282 THEORIES ON THE ORIGIN OF VERTEBRATES 



show a closer resemblance to Vertebrates than his beloved 

 Annelids. Amphioxus, he thinks, is not a Vertebrate, and 

 Ascidians, though sharing with Annelids the possession of a 

 notochord, gill-slits, and a " dorsal " nervous system, yet are 

 further removed from Vertebrates than the latter by reason 

 of their lacking that essential characteristic of Vertebrates, 

 metameric segmentation. 



Not content with establishing the unity of plan of 

 Annelids, Arthropods, and Vertebrates, Semper tries to 

 link on the Annelids, as the most primitive group of the 

 three, to the unsegmented worms, and particularly to the 

 Turbellaria. His speculations on this matter may be 

 summed up somewhat as follows: The common ancestor 

 of all segmented animals is a segmented worm-like form, not 

 quite like any existing type, resembling the Turbellaria in 

 having two nerve strands on the dorsal side and no 

 cesophageal ring, potentially able to develop either the 

 Vertebrate or the Annelid mouth, and so to give origin both 

 to the Articulate and to the Vertebrate series. The common 

 ancestor alike of unsegmented worms and of all segmented 

 types is probably the trochosphere larva, which in the Verte- 

 brates is represented by the simple Keiinblase or blastula. 



The Annelid theory of Dohrn and Semper was perhaps 

 not so widely accepted as the rival Ascidian theory, but it 

 counted not a few adherents and gave a certain stimulus 

 to comparative morphology. F. M. Balfour, who pointed 

 out about the same time as Semper the analog)- between 

 the nephridia of Annelids and the mesonephric tubules of 

 Vertebrates, 1 while not accepting the actual theories of 

 Dohrn and Semper, took up a distinctly favourable attitude 

 to the general idea that Annelids and Vertebrates were 

 descended from a common segmented ancestor. Discussing 

 this question in his classical work on the development of 

 Elasmobranch fishes,- Balfour came to the conclusion " that 

 we must look for the ancestors of the Chordata, not in allies 



1 "On the origin ;md history of the urino-genital organs of 

 Vertebrates," Journ. Anat. P/iys., x., 1876. The conclusions of Balfour and 

 Semper were adversely criticised by M. Fiirbrmger (MorpJi. Jahrb., iv., 

 1878), and were negatived by later research. 



- A Monograph on the Development of Elasmobranch Fishes, London, 

 1878. 



