;}74 METAMORPHOSIS. 



In the third sta*>"o the sliell lias fallen oif, and the "'eneral 

 shape is that ol" the parent ; but the veils still remain, in 

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

 ropod fashion, and the branehiit> and ecxK'a hegin to s])rout. 

 "^IMiere are now also visibh> pulsations in the heart ; and the 

 mouth arms itself with jaws and with a spinous tongue. 

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

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

 branchiix;; and the full evolution of these organs completes 

 the metamorphosis, and entitles the animal to the privileges 

 of maturity.* 



In all (jaster()])o(ls whose develo])ement in the c(^g Milne- 

 I'idwards has had the o])portunity of tracing, the embryo 

 presents, in its tirst stagcvs, the same characters; and it is 

 only in the latter period of its metanu)r])hosis, 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. 'J'hus, up to a certain age, the larv;ie 

 of the Vermetus, Cerithiuni, Pleurobranchus, Doris, and 

 of the A])lysia, have the sanu' manner of conformation ; aiul 

 it was only when they became recognizable as (jiasteropod 

 Mollusca that scnne differences of a secondary order were 

 observable in their structure. Milne-Edwards' researches 

 have likewise satisiied him that in all Mollusca the series 

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

 tebrati'd animals ; and he is convinced of the existence of a 

 ci'rtain relation between the degree of imjxjrtance which the 

 leading systems of the economy offer, considered undtM- 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 ])ermanent condition of each 

 of the j)rincipal inferior types of the animal kingdom, so 

 that the Mollusk, for exam})le, is the ])ermani'nt representa- 

 tion of one of the transition forms of the young nuunmal 

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

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

 is ])eculiar to it; and the iirst characters of animality visible 

 in the end)ryo of the mannnal are, on the contrai'y, those 

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

 si(m of Vertebrates. The dillerences are, tlu-refore, prim- 

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



* Noidmunn in Ami. dcs Sc, Nat. (184(J) v. 155 and 158. 

 t Ami. (K'.s 8c. Nat. (1 845). iii. i;58. 



