INTRODUCTION. xix 



in a sac or capsule, and vary both in form and 

 number. In ' British Conchology,' Introduction, 

 p. 28, it is stated that Frey " counted as many as two 

 hundred otolites of different sizes in one of the audi- 

 tory vessels (of which there were two) in an adult 

 snail." 



The mantle consists of a single flexible lobe 

 which covers the front part of the body and usually 

 forms a sort of collar round the head. The foot 

 consists of a muscular disc adapted for crawling or 

 sometimes for floating. The reproductive system of 

 the Cephalic molluscs is very remarkable : in some 

 cases the sexes are distinct, each animal being either 

 male or female only ; in others the sexes are united 

 in the same individual which, though capable of per- 

 forming the functions of both, is unable to fertilize 

 itself. In a few instances the animal which at its 

 birth is either male or female only, on arriving at 

 maturity finds itself to be both. 



A great number of Cephalic molluscs are marine, 

 others inhabit fresh water, many live entirely on land, 

 and some are amphibious ; this being the case, their 

 respiratory organs are, as might be expected, variously 

 constructed, according to their requirements. All the 

 British Cephalic molluscs which inhabit land or fresh 

 water belong to the Gasteropoda, and are comprised in 

 two of the many orders into which that class has 

 been divided, viz. Pectinibranchiata and Pulmono- 

 branckiata. 



The respiratory apparatus of the animals belonging 

 to the first of these orders consists of comb-like gills 

 which have a single, or more rarely, a double plume, 



b 2 



