io8 THE MOLLUSC'A 



Walls, etc. Slugs usually excavate a chamber in the earth into 

 which they retire alone, curling themselves up almost into a ball 

 and encrusting themselves with their own slime. 



Snails are of a more sociable disposition, and several may 

 often be found clinging together in a mass during their winter 

 sleep. In extreme cold weather, or in periods of drought, fresh- 

 water snails bury themselves deeply in the mud, but do not 

 appear actually to hibernate, for even when the ponds are covered 

 with a coating of ice some species have been noticed creeping 

 about below. 



With the exception of a single family the Pulmonata have no 

 operculum, but before hibernation, or shorter periods of rest, most 

 land snails form a temporary lid to the opening of the shell, called 

 the epiphragm. This covering varies in thickness and com- 

 position in different species. It may be quite hard and thick, 

 and contain a considerable quantity of lime ; or a simple, thin layer 

 of hardened mucus. 



Many land and fresh-water molluscs are capable of enduring 

 extreme cold and heat. They live for several years, and are 

 very tenacious of life, being able to survive treatment that 

 would prove fatal to most creatures. The story of the snails 

 that lived for four years gummed to a tablet in the British 

 Museum is well known, but is so curious that it is worthy of 

 repetition : 



" On the 25th March, 1846, two specimens of Helix desertorum, 

 collected by Charles Lamb, Esq., in Egypt some time previously, 

 were fixed upon tablets and placed in the collection among the 

 other mollusca of the museum. There they remained, fast gummed 

 to the tablet. About the I5th March, 1850, having occasion to 

 examine some shells in the same case, Mr. Baird noticed a recently 

 formed epiphragm over the mouth of one of these snails. On 

 removing the snails from the tablet and placing them in tepid 

 water, one of them came out of its shell, and the next day 

 ate some cabbage leaf. A month or two afterwards it began re- 

 pairing the lip of its shell, which was broken when it was first 

 affixed to the tablet." 



Several other instances are on record of snails reviving after 

 having been kept shut up in boxes and bottles or fastened to cards 



1 Ann. Mag. Nat. Hist, (a) vi. (1885), p. 68. 



