210 SNAILS AND SLUGS [CH. VII 



reached a length of 18 mm. in the same period. It is 

 perhaps hardly necessary to point out that the amount of 

 calcareous matter dissolved in 2000 c.c. of water is greater 

 than that in 100 c.c. This is probably an -important factor 

 in determining growth. It would be interesting to vary 

 Semper's experiments by dissolving (either at once or 

 gradually during the course of experiment) in 100 c.c. of 

 water as much calcareous matter as is actually contained 

 in 2000 c.c. of the same water. This would, ceteris paribus, 

 enable us to determine the influence of volume alone upon 

 the size of the shells. 



The Helicidce, Limnceidce and other air-breathing or 

 (pulmonate) gastropods have no lid (operculum) by which 

 the aperture of the shell can be closed when the animal 

 has retreated within. Such lids are well known in the 

 common periwinkles and whelks of our shore pools and may 

 alsobe seen upon the fresh-water snails Vivipara(Paludina), 

 Bithynia and others ; they are horny, sometimes calcined 

 structures developed upon the " foot " of the animal ; they 

 are marked with concentric lines caused by additions to the 

 margin as need arises for a larger operculum to fit the 

 increasing shell-mouth. The land snails however can 

 produce somewhat analogous structures at certain times. 



When a snail hibernates it retires into some sheltered 

 spot or even excavates a tunnel in the earth or rock (e.g. 

 H. aspersa in limestone rocks) and closes the aperture of 

 the shell by a disc, the " epiphragm 1 " or " hibernaculum/' 



1 Keferstein, Bronris Klass. u. Ord. in. 2. Binney, Terr. Moll, of 

 U. S. n. 1851. Barfurth, Arch. f. Mikrosk. Anat. xxn. 1883. Allman, 

 Jour. Linn. Soc. Zool. xxv. 



