XX11 INTRODUCTION. 



shell-gland, the edges of which secrete and form the shell. 

 Between the mantle and the rest of the body is a cavity, the 

 pallial chamber, the seat of the respiratory organs, which may 

 consist of gills (branchiae), or, as in the Pulmonates, of a sort of 

 lung formed by a network of blood-vessels developed on the roof. 

 The pallial margin forms a complete zone with only a small 

 aperture for respiratory and excretory purposes, and thus the 

 mantle becomes divided into lobes, an anterior left and a posterior 

 right, the dorsal lobes being below them. 



Gasteropods with conical or spiral shells, with gills placed in 

 front of the heart, and sexes distinct that is to say, nearly all the 

 marine and a large series of terrestrial genera belong to the order 

 Prosobranchia. 



In the Pulmonata, with which this volume deals, the mantle is 

 large as well as the unsymmetrically coiled visceral sac, in which 

 the intestines, liver, and part of the genital organs lie. The 

 mantle-cavity lies on the fore part of the sac, and the anus (a) 

 opens at its margin, generally on the right side. Thus in all the 

 ordinary Pulmonata the end of the intestine is twisted from its 

 primitive position at the hinder end forwards to the right dorsal 

 side of the body. 



In the cephalic area all the organs of sense are situated, the 

 common generative aperture lying on the right side at a varying 

 distance behind the right eye-tentacle. The head or prosoma is 

 symmetrical, bearing on the upper side two cylindrical, tapering, 

 hollow, reversible tentacles, with swollen or bulbous tips carrying 

 the eyes, also two much shorter below them, the oral ; both pairs 

 are contractile within the body-cavity. On the anterior ventral 

 side is the mouth, with the more or less solid jaw above and visible 

 externally. The radula with its teeth below is not usually seen 

 unless when protruded. The mouth is quite distinct from the 

 respiratory system and is used for feeding only. The upper lip 

 has a ring of rounded tubercles on its edge ; the lower lip is simpler 

 and more fleshy, but in certain carnivorous forms the corners are 

 produced into lappets. 



The pedal area consists of the muscular foot or podium, and is 

 characteristic of the Land-Mollusca and all Gasteropods. It varies 

 greatly in size and form in different genera. "When fully extended 

 the portion behind with respect to the shell or visceral sac may be 

 very short, that in front very long, or they may be equal in length. 

 It may be either narrow or broad and flat, almost circular, 

 possessing thus great power of adhesion to the surface on which 

 the animal rests. The foot-sole in the Cyclostomidae is divided 

 longitudinally into two portions, and the animal crawls by 

 advancing the halves alternately. The sole has either a smooth 

 uniform surface or it is divided into three nearly equal parts 

 (trifasciate) longitudinally, the central area being the true ambu- 

 latory one. This is the type generally found in genera of the 

 Zonitidae, and has as well, on the outer side of the foot, a border 

 broken up into segments by lines giving it a fringed appearance. 



