, resulting from the nature 

 of their respective habitats. All the land Pulmono- 

 branclis are more slimy than their aquatic representa- 

 tives ; and they appear to be less inactive in their habits. 



The first section comprises four families, viz., — 



* 'Tentacles, almost in every case, 4 : eyes placed on the tips of 

 the upper, or single, pair : shell rudimentaiy, shield-like, 

 or complete and spiral, 



I. LiMACIDiE. 



II. Testacellid^. 

 III. Helicid^. 



** Tentacles 2, besides rudiments of a second or lower pair : eyes 

 placed at the internal base of the*developed pair : sliell 

 spiral, elongated. 



lY. Cakyciiiid^. 



