HAEITS AND ECONOMY OF THE MOLLTJSCA. 13 



winter they retire to deeper water, and in the following spring 

 return to the tidal rocks, attain their full growth early in the 

 summer, and after spawning-time disappear. 



The land-snails are mostly biennial ; hatched in the summer 

 and autumn, they are half-grown by the winter time, and 

 acquire their full growth in the following spring or summer. 

 In confinement, a garden-snail will live for six or eight years ; 

 but in their natural state it is probable that a great many die 

 in their second winter, for clusters of empty shells may be 

 found, adhering to one another, under ivied walls, and in other 

 sheltered situations ; the animals having perished in their 

 hybernation. Some of the spiral sea-shells live a great many 

 years, and tell their age in a very plain and interesting man- 

 ner, by the number of fringes {varices) on their whorls ; the 

 contour of the ranella and murex depends on the regular re- 

 currence of these ornaments which occur after the same inter- 

 vals in well-fed individuals, as in their less fortunate kindred. 

 The ammonites appear by their varices, or periodic mouths 

 (PI. III., fig. 3), to have lived and continued growing for many 

 years. 



Many of the bivalves, like the mussel and cockle, attain their 

 full growth in a year. The oyster continues enlarging his shell 

 by annual " shoots," for four or five years, and then ceases to 

 grow outwards ; but very aged specimens may be found, espe- 

 cially in a fossil state, with shells an inch or two in thickness. 

 The giant-clam {tridacna), which attains so large a size that 

 poets and sculptors have made it the cradle of the sea-goddess, 

 must enjoy an unusual longevity ; living in the sheltered 

 lagoons of coral islands, and not discursive in its habits, the 

 corals grow up around until it is often nearly buried by them ; 

 but although there seems to be no limit to its life (though it may 

 live a century for all that we know), yet the time will probably 

 come when it will be overgrown by its neighbours, or choked 

 with sediment. 



The fresh-water molluscs of cold climates bury themselves 

 during winter in the mud of ponds and rivers ; and the land- 

 snails hide themselves in the ground, or beneath moss and 

 dead leaves. In warm climates they become torpid during the 

 hottest and driest part of the year. 



Those genera and species which are most subject to this 

 " summer sleep " are remarkable for their tenacity of life ; and 

 numerous instances have been recorded of their importation 

 from distant countries in a living state. In June, 1850, a 

 living pond-mussel was sent to Mr. Gray from Australia, which 



