Conchology. 139 



pher, who in the course of it has clearly expressed a contrary 

 opinion. ' I have frequently,' says Reaumur, 'compared the 

 shells of snails which are just hatched, and even with those which 

 I had taken from the eggs before they were hatched, with other 

 shells of full grown snails of the same species, with which I had 

 Jeft only the same number of whorls of the spire with the same 

 shells, and then they appeared in all respects the same.' He 

 farther observes, ' that what has been said with regard to the in- 

 crease of shells, renders it unnecessary to enter into the detail of 

 their original formation; for it is easy to conceive, that when the 

 body of a small embryo which is one day to fill a large shell, has 

 arrived at a certain state, in which the different teguments in 

 which it is included, have sufficient consistence to secrete from 

 their pores the pecuhar fluid which is destined to the formation of 

 the shell, this fluid may be deposited on the surface, may thick- 

 en, and at last become firm and sohd.' And thus commences the 

 formation of the shell, in the same way as its increase is continu- 

 ed. Snails do not proceed from the egg, without being previous- 

 ly furnished with this shell, which then has one turn and little 

 more of the spire. 



When the eggs of testaceous animals are hatched, the young 

 appears withjits shell already formed, and according to the observ- 

 ations of Reaumur, it has then one complete turn of the spire 

 and a little more; but at that period the shell is extremely thin. 

 It seems probable that the formation of the shell is posterior to 

 that of the principal organs of the animal, as the bones in the foe- 

 tus of other animals are formed after the brain and the heart. 



Reaumur has suspected that the shell is the last formed, and if 

 proofs are wanting to establish this fact, it is certain that at par- 

 ticular periods, if the eggs of testaceous animals are opened, the 

 external parts of the embryo are found already developed, with- 

 out any appearance of the shell. But whatever may be the peri- 

 od of the formation of the shell, it may be received as an estab- 

 hshed fact., that the animal is furnished with it at the time it leaves 

 the egg. Leuwenhoek first observed this fact with regard to oys- 

 ters; the same observation was afterwards made by Lister, and 

 extended to others, both land and river shells. This observation 

 has been confirmed by other naturahsts, and particularly by Rum- 

 phius, Swammerdam, Reaumur and Adanson. From the inves- 

 tigations of the latter, it appears, that although there are many, of 

 the marine testaceous animals which are viviparous, they resem- 

 ble those which are oviparous, in being furnished with the shell 

 when they separated from the parent. 



Since then it appears, that the shell of testaceous animals is 



