xliv INTRODUCTION. [CH. 



Mollusca, both univalve and bivalve, possess also a cer- 

 tain power of endurance under altered conditions. Thus 

 Trochus lineatus, which inhabits rocks only uncovered 

 at low water, can live in a warm room for a fortnight ; 

 and specimens of Mya arenaria, which burrows into 

 muddy gravel in the sublittoral zone, have been noticed 

 by Mr. Rich (an intelligent collector and dealer) to sur- 

 vive their captivity for three weeks, being all that time 

 in apparently a healthy state (evidenced by the with- 

 drawal of their tubes when touched), at the end of which 

 period they were killed for commercial purposes. No 

 sea- water was supplied in any of the above cases. The 

 gills must have been kept moist by the fluid contained 

 within the mantle — a provision nearly similar to that by 

 which the camel is able to endure the heat and fatigue of 

 a journey across the desert after having filled its paunch 

 with a stock of water. M. Joly observed with respect to 

 some freshwater mollusca (^/iof/ow/a cygnea and Paludina 

 vivipara), that they maybe frozen up, and kept for some 

 time enclosed in ice, without being killed. Some of the 

 Paludincs even produced young after being thawed *. 



Age. — Little or nothing is known with respect to the 

 duration of life in the Mollusca. According to Sir 

 Emerson Tennent, the pearl oysters of Ceylon only live 

 seven or eight years ; and it is said that snails do not 

 attain a greater age. This is not improbable as regards 

 the latter, because most of them become adult at the 

 end of their first year. Whether the numerous laminae 

 of old oyster-shells afford the same indication of an- 

 nual growth as the rings of a forest tree is another 

 question. 



Resume. — In concluding this chapter, I cannot do 

 better than quote the resume given by M. Moquin-Tandon 

 * Comptes Rendus, 1843, xvi. p. 460. 



