18 MANUAL OF THE MOLLUSCA. 



I 



or periodic mouths QdI. 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 outwai^ds ; but very aged specimens may be found, especi- 

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

 The giant-clam (tridacna), which attains so lai'ge 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 bm-ied by them ; but although 

 there seems to be no certain limit to its life (though it may live 

 a century for aU that we know), yet the time wiU probably come 

 when it wiU be overgrown by its neighbom's, or choked with 

 sediment. 



The fresh-water molluscs of cold climates buiy themselves 

 dmring winter, in the mud of their ponds and rivers ; and the 

 land-snails hide themselves in the gi'ound, or beneath moss and 

 dead leaves. In warm cKmates 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 had 

 been more than a year out of water.* The pond-snails {ampul- 

 laria) have been found alive in logs of mahogany from Honduras 

 (Mr. Pickering) ; and M. Caillaud carried some from Egypt to 

 Paris, packed in saw-dust. Indeed, it is not easy to ascertain 

 the limit of their endurance ; for Mr. Laidlay having placed a 

 number in a drawer for this purpose, found them alive after Jive 



* " It was alive 498 days after it was taken from the pond ; and in the 

 interim had been only twice for a few hours in water, to see if it was alive." — 

 Rev. W. 0. Newnham. 



