DOHRN 275 



medial coalescence of a pair of gill-slits. 1 Probably the two 

 mouths at one period co-existed, and the older one was 

 ousted by the growing functional importance of the newer 

 mouth. 



The gill-slits were considered by Dohrn to be derived 

 from the segmental organs of Annelids, which were present 

 originally in every segment of the primitive ancestor. The 

 gills were at first external, like the gills of many Chaetopods 

 at the present day. For their support cartilaginous gill-arches 

 naturally arose in the body-wall, and the superficial muscu- 

 lature became attached to these bars. " There existed in all 

 the segments of the Annelid-ancestors of Vertebrates gills 

 with cartilaginous skeleton and gill-arches in the body wall. 

 Each gill had its veins and arteries, each had its branch of 

 the ventral nerve-cord, and between each successive pair of 

 gills a segmental organ opened to the exterior" (p. 14, 1875). 

 The paired fins and limbs of the Vertebrate arose by the 

 functional transformation of two pairs of these gills. The 

 anterior gills became the definitive internal gills of the 

 Vertebrate, for they gradually shifted into the mouths of the 

 anterior segmental organs, which had already acquired an 

 opening into the pharynx and had been transformed into 

 true gill-slits. The posterior gills degenerated and disap- 

 peared, but their arches remained as ribs. Gill-arches and 

 ribs were accordingly homologous structures and formed 

 a parietal skeleton. The vertebrate anus, like the mouth, 

 was probably secondary and formed from a pair of gill-slits, 

 the post-anal gut of vertebrate embryos hinting that the 

 original anus was terminal as in Annelids. The unpaired fins 

 of fish were originally paired and possibly arose from the 

 coalescence of rows of parapodia. Dohrn assumed also that 

 the primitive Annelid ancestor must have possessed a 

 notochord to give support in swimming. 



If Vertebrates arose from primitive Annelid ancestors, 

 how account for Amphioxus and the Ascidians, which seem to 



1 Leydig (Vom Bauc des thierischen Kbrpers, Tiibingen, 1864), who, 

 in a measure, forestalled Dohrn and Semper by comparing Vertebrates 

 with reversed Arthropods, specially insects, supposed the old mouth to 

 pass between the crura cerebri. 



