THE ANNALS 



AND 



MAGAZINE OF NATURAL HISTORY. 



[FOURTH SERIES.] 

 No. 92. AUGUST 1875. 



VIII. — On the Position of Sagitta, and on the Convergence 

 of Types by Pelagic Life. By M. A. Giard*. 



No animal has been move frequently shifted from one syste- 

 matic division to another than Sagitta. Some have regarded 

 it as a degraded vertebrate, and have placed it beside Amphi- 

 oxus ; others have considered it a heteropod mollusk ; Oscar 

 Schmidt declares that it " is neither a true annelid nor a legi- 

 timate mollusk "f; Leuckart, Schneider, and Clans approxi- 

 mate it to the Nematoidea. 



Hackel, in his ' Generelle Morphologie,' also places the 

 Chsetognatlia among the Nemathelmintha, and, further, he 

 takes up the idea of Meissner with regard to the relationship 

 of Sagitta and the Vertebrata. If we make a perpendicular 

 section of the tail of a fish, we see clearly, he says, that the 

 trunk of a vertebrate is formed originally of four antimera, 

 and not of two. The primitive form of the lower Vertebrata, 

 like that of the Nematoidea, is the eutetrapleural interradial 

 form. Thus we may put forth, with some appearance of rea- 

 son, the hypothesis that the Vertebrata have issued from the 

 Chaetognatha by a progressive metamorphosis, whilst the 

 Nematoidea have been produced from them by a retrograde 

 metamorphosis. 



Since the admirable researches of Kowalevsky upon the 



* Translated bv W. S. Dallas, F.L.S., from an article in the ' Revue des 

 Sciences Natiuelies,' tome Hi. March 1875, communicated by the Author. 

 t The Doctrine of Descent and Darwinism, p. 37. 



Ann. & Mag. N. Hist. Ser.4. Vol. xvi. 6 



