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LETTER XXI. 



ON THE AGE OF THE MOLLUSCA, THEIR RENEWAL OF LOST 

 ORGANS, AND THEIR DISEASES. 



The young of the Mollusca, immediately on their exclu- 

 sion from the egg, or from their brief span of a larvated life, 

 enter into all the habits and instincts of their parents. Their 

 growth, like that of the young of animals in general, is 

 rapid ; and the secretion of the shell proceeds at an equal 

 pace, the proportion of its earthy constituents being at the 

 same time increased in most of them, that its strength and 

 density may have a certain correspondency with the growing 

 strength and bulk of the inmate. How long the time may 

 be which the Mollusk requires to finish its house is yet 

 unknown, or only known in a very few terrestrial species. 

 A Helix pomatia born 15th of September, 1825, grew pro- 

 gressively until checked by the approach of winter on 26th 

 of November. It resumed its growth 1st of April, 1826, 

 and completed the shell on the 3 1st of July, or about six 

 weeks within the year.* I have been led to believe that 

 some of our Helices require two years for this purpose, — 

 that is to say two summers ; but probably the time is 

 influenced both by the temperature and moisture of the 

 seasons. The oyster is said to require four years to attain 

 its fullest size. Steno and others have asserted that we 

 may measure the time which a Mollusk occupies in com- 

 pleting its structure by counting the number of layers in 

 the bivalvular shells, and the number of whorls in the tur- 

 binated univalves, a layer or a whorl being the work of one 

 year, but I am certain that Muller is right in pronouncing 

 the test to be altogether fallacious. It would give a slow- 

 ness of growth to the oyster, which we know to be contrary 

 to fact ; and with reference to the univalves it should be 

 remembered that at least two whorls are completed previous 

 to birth ; and I know that the remainder may be completed, 

 in our common snails, in the course of a single summer. 

 And in those marine tribes which, from the depth of their 

 localities," suffer no alternations of temperature, and con- 



* Pfeiffer, Bull, des Sc. Nat. Jan. 1829, p. 145. 



