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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 cemplete that 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 ín 
both Annelids and Vertebrates correspond. We see the blood 
circulating from ín 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 
