372 METAMORPHOSIS. 



Sars' discovery in the Aplysia, the representative of the 

 order Tectibranches ; and it was only at a little later period 

 that it was extended to the Pectinibranchial Gasteropods by 

 Loven, Peach, and Milne-Edwards. Loven has recently 

 been pretty successful in proving that the bivalves undergo 

 a metamorphosis in all respects analogous to that of the 

 Gasteropods.* Hence it seems fair enough to conclude that 

 the class metamorphose ; but the conclusion awaits confir- 

 mation by an appeal to one or two orders, and several large 

 families, yet unexamined. It will only apply, under a modi- 

 fied form, to the Tunicata ; and finds the Cephalopods not 

 amenable to any changes. 



The metamorphosis is very much alike in all the genera 

 in which it has been witnessed. Messrs. Alder and Han- 

 cock thus describe its progress in the Nudibranches : — 

 " The spawn of the Nudibranchiate Mollusca is deposited 

 in the shape of a gelatinous band, always arranged in a more 

 or less spiral form, and fastened to corallines and the under 

 sides of stones by one of its edges. The ova are minute 

 and very numerous, amounting in some species to several 

 thousands. Before the period of exclusion, the young may 

 be seen revolving on their own axis by means of vibratile 

 cilia, and on escaping from the egg, they swim about freely 

 in the water by the same means. The larva is extremely 

 minute, and has more the appearance of a rotiferous animal- 

 cule than a Mollusk. It is inclosed in a transparent, calca- 

 reous, nautiloid shell, with an operculum. Its structure 

 is very simple, showing no signs of the external organs that 

 distinguish the future adult. The principal portion visible 

 outside the shell is composed of two flat discs or lobes, 

 fringed with long cilia, by the motion of which it swims 

 freely through the water. These are often withdrawn into 

 the shell, and the operculum is closed upon them when the 

 animal is at rest. We have not been able to trace the 

 animal further than the first stage of its developement, and 

 are therefore unable to say by what process it assumes the 



* Loven's observations were made on Anodonta, Modiola discors, and 

 Kellia rubra. — De Quatrefages thus concludes some researches into the deve- 

 lopement of the Teredo : — " II est bien e'vident d'apres ce qui precede que 

 les Tarets subissent de veritables metamorphoses avant d'atteindre leur 

 forme definitive. Des observations analogues out ete recueillies sur les Ano- 

 dontes, il y a une douzaine d'annees, par MM. Carus, Jacobson et moi- 

 meme. En voyant ce fait de produire en quelque sorte aux deux extr£mites 

 de la classe des Acephales, il est, je crois, permis de penser qu'on le retrou- 

 vera dans le groupe tout entier." — Ann. des Sc. Nat. (1848) ix. 36. — See 

 also a short notice of Mr. Caillaud's observations on Gastrochsena modiolina 

 in Ray Rep. on Zoology, 1847, p. 230. 



