568 LECTURE XXU. 



entirely covered, occasions tlie usual rotations of that body. The 

 211 212 progress of transformation of this monad-like 



\ embryo to the Gastropodous form, resembles 

 closely that which has been described in 

 Tritonia. The remains of the vitelline mass 

 {fig. 212, a), not yet metamorphosed into 

 special organs, indicates the expanded alimen- 

 tary sac ; h is the apex of the rudiraental foot, and c the velum with its 

 strongly ciliated border. These parts protrude from a rudimental, 

 thin, pellucid, and flexible shell, which covers all the rest of the sur- 

 face of the body. The arrows indicate the direction of the rotatory 

 movements of the embryo, which now likewise describes its elliptical 

 revolutions in the chorionic cavity. As development proceeds and 

 the embryo increases in size, the shell acquires a more distinctly tur- 

 binated form, and is slightly bent out of its vertical plane {fig- 213). 

 An operculum (e) is formed upon the pro- 

 truded surface of the foot : the course of in- 

 ternal development accords with that in 

 Tritonia. The ciliated branchial surface (c) 

 begins to be withdrawn more into the inte- 

 rior ; and, in this state, protected completely 

 by an external shell, the young Aplysia is 

 ^«* ^\o launched into the ocean. 



''^^'^"'"^' Truly may the subsequent growth, which 



effects an entirely internal position of the shell, with such a mutation 

 of its form that the primitive 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 

 Gastropods, which are not only naked, like the Aplysia, are devoid of 

 any internal rudiment of a shell, and yet are provided with a delicate 

 little operculated nautiloid horny external shell in their young state. 

 The same general course of development, in which the embryo or 

 larval mollusk is provided with the ciliated lobes and operculated 

 shell, has been traced by Loven in jEolis, Bulla, Cerithium * ; by 

 Nordmannf in Tergipes ; by Allman| and Vogt§ in Actceon ; by 

 Lund II in Murex and Nalica ; by Carusif in Paludina, and by 

 Siebold in Vermetus.** Rudiments of the vela are retained in Ter- 

 gipes, uEolis, Doris, Tritonia, and Aplysias ; and in Thetys they 



* CCCLXIX. t CCCXL. X CCCLXX. § CCCLXXL 



II CCCLXV. f CCCLXXII. •♦ XXIV. p. 360. 



