374 METAMORPHOSIS. 



In the third stage the shell has fallen off, and the general 

 shape is that of the parent ; but the veils still remain. In 

 the fourth stage the creature begins to crawl in the Gaste- 

 ropod fashion, and the branchiae and coeca begin to sprout. 

 There are now also visible pulsations in the heart ; and the 

 mouth arms itself with jaws and with a spinous tongue. 

 Another stage is marked by the fall of the veils, and by the 

 budding forth of the anterior tentacula, as well as of the 

 branchiae ; and the full evolution of these organs completes 

 the metamorphosis, and entitles the animal to the privileges 

 of maturity.* 



In all Gasteropods whose developement in the egg Milne- 

 Edwards has had the opportunity of tracing, the embryo 

 presents, in its first stages, the same characters ; and it is 

 only in the latter period of its metamorphosis, that the 

 young animal acquires those peculiarities of structure upon 

 which the class of which it is a member is subdivided into 

 families and genera. Thus, up to a certain age, the larvas 

 of the Vermetus, Cerithium, Pleurobranchus, Doris, and 

 of the Aplysia, have the same manner of conformation ; and 

 it was only when they became recognizable as Gasteropod 

 Mollusca that some differences of a secondary order were 

 observable in their structure. Milne-Edwards' researches 

 have likewise satisfied him that in all Mollusca the series 

 of the organic developements is not the same as in the ver- 

 tebrated animals ; and he is convinced of the existence of a 

 certain relation between the degree of importance which the 

 leading systems of the economy offer, considered under a 

 zoological view, and the chronological order of their appear- 

 ance in the growing organism. All the phenomena of their 

 genesis, too, are opposed to the opinion of those physiolo- 

 gists, who maintain that the embryo of the superior animals, 

 even that of man himself, offers in succession modes of 

 organization analogous to the permanent condition of each 

 of the principal inferior types of the animal kingdom, so 

 that the Mollusk, for example, is the permanent representa- 

 tion of one of the transition forms of the young mammal 

 in the course of its formation. It is far otherwise. The 

 Mollusk, from its origin, is constituted on a model which 

 is peculiar to it ; and the first characters of animality visible 

 in the embryo of the mammal are, on the contrary, those 

 by virtue of which the mammal belongs to the great divi- 

 sion of Vertebrates. The differences are, therefore, prim- 

 ordial, and do not justify the hypothesis alluded to.f 



* Nordmann in Ann. des Sc. Nat. (1846) v. 155 and 158. 

 t Ann. des Sc. Nat. (1845). iii. 138. 



