CHAPTER XII. 



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 Chord ata ; 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 tlie view, already tentatively put forward by rarlier 

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

 amongst the Chsetopoda, and that the dorsal surface of the Chordata with 

 the spinal cord corresponds morphologically with the ventral surface of 

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

 some time ago 1 I suggested that we must look for the ancestors of the 

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

 forms descended from the same unsegmented t^ypes as the Chsetopoda, but 

 in which two lateral nerve-cords, like those of Nemrrtinps, coalesced dorsallv, 

 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 

 Nernertmes 2 Lave, 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 



3 Monograph on the ilcn-liijnin'nt of Elamnobranch Fishex, pp. 170 173. 



- Hubrecht, " Zur Auat. u. Phys. d. Nervensystenis der Nemertinen." Ki'm. A kail. 

 IFV.v.s. Amsterdam : aud "Researches on the Nervous System of Neuiertines.'' 

 Journ. of Slier. Science, 18HO. 



