284 THEORIES ON THE ORIGIN OF VERTEBRATES 



The hypothetical ancestral forms (Protochordata) pos- 

 sessed a notochord, a ventral suctorial mouth and numerous 

 gill-slits, and were presumably descended from the common 

 ancestor of Annelids and Vertebrates. Amphioxus and the 

 Ascidians found their place in this schema as degenerate 

 offshoots of the ancestral Protochordates, while the Cyclo- 

 stomes were in the same way the degenerate modern 

 representatives of the ancestral Protovertebrates. 



Balfour's suggestion, that the nervous system in Annelids 

 and Vertebrates might have arisen by the dorsal or ventral 

 coalescence of the lateral nerve cords found in their common 

 ancestor, bore fruit in the speculations of Hubrecht, 1 on the 

 relation of Nemertines to Vertebrates. 



The Annelid theory was firmly supported by Eisig, who 

 in his elaborate monograph on the Capitcllidie" maintained 

 against Fiirbringer the genetic identity of the Annelidan 

 nephridia with the kidney tubules of Vertebrates. The 

 independent discovery by E. Meyer :? and J. T. Cunningham, 4 

 of an internal segmental duct in Lanicc, into which several 

 nephridia opened, seemed to strengthen this view. 



Following Ehlers/' Eisig found the homologue of the 

 notochord in the accessory intestine of the CapitellidcB and 

 Eunicidtc, which he supposed might easily be transformed, 

 according to the principle of function -change, from a 

 respiratory to a supporting organ. lie finally disposed of 

 the alternative notion that the notochord was represented 

 in Annelids by the "giant-fibres" or neurochordal strands 

 which lie close above the nerve-cord, a view held by 

 Kowalevsky, (i and for a time by Semper. These strands were 



1 "On the Ancestral Form of the Chordata," Q.J.M.S., xxiii., 1883. 

 "The Relation of the Nemertea to the Vertebrata," ibid., xxvii., 1887. 

 Hubrecht gives the credit for the first indication of the relationship of 

 Nemertines and Vertebrates to Harting (Lccrbock van dc Grondbegimelen 

 dcr Dicrkundc, 1874). 



2 " Monographic der Capitelliden des Golfes von Neapcl," Fauna 

 11. I-'lora dcs Golfes von A'w/V/, Monog. xvi., Berlin, 1887. 



3 Mitt. tool. Stat. Neapcl, vii., 1887. ' Nature, xxxvi., p. 162, 1887. 

 " Ncbendarm und Chorda dorsalis," Nachr. Gcx. ll'iss. Gottingcn, 



p. 390, 1885. 



11 " Embryologische Studicn an Wiirmern u. Arthropoden, :: J /<>//. 

 Acad. Sri. \/ r.'tersbourg (I'etrograd), (7), xvi., 1870. And in Arch f. 

 inikr. Anat., vii., p. 122, 1871. 



