THE ANCESTRY OF VERTEBRATES 



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Vertebrates to Chordonia was a necessary consequence of 

 KOWALEWSKY's (1866) embryological work. GroBBEN now 

 designated the Zygoneura as Protostomia, and opposes them 

 to both the other groups as Deuterostomia, since in the 

 former the blastopore directly passes into the ingestion- 

 opening, whereas in Deuterostomia it passes into the 

 egestion-opening, the anus, or at least exhibits certain 

 relations to the latter and none to the mouth, which is 

 formed as a secondary perforation quite independent from 

 the blastopore. The first to attribute such a primary 

 importance to the fate of the blastopore has been GOETTE 

 (1884), who distinguished the hypogastric from the pleuro- 

 gastric Bilateria. In the former, to which Annelids, Nemertines, 

 Nematodes etc. belong, the blastopore passes into the mouth, 

 in the latter, comprising Vertebrates, Echinoderms and 

 probably also Enteropneusts, into the anus. 



Thus the place assigned to the Chordates in this system is 

 in agreement whh he attempts of BatesoN (1886) and his 

 followers to found a relationship between Chordates and 

 Enteropneusts and by means of the latter again with 

 Echinoderms (cf. GarstanG, 1894), to which the Enterop- 

 neusts are undoubtedly related. I will not go here into a 

 criticism of BateSON's theory. Others, more competent 

 than I, as e.g. SpenGEL (1893, p. 721), have done this 

 already and have shown how unconvincing are the homo- 

 logies proposed by BATESON. Indeed, if we ask ourselves 

 with which group the Chordates show closer affinity, 

 whether with the Annelids, a trunk to which more than 

 one branch showing tendency to higher development — like 

 Arthropods and Molluscs — is related, or with the dull 

 Echinoderms with their lack of intellectual and other develop- 

 ment, the answer can be hardly doubtful. 



My theory. — The theory on the origin of Vertebrates,, 

 which 1 published some years ago (1913), can be ranked 

 among those theories which in different ways try to find 

 a solution of the difficulties connected with the derivation 

 of Vertebrates from Annelids; difficulties which have resis- 

 ted for so long all attempts to overcome them, that the 

 case seemed to appear almost hopeless. 1 must emphasize, 

 however, that, unlike most of the authors cited in the 

 beginning of the next chapter, it was not in the least 

 my intention to make an attempt to render DOHRN's and 

 Semper's theory acceptable, nor to look for a solution of the 



