146 HABITS AND ECONOMY OF THE MOLLUSCA. 



1863 lie collected Mijtilus edidis, four inches in length, from a 

 buoy at Arcachon, which had been cleaned under his eyes a year 

 previously : in this case the species had grown to nearly double 

 the size which it normally attains within the same period in the 

 mussel-banks of this locality. 



The oj^ster continues enlarging its shell bj^ annual "shoots," 

 for four or five years, and then ceases to grow outwards ; but 

 very aged specimens may be found, especially in a fossil state, 

 with shells an inch or two in thickness, and very heavy. 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 (it may live a centur}^ for 

 all that we know), yet the time will probably come when it will 

 be overgrown by its neighbors, or choked with sediment. 



The Meleagrina or pearl-oyster of Ceylon lives seven or eight 

 years (Tennent). 



The fresh-water mollusks 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 dviring the hottest 

 and driest part of the year. Among those which are inopercu- 

 lated, the mantle at this time secretes a temporary door to the 

 aperture, which is sometimes glutinous (Orthalicus), sometimes 

 thin, like a lamina of isinglass, sometimes calcareous and 

 thickened (Helix pomatia, -E naticoides, etc.). Occasionally, 

 besides this calcareous plate, there are found one or several 

 membranaceous epiphragms farther within the aperture. 



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 Dr. Gray from Avistralia, which had 

 been more than a year out of water. In December, 1874, 

 Deshayes, in opening a box of Anodontas, collected eight months 

 previously by the naturalists of the French expedition to 

 Cambodia, found two individuals still living within their paper 

 covering's. He has given to this species the name of '•'• Anodonta 

 sempervirens.''^ The pond-snails (Ampullariae) have been found 

 alive in logs of mahogany from Honduras (Mr. Pickering) ; and 

 M. Cailliaud 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 five years^ although in 

 the warm climate of Calcutta. The Cyclostomas, which are also 



