METAMORPHOSIS. 371 



literally to sit upon and hatch its eggs. The mother dis- 

 poses them under her belly, and preserves them, as it were 

 imprisoned, between the foot and the foreign body to which 

 she adheres, her patelloid shell thus serving not only to cover 

 and protect herself, but as a shield to her offspring. The 

 young Calyptraese are developed under this kind of maternal 

 roof, and do not quit it until they have strength to attach 

 themselves to the rock, and until their own shell is hard 

 enough to afford protection when so attached. The eggs 

 themselves are ovular corpuscules enclosed, like the zoopha- 

 gous Mollusks, in membranous elliptical flattened capsules 

 filled with an albuminous matter. The number of capsules 

 varies from six to ten, each enclosing from eight to twelve 

 eggs ; and they are all joined together by a pedicle in such 

 a manner as to make up the representation of a sort of 

 cockade.* 



Until very recently nothing was deemed, amongst mala- 

 cqlogists, more certain than the axiom that no species in the 

 sub-kingdom Mollusca was subject to a metamorphosis, — - 

 but now the vivid language of some naturalists might lead 

 you to suppose that they do all of them exhibit, in their 

 progress from quickening to maturity, metamorphoses as 

 striking and marked as those which insects offer to our study. 

 This, however, is not exactly the case. It is admitted that 

 the Mollusca have, indeed, in their embryo condition, or at 

 birth, an appearance and a form very dissimilar to what they 

 have in adolescence and adult age, and hence they may justly 

 be said to metamorphose ; but in their developement there 

 are no stages — no arrests of growth, as in insects, but a gra- 

 dual and imperceptible advance of transformation, which is 

 completed in the very earliest days of life, and before the 

 body has outgrown an almost microscopical minuteness, f 

 " Semper ad eventum festinat." 



The discovery of a metamorphosis in the class was first 

 made, I believe, by M. Sars, a distinguished Norwegian 

 naturalist, who has therefore the merit of having inserted 

 the lever, which was to overturn and erase the long unques- 

 tioned error. The discovery was made on a species of the 

 order Nudibranches ; and it was soon shown to be general 

 in this order by Alder and Hancock, Nordmann, Quatrefages, 

 Loven,J Peach, § and Allman. Van Beneden earlier verified 



* Auclouin and M. Edwards, Hist. Litt. dc la France, i. 133. 



t M. A. do Quatrefages lias, in an essay just published, pointed out this 

 distinction. " Embryogtnic des Tarets'' in Ann. des Sc. Nat. (1849) xi. 

 226. % Ray Reports on Zoology, 1847, p. 430. 



§ Ann. and Mag. N. Hist. xv. 445. 



I! it 2 



