312 LECTUEE XXIII. 



cavity. As development proceeds and the embryo increases in size, 

 the shell acquires a more distinctly turbinated form, and is slightly 

 bent out of its vertical plane (^fig. 128.). 

 ^ An operculum (e) is now observed to be 



formed upon the protruded surface of the 

 foot ; the oesophageal ganglion is visible at 

 d. The ciliated branchial surface (c) begins 

 to be withdrawn more into the interior; 

 and, in this state, protected completely by 

 an external shell, the young Aplysia is 

 ^P'^''*- launched into the ocean. 



Well may the subsequent growth, which effects an entirely internal 

 position of the shell, with such a mutation of its form that the primi- 

 tive nucleus can scarcely be detected upon the almost flattened plate 

 now destined to protect the equally internal respiratory organs of the 

 mature animal, justify us in applying to it the term metamorphosis. 

 This term is still more applicable to the developmental phenomena 

 in the Tritonia and Doris ; since these Gasteropods, which are not 

 only naked, like the Aplysia, but devoid of any internal rudiment of 

 a shell, are provided with a delicate little nautiloid horny external 

 shell in their young state. 



LECTURE XXIII. 



CEPHALOPODA. 



We trace the progressive diminution of the existing species of Gas- 

 teropodous MoUusca by their fossil shells through the descending 

 strata of the tertiary periods of geology, beyond which such indi- 

 cations become very doubtful and obscure.* In the oldest tertiary 

 deposits, not more than 2)\ per cent, of the remains of any class of 

 Mollusca have been identified with species now living. From this 

 striking fact, which Mr. Lyell looks upon as indicating the dawn of 

 the existing state of the testaceous fanna, he has proposed the term 

 ' eocene ' for these strata. In the next, or ' miocene ' tertiary 

 period, there are about 17 per cent, of fossil shells, identical with 



* The supposed recent species of Trochus observed by Defrance in chalk, and 

 the Paludina and Cyclas, described by Dr. Fitton from the Wealden, are not con- 

 sidered to be identical with existing species by Deshayes and Lyell. 



