274 THEORIES ON THE ORIGIN OF VERTEBRATES 



Some years after the genetic relationship of Ascidians and 

 Vertebrates had been established, a rival theory of the origin 

 of Vertebrates made its appearance a theory which was 

 practically a rehabilitation in a somewhat altered form of the 

 old Geoffroyan conception that Vertebrates are Arthropods 

 walking on their backs. This was the so-called Annelid 

 theory of Dohrn and Semper. Both Dohrn and Semper 

 started out from the fact that Annelids and Vertebrates are 

 alike segmented animals, and it was an essential part of their 

 theory that this resemblance was due to descent from a 

 common segmented ancestor. Both laid great stress on the 

 fact that the main organs in Vertebrates are arranged in the 

 same way as in an Annelid lying on its back, the nervous 

 system being uppermost, the alimentary. system coming next, 

 and below this the vascular. 



Dohrn's earlier views are contained in the fascinating little 

 book published in 1875, which bears the title Der Ursprungder 

 WirbcltJiiere nnd das Princip des Functiotiswechsel (Leipzig). 

 He followed this up by a long series of studies on vertebrate 

 anatomy and embryology, 1 in which he modified his views 

 in certain details. We shall confine our attention to the first 

 sketch of his theory. 



If the Vertebrate is conceived to have evolved from 

 a primitive Annelid which took to creeping or swimming 

 ventral surface uppermost, a difficulty at once arises with 

 regard to the relative positions of the " brain " and the 

 mouth. In Vertebrates the brain, like the rest of the nervous 

 system, is dorsal to the mouth and the alimentary canal ; in 

 an inverted Annelid, however, the brain is ventral to the 

 mouth and is connected with the dorsal nerve cord by 

 commissures passing round the oesophagus. It would seem, 

 therefore, that the primitive Vertebrate must have acquired 

 either a new brain or a new mouth. Dohrn took the latter 

 view. He supposed that the original mouth of the primitive 

 ancestor lay between the cnira ccrcbclli in the A'.ow rlioinboiJca, 

 and that in Vertebrates this mouth has been replaced 

 functionally by a new ventrally placed mouth, formed by the 



1 "Studien zur Urgeschichte des \Virbelthierkorpers," MitthciL 

 Zool. Staf. ,V,v//"/, 1882-1907. 



