88 M. A. Giard on the Position of Sagitta, and on 



Thus it seems to us advisable to leave the Sac/ittce in a 

 special group, which, under the name of Chgetognatha, must 

 take its place at the base of the phylum of the Annelida, of 

 which this group represents a divergent branch adapted for a 

 pelagic existence. 



Other examples will show still better the practical import- 

 ance that may attach to speculative considerations such as 

 those which we have expounded with regard to the convergence 

 of types by pelagic life. 



The illustrious Von Baer, in a memoir dated last year, has 

 endeavoured to demonstrate that the Ascidia and the Salpce 

 are Mollusca presenting the same typical structure as the 

 Heteropoda ; but the smallest acquaintance with the develop- 

 ment of these animals suffices to prove, as we have endeavoured 

 to do elsewhere, that the resemblance between a Biphora and 

 a Firoloides is a result of adaptation, and that the analogies 

 of the Tunicata with the Gasteropoda are no more real than 

 those w T hich have been attempted to be established between 

 the same animals and the Lamellibranchiata*. 



Forbes thought he could find great affinities between the 

 larvae of the Ascidia and the Hydroida. On the other hand, 

 Carl Vogt formerly placed the Ctenophora among the Mol- 

 luscoida. An English naturalist, Macdonald, taking up a few 

 years ago these ancient ideas, gave the following classification 

 of the Molluscoida : — 



Molluscoida. 



r , ,. , ! ( Curvature primitively haemal, 



Intestine separated I finaU / ne ural . . . Ascidiozoa. 



from the cavity { \ \Brachiop6da 



ot the body. / Curvature simply neural . . . . j & ^ Pohjzoa 



Intestine straight and communicating with the cavity 



of the body Ctenophora. 



Macdonald regards the Ctenophora as a central type, from 

 which are derived on the one hand, by progression, the Mol- 

 luscoida, on the other, by degradation, the Hydrozoa. This 

 curious classification also has evidently for its starting-point 

 false homologies due to adaptation, which have appeared to 

 the author of more importance than the fundamental differences 

 presented by the embryogeny of these animals. The com- 

 parison of the pelagic types (natatory Tunicata, Ctenophora, 

 and Hydroida) is evidently the starting-point of these lucu- 

 brations, which look as if they were a century old, and never- 

 theless were published in 1864. 



* See Giard, " Embryogenie des Ascidifis, et l'origine des Vertebras," 

 Revue Scieutirique, 4 e annee, No. 2, July 11, 1^74. 



