23(5 ON THE CLASSES OF THE 



vestige of \heAcephala should disappear, it will still be ne- 

 cessary to account for its own univalve shell. This is in 

 my opinion to be understood by the examination of the 

 genus Fistulana, consisting of animals which Lamarck 

 places in the same family with Teredo. The tube which 

 covers the bivalve shell of Fistulana is swelled or dilated 

 at its hinder extremity and closed all round except in front. 

 Now let this tube be shortened, and the consequence will 

 be that we shall have the form of the shells of Sulla ligna- 

 ria or hydatis. So that if this principle of connexion be 

 correct, paradoxical as it may appear, a Teredo is an Uni- 

 valve Mollusque so far as regards its tube, and the genus 

 Bulla is bivalve as to its gizzard ; and in this curious man- 

 ner may Nature have chosen to pass from the form of an 

 Oyster and Pholas to that of an Aplysia and Limax. 



But it must not be overlooked that great changes have 

 undoubtedly taken place towards perfection in the anato- 

 mical structure of the Acera, and that a Bulla belongs 

 properly to a very different class from the true bivalve 

 Mollusca, though it may serve to unite them with the 

 Gasteropoda of Cuvier. The branchiae of the genus 

 Bulla are of a more complex structure than those which 

 we have observed among the Acephala. They are now 

 transverse leaves subdivided each into still smaller folioles, 

 and are attached to the two sides of a triangular mem- 

 brane which adheres by one of its sides to the back of the 

 animal. The oesophagus leads us to the gizzard, which 

 opens into a membranaceous canal, still sufficiently in- 

 flated to deserve the name of a second stomach, and the 

 rather because it diminishes at once after having received 

 the biliary ducts. The lower side of the oesophagus is fur- 

 nished with a rounded tubercle armed with teeth, which 



