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LETTER XXIII. 



ON THE FORMATION AND STRUCTURE OF SHELLS. 

 BY JOHN EDWARD GRAY, ESQ., F.R.S., &c. 



(p # * Reprinted by permission of the Author from the Philosophical 

 Transactions for 1833. 



1. First Formation of Shells. 



The shells of Mollusca appear to be coeval with the first 

 formation of the animal : they may be observed covering the 

 embryo on its first development in the egg, even before it 

 has acquired its proper shape or any of its internal organs. 

 The accurate Swammerdam observed them in the eggs of 

 several of the garden and pond snails. His observations 

 have been recently verified and extended by Pfeiffer, on 

 many species of land and fresh-water Mollusca ; and I have 

 myself observed the same fact in the eggs of several animals 

 belonging to the different orders of marine shells ; there is 

 reason, therefore, to believe that this circumstance is general 

 throughout the class. These observations are most easily 

 made on the embryo of the fresh-water shells, such as the 

 Lymnaeae, Physae, Ancyli, and Bithyniae, the eggs of these 

 animals being covered with a transparent coat ; while the 

 viviparous Mollusca, and especially the Littorinas, Palu- 

 dinae, and Cyclades,* offer the additional advantage of ex- 

 hibiting the embryos of their animals in all the different 

 states of development at the same time. 



* Between the laminae of the branchiae of the Anodontes and Uniones 

 are found small cordate, bivalve bodies, which have been considered as their 

 young ; but they differ so much in external form and internal structure from 

 the adults, that many excellent naturalists, and especially Professor Jacob- 

 son, of Copenhagen, have considered them as parasites. It is, however, 

 remarkable that they are found in abundance in almost every specimen, and 

 Pfeiffer has apparently proved that they are the young, he having found 

 them constituting the umbones of very minute Uniones. I have searched 

 for them in vain in this situation ; perhaps because I have never been so 

 fortunate as to discover specimens of the young shell so small as those 

 fio-ured by this author. If Pfeiffer should prove to be correct, this remark- 

 able change of form and structure will be the only approach towards a 

 metamorphosis that has been hitherto observed in this class of animals. 



