34 LIMN&IDsE. 



respiration is required, it is also provided with lamellar (leaf-like) 

 branchiae. 



Shell usually external and spiral, occasionally internal, and 

 sometimes altogether wanting. 



Some of the members of this order are inoperculate, while 

 others are provided with an operculum ; in the former case both 

 sexes are united in the same animal, but fertilization by another 

 individual is necessary ; in the latter case, i. e. when the animal 

 is operculate, each individual is male or female only. 



By far the greater number of the British Pulmo- 

 nobranchs are terrestrial. The aquatic kinds inhabit 

 slow rivers, ponds, ditches, &c., but they all require 

 atmospheric air from time to time, and frequently rise 

 to the surface to inhale it; some species leave the 

 water altogether for a considerable period, and often 

 travel some distance away from it. 



Most of the molluscs comprised in this order are 

 herbivorous, but some are also carnivorous, and those 

 of one genus at least (Testacella) are entirely so. 



In this country the Pulmonobranchs are repre- 

 sented by six families ; the members of five of them 

 are terrestrial, and those of the remaining one, with 

 which we have first to deal, are aquatic. 



FAMILY I. LIMN-ffilDJE. 



Body usually long and spiral, rarely (as in Ancylus) short and 

 hood-shaped ; mantle covering the front part ; tentacles 2, con- 

 tractile, more or less pointed at their tips ; eyes situated on the 

 inner side of the tentacles at their base, towards the front ; foot 

 distinct from the body, oval, adapted for crawling or floating ; 

 lingual dentition various. 



Shell spiral or hood-shaped. 



The Limnaeidae are comprised in the following four 

 genera : 



