CHAPTEK XIL 



OBSERVATIONS ON THE ANCESTRAL FORM OF 

 THE CHORDATA, 



The present section of this work would not be complete without 

 some attempt to reconstruct, from the materials recorded in the 

 previous chapters, and from those supplied by comparative anatomy, 

 the characters of the ancestors of the Chordata ; and to trace as far 

 as possible from what invertebrate stock this ancestor was derived. 



The second of these questions has been recently dealt with in a 

 very suggestive manner by both Dohrn (No. 250) and Semper (Nos. 

 255 and 256), but it is still so obscure that I shall refrain from any 

 detailed discussion of it. 



While differing very widely in many points both Dohrn and Semper 

 have arrived at the view, already tentatively put forward by eai'lier 

 anatomists, that the nearest allies of the Chordata are to be sought for 

 amongst the Clifetopoda, and that the doi'sal surface of the Chordata with 

 the spinal cord corresponds morphologically with the ventral suiface of 

 the Chietopods with the ventral ganglion chain. In discussing this subject 

 some time ago* I suggested that we must look for the ancestors of the 

 Chordata, not in allies of the present Chsetopoda, but in a stock of segmented 

 forms descended from the same unsegmented types as the Chsetopoda, but 

 in which two lateral nerve-cords, like those of Nemertines, coalesced dorsally, 

 instead of ventrally to form a median nervous cord. This group of forms, 

 if my suggestion as to its existence is well founded, appears now to have 

 perished. The recent researches of Hubrecht on the anatomy of the 

 Nemertines^ have, however, added somewhat to the probability of my views, 

 in that they shew that in some existing Nemertines the nerve-cords 

 approach each other very closely in the dorsal line. 



With reference to the characters of the ancestor of the Chordata 

 the following pages contain a few tentative suggestions rather than 



^ Monograph on the development of Elasmohranch Fishes, pp. 170 — 173, 



- Hubrecht, " Zur Anat. u. Pliys. d. Nervensystems der Nemertinen." Kon. Akad. 



JViss. Amsterdam; and "Researches on the Nervous System of Nemertines." Quart. 



Journ. of Micr. Science, 1880. 



