376 MOLLUSCA THEIR AGE. 



tinue actively at work throughout the year, it is reasonable 

 to infer even a quicker growth. Hence you may reject as 

 worthless all calculations of the age of Mollusca founded on 

 the basis in question ; and when fifty or sixty years are 

 stated to be a moderate computation of the average life of 

 the Unio margaritiferus,* and thirty years of adolescence are 

 assigned to the Tridacna gigas, I do not hesitate to state my 

 belief that the calculations are greatly exaggerated, although 

 an eminent geologist, proceeding on the latter statement as 

 on a fact, has thence deduced an argument for the slow 

 growth of coral, and the slow rate of increase of calcareous 

 reefs, -f The rapid renewal of exhausted beds of oysters, 

 and the annual harvests yielded in many places by cockles, 

 prove more certainly the rapid attainment of maturity in the 

 class. Mussels of small size, and probably of only a few 

 weeks' age, when deposited in a favourable site, reach their 

 maximum size within the year ; and, according to the fisher- 

 men on our shore, this is also the case with the Limpet. 

 The fishermen at Marseilles told Cuvier that the Aplysia 

 attains its full size of about four inches in length in one or 

 two months. J Having attained maturity I know of no data 

 which inform us to what length of days the life of the Mol- 

 lusk is prolonged. Of land snails, Swammerdam says : — 

 " The period of their natural lives cannot be certainly deter- 

 mined, but I am inclined to think they live to a very great 

 age, which may probably be conjectured from the slow 

 increase of their shell." You may arrive at the same con- 

 clusion on the surer ground of their peculiar organization, 

 which necessitates a certain degree of sluggishness in the 

 action of every organ, and a low insensate nature, that, as 

 in reptiles, is favourable to the prolonged continuance of life. 

 It is this low organization which enables them to recover 

 from injuries that would annihilate more lively entities, and 

 which endows them with the property of renewing organs 

 lopt off by accident or an enemy. Snails (Helix) will live 

 six or seven days under water; and if bruised, and taken 

 from their shell, they will still live four days provided they 

 are put into water. Muller froze some living specimens of 

 Physa hypnorum by congealing the water in which they 

 were, but on thawing the water the snails began to move 

 again as usual. § Still more marvellous instances are on 



* Maton's Life of Linnpeus, &c, p. 93. 



t Lyell's Geology, ii. 287. + Mem. ix. 11. 



§ " Joly has observed with respect to certain Mollusca (Paludina vivi- 

 para, Lam., and Anodonta cygnea, Lam.) that they may be frozen up in 

 ice without being killed. Some of the Paludinse even produced young 



