XX INTRODUCTION. 



and are situated on the upper part of the head or on 

 either side of it, and covered by the mantle. 



The P ectinibranchs treated of in this volume, inhabit 

 fresh water only, with the exception of NeT-itina, 

 which occurs sometimes also in brackish or even salt 

 water. 



The organs of breathmg in molluscs which belong to 

 the second order (PtdmojtobrancJdata) consist chiefly 

 of a network of vessels seated within a sac formed by 

 a fold of the mantle, and are adapted for the respira- 

 tion of atmospheric air. The greater number of the 

 British P?ilmonobranchs live on the land (very few are 

 marine), but some inhabit fresh water, in which case 

 they are provided with additional branchial organs 

 which enable them to extract air from the water; but 

 as they require atmospheric air also, they frequently 

 rise to the surface to inhale it, or sometimes leave the 

 water altogether for a time. 



Jaw and Lingual Ribbon. — It has already been 

 stated that all the British land and fresh-water mol- 

 luscs which are provided with a head belong to the 

 Gasteropoda. The mouths of Gasteropodous molluscs 

 are furnished with arched jaws of a horny substance, 

 and frequently with a tongue-like organ armed with 

 teeth, which is called the lingual ribbon. 



The jaw varies in form ; in some cases it is more 

 strongly arched than in others ; sometimes it is 

 coarsely ribbed and has a notched margin, and in 

 other instances it is smooth and more or less promi- 

 nently beaked in front. 



The lingual ribbon is also very variable, in different 



