§ 229. THE CEPiiALornoRA. 2G5 



a delicate ciliated epithelium. By the means of these cilia, the embryo 

 rotates upon its axis for a lon:^ time. From this period, the aquatic differ 

 widely from the pulmoiiute Grasteropoda. With the Apneusta, and the 

 HeteroSranchia, the two lobules producel by the indentation just indi- 

 cated, enlarge and chaniie into round pinions (Vela), upon whose borders 

 very long cilia are gradually developed. A third eminence is developed 

 between tliese two pinions, and, ultimately, changed into the foot. 



Although the ciliated epithelium is always most widely spread around 

 these two pinions, which should be regarded as situated on the anterior 

 extremity of the body, yet there is formed a thin shell upon the posterior 

 extremity of the embryo, whether this last belongs to a conchiferous 

 species or not. At the same time, there appears upon the dorsal part' of 

 the foot, an operculum corresponding as to size with the opening of the shell. 



Among the internal organs, the two auditive capsules appear first; and 

 when these have become ([uite distinct, the eyes are seen. FoUowino- these, 

 are developed the tentacles, the border of the mantle, and the mouth which 

 appears between the two pinions. At the same time, the stomach, the 

 intestine, and the liver, individually appear in the interior. At this epoch, 

 the young leave the egg and swim freely about by means of the long cilia 

 which are situated on their extended and rigid pinions.*-' Subsequently 

 these pinions disappear, or are changed into two tentacular prominences 

 situated on each side of the mouth. '■''' At the same time, also, the naked 

 Gasteropoda lose their shell and operculum. From the isolated facts hith- 

 erto published upon the embryology of other branchiferous Gasteropoda, it 

 may be concluded that they experience a similar metamorphosis, only the 

 shell of the embryo, at this time, usually presents some convolutions.'*' 



In the development of the operculate Pulmonata, there is no analoo-ous 

 metamorphosis.'^' The embryo lengthens a little when it begins to rotate * 



2 The embryology of the Apneusta, and the He- bo, Nerita, Buccinum, and Purpura. Judging 

 terobranchia owes its progress principally to the from Cams'' figure (Nov. Act. Acad. Nat. Cur. 

 followin;; worlis : Sirs, in IVie^mann^s Arch. 1837, XIII. 1827, p. 767, Tab. XXXIV. fig. 2) of the 

 I. p. 402; 1840, I. p. 196, Taf. V.-VII. 18i5, I. p. embryo of Paladina vivipara, it also lias at this 

 4, Taf. I. fig. 7-11 (Tritonia, Doris, Apli/sia, an! age a pinion. 



Aeolis) ; Loven, in the Kougl. Vetensk. Akad. This remark is also applicable to the young animals 



Ilandl. 183a, p. 227, or Isis, 1842, p. 360, Taf. I. found by Lund (Ann. d. Sc. Nat. I. 1834, PI. VI. 



(Aeolis); yan Bene.den, Ann. d. Sc. Nat. XV. fig. 9-14) in the egg-capsules of a MMrea7(.') and a 



1841, p. 123, PI. I. (Aplasia) ; Nordm'in n, loc. cit. Natica (.')• 1 have found, in tiie pyriforni oviger- 



p. 71, Taf. IV. V. (fersipes) ; Allmaii, loc. cit. ous capsules adhering to the orifice of the shell of 



p. 152, PI. VII. fig. 10-12 ; l-'ost, Conipt. Rend. Vermetus, young with highly-developed jiinions 



XXI. 1845, No. 14, XXII. No. 9, or Froriep's having 1 mg cilia, and with a regularly convoluted 



neue N )t. N J. 7 J5, 820 (.Jc^af on) ; and Re/i, Ann. shell, such as has been described by Pliitippi 



of Nat. Hist. XVII. 1846, p. 377, PI. -V. {Doris and (IFies^mann's Arch. 1839, I. p. 12S, Taf. IV. fig. 



Polijcera). yo^t has since published his entire 8). Loven has observed similar embryos swim- 



Menwir on the devel,)|)ment of Actaeon viridis in ming with two pinions, with the Ileterobranchia 



the Ann. d. Sc. Nat. VI. 1846, p. 5, PI. I.-IV. ; see of the genera Eti/sia, Bulla, Bullaea, and with the 



also Sc/ileiden, auJ Froriep's Not. II. p. 77, fig. Pectlnibranchia of the genera incuna, Cerit/iium, 



1-12. and F.ulima ; see Arch. Skandinav. Beitr. &c. I. 



3 These remains of the two jiinions are easily seen 1846, p. 154, Taf. I. fig. 1-8. 



with Ter^ipes, Aenlis, Doris, Tritonia, Aplysia, 5 Tlie development of the Pulmonata which have 



and other H -terobranchia. Tht; ciliated I ib -s on the a shell, has often fixed the attenti)n of naturalists, 



head of Thetis, are only thes,' pinions persisting in Sje Stiebel, loc. cit. p. 38, Tab. 11. and in Meek- 



an embryonic firm ; see Lovin, loc. cit. eVs Arch, deutsch. I. p. 423, II. p. 557, Taf. VI. ; 



4 Ace rdiug to Lovin (loc. cit. or Isis, 1842, p. Hugi, Isis, 1823, p. 213 ; Varus, Von den '.iusseren 

 366, Taf. I. fig. 22), the young of Rissoa have a Lebensbed. loc. cit. p. 60, Taf. I. ; Prevost, Ann. 

 very large pini m. Nordmann (loc. cit. p. 93) has d. Sc. Nat. XX.X. 1833, p. 40 (Li/wnrieus) ; Pfeif- 

 contirmej this, and found an analogous one with fer, Naturg. deutsch. Land-und Susswass.r-Mol- 

 Littorina,\uu\ Phasianella. The small .Mollusks lusk. Abth. III. p. 70, Taf. I. (Helix); (^natre- 

 with a pinion a.id a turbinated shell, of which Sirs faeces, Ann. d. Sc. Nat. II. 1834, \>. 107, PI. XI. 

 (Beskrivel. loc. cit. p. 77, fig. 38, 3J) h:i3 f )rmed the B. (Li/mnaeus and Planorbis) , Jacquemin, Ibid, 

 genus Cirropteron, have since been f lund by him- V. 1836, p. 117, 119, and in the N iv. Act. Acad, 

 self, to be young indivi.luals of Turbo, Trochus, kc. .Will. 1S38, p. 636, Tab. .VLIX. L. (Planor- 

 or Nerita ; this accords with Grant's observations bis) ; Dumortier, N mv. Mtm. do I'Aead. Koy. de 

 (Edmb. new I'hilos. Jour. No. 13, 1827) upon Tar- BruxeUes. X. 1837, PI. I.-IV. aud Ami. d. Sc. Nat. 



23 ^ 



