THE ANCESTRY OF VERTEBRATES 



both groups so wholly agree, that, as BOAS (1914, p. 532)* 

 remarks, "der Gedanke an blosse Analogie entschieden von 

 der Hand gewiesen werden muss". Indeed, the agreement 

 of a protonephridium of Amphioxus with that of a worm 

 like Phyllodoce is so complete thai it is hardly possible to-, 

 assume that two so similar structures could have originated 

 independently. 



Another point of agreement is found in the circulatory 

 system, if only we assume again that the neural side in 

 both Annelids and Vertebrates correspond. We see the blood 

 circulating from in front backwards in the ventral vessel of 

 the Annelid and in the dorsal aorta of the Vertebrate, and 

 in the reverse direction dorsally in the Annelid and ven- 

 trally in the Vertebrate. Also the structure of the main sense- 

 organs and their situation at the anterior end of the body 

 shows affinity between the Vertebrates and the Annelids. 



Other theories. — What then is the reason that the theory 

 of the Annelidan ancestry of the Vertebrates, which many 

 have adhered to, has yet not given the complete satisfac- 

 tion we might have expected? How is it possible that other 

 theories have grown up at its side like mushrooms, deriving 

 the Vertebrates from anemones (Lameere, 1891, Hubrecht, 

 1902), Nemerteans (HUBRECHT, 1883), Turbellarians(GOETTE: 

 1884. 1895), Arachnids (PATTEN, 1890), Palaeostracans 

 (Gaskell, 1908), Enteropneusts(BATESON, 1886), nay, from 

 nearly every group of Invertebrates, often along the most 

 unexpected ways? How could one of these theories, viz. the 

 last of those mentioned above, gain such a wide acceptance 

 in recent time, that in text-books of zoology we find more 

 and more the Chordates placed behind the Enteropneusts- 

 and the latter often designated as Prochordates? 



The old mouth. — One difficulty connected with the deriva- 

 tion of Vertebrates from Annelids has never found a satisfactory 

 solution, and has remained a serious obstacle to this theory 

 ever since DOHRN first encountered it. It is the old question, 

 that in Annelids the central nervous system is situated on 

 both sides of the gut — the cerebral ganglia at the dorsal^ 

 the ventral ganglion chain at the ventral side — , while in 

 Vertebrates the whole nervous system, brain as well as 

 medulla, is situated dorsally from the gut and is not pierced 

 by the latter. DOHRN tried to account for it by assuming, 

 as suggested already by LEYDIG (1864), that formerly in 

 Vertebrates also such a passage of the enteron through the 



