the Convergence of Types by Pelagic Life. 89 



Under other circumstances adaptation to pelagic life causes 

 certain adult animals to resemble embryonic forms of other 

 animals belonging to higher types, or produces apparent 

 analogies between larval forms pertaining to different groups. 

 In his fine work on the Metamorphoses of Man and Animals 

 (1862), M. de Quatrefages, speaking of tlieAmphioxus, says: — 

 " It is allowable to. ask ourselves whether this animal, which is 

 placed in the lowest rank of Vertebrates, and which in many 

 respects approaches the Annelida Errantia, is really a perfect 

 animal. In some parts of its organization it reminds us of 

 the Ammoccetes of our brooks. May it not be the larva of 

 Petromyzon marinus or of some other species?" In 1867, 

 in a memoir upon this singular vertebrate, M. Bert insisted 

 on the facts which show that it is an adult creature ; and in 

 the same year Kowalevsky gave a complete embryogeny of 

 it. Nevertheless in 1871, at the Academy of Boston, in 

 presence of Louis Agassiz, the question whether Amphioxus 

 is not the larva of a Myxinoid fish was discussed over 

 again *. 



We have several times observed, in the neighbourhood of 

 Boulogne, troops of young Clupeceoi astonishing transparency, 

 and resembling the Leptocepliali in general aspect. Now it 

 is well known that zoologists have not yet completely solved 

 the question whether these Leptocephali are or are not adult 

 forms. Gill and several other ichthyologists assert that they 

 are embryonic forms ; Peters, on the contrary, affirms f that 

 they cannot be regarded as the embryos of the Cepolce or of 

 other fishes. According to Gill, Leptocephalus Morrisii is the 

 young of Conger vulgaris, Hyoproprus messinensis belongs to 

 Nettastoma melanura, and Stomasunculus is the larva of a 

 Clupeoid \. 



If such questions are difficult to solve in the case of animals 

 so high as the fishes, it will easily be understood how much 

 greater are the difficulties met with by the zoologist when he 

 tries to establish the true homologies which may exist between 

 the larvae of the lower animals. 



It seems to me that one of the most important and necessary 

 investigations for the progress of embryogeny would be to 

 distinguish what is due to heredity and what is the result of 

 adaptation to pelagic life in embryonic forms, such as the larva? 

 of the Echinoderms, the Pilidium of the Nemertians, the Acti- 

 notroclue, Mitrarue, Cyphonautes, certain larva? of Planarias, 



* Similar difficulties occurred formerly with regard to Phylhsoma, 

 Cuma, <fcc. 

 t Monatsb. Akad. Wis?. Berlin, 1864, p. 399. 

 X Proc. Acad. Nat. Sci. Philad. 18C4. 



