70 COMPARATIVE ANATOMY 



illustrate some important characters of the class, it will be 

 necessary, from time to time, to refer to two equally common 

 species of fresh-water snails, Paludina vivipara and LimncEa 

 stagnalis. The common garden snail. Helix aspersa, is found 

 everywhere in England; the edible snail. Helix pomatia, is 

 tolerably abundant in limestone districts, and is cultivated in 

 large quantities on the Continent for culinary purposes. Both 

 species are herbivorous, and their depredations are only too 

 well known to gardeners. They rasp holes in the leaves and 

 succulent plants by means of the odontophore, an organ beset 

 with numerous minute teeth which will be described further 

 on. The expanded snail is familiar enough to everybody. 

 It has an oblong body, the base of which is thick and muscular 

 and forms an elongate sole-like foot. At the anterior end is a 

 distinct head-region, bearing two pairs of cephalic tentacles. 

 The spiral shell rests on the middle of the back of the 

 extended animal, its opening directed downwards and its apex 

 turned to the right. The visceral hump is permanently con- 

 cealed in the shell and is coiled in a spire corresponding with 

 that of the shell, but it does not occupy the whole of its cavity, 

 for the shell is capable of containing the whole animal when 

 retracted. 



Let us first examine the shell. It consists of from four and 

 a half to five whorls increasing in size from the summit to the 

 base. The whorls are in close contact with one another, and 

 each lower whorl overlaps the one next above it by more than 

 half its extent. The lines of junction between the whorls are 

 called sutures. The cavity of the shell is continuous — i.e. 

 it is not broken up into chambers by partitions as are the 

 coiled shells of the cephalopodous mollusca {Nautilus, Spirula 

 and Ammonites). The axis of the shell is occupied by a 

 spirally twisted central pillar, the columella, which is hollow 

 and opens below by a narrow fissure called the umbilicus. 

 The opening of the shell is called the aperture ; it is round, 

 and its lips are even and without a notch. The outer hp of 

 the aperture is distinguished as the peristome ; it is thickened, 

 and its lower end is reflected so as to partially conceal the 

 umbilicus. The inner lip is formed by the body whorl and 

 columella. The surface of the whorls is marked by a number 

 of transverse ridges or growth lines and there are also several 

 coloured lines which wind longitudinally round the spiral. At 



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