IN GENERAL. 689 



reason why the formation of pearls should not be ascribed also to 

 other irritants affecting the mantle. It has been observed at least 

 that injuries of the shells and wounds, caused by boring worms, 

 have had the production of pearls for a consequence ; and the secret 

 of LINNAEUS for favouring the production of pearls (in Unio), con- 

 sisted, most probably, in boring the shell in different places in con- 

 chifers, which were submitted to these experiments 1 . 



The shells of molluscs, from their variety of colour and form, 

 constitute no small part of the ornament of natural-history collec- 

 tions. The knowledge of conchology is of the highest interest to 

 the Geologist, since the petrified and extinct species afford im- 

 portant characters for distinguishing the different strata. But, 

 more than this, the knowledge of the molluscs is of great value to 

 general physiology. To the celebrated Danish zoologist of the last 

 century, O. F. MUELLER, the honour is principally due of having 

 raised this part of natural history from the fondness of collectors to 

 the scientific contemplation of naturalists; it was his impressive 

 exhortation that thenceforward attention should not be confined 

 solely to the house or the shell, but, above all, be extended 

 to an accurate investigation of its inhabitant 2 . Already had SWAM- 

 MERDAM in Holland, and MARTIN LISTER in England, investi- 

 gated the internal structure of some molluscs. POLI and CUVIER 

 made this subject a primary object of their numerous inquiries, and 

 thus, in the course of the last fifty years, and even in our own day, 

 i through the labours of DELLE CHIAJE, OWEN and others, a clear 

 and extensive view has been obtained of a field of comparative 

 I anatomy that previously was almost unknown. This was an inesti- 

 I mable gain for a science which, if it is indeed to exercise an 

 [important influence on physiology, must not, in any sense, be 

 limited to a few classes of animals, but must, in reality, be com- 

 paring, and must compare generally. 



1 Compare CHEMNITZ Vom Ursprunge der Perlen, Naturforscher, xxv. Halle, 1791, 

 s. 122 130, and BECKMANN'S Geschichte der Erfindungen, cited there. On the origin 

 of pearls from eggs of Conchifers see Phil. Trans. 1674, Vol. IX. pp. n, 12, and 

 especially E. HOME in Phil. Transact, for the Year 1826, Part 3, pp. 338341. This 

 peculiarity had already been observed in 1673 by a Dane, H. ARNOLDI, at Christiana 

 in Norway. 



2 See his Vermium terrestrium et fluviatilium Historia, Hauniae, 1774, 4to. Tom. u. 

 Praefat. p. i. 



VOL. I. 44 



