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 compara- 

 tive 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 earlier anato- 

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

 the Chaetopoda, and that the dorsal surface of the Chordata with the spinal 

 cord corresponds morphologically with the ventral surface of the Chastopods 

 with the ventral ganglion chain. In discussing this subject some time ago x 

 I suggested that we must look for the ancestors of the Chordata, not in 

 allies of the present Chaetopoda, but in a stock of segmented forms descend- 

 ed from the same unsegmented types as the Chaetopoda, 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 sug- 

 gestion as to its existence is well founded, appears now to have perished. 

 The recent researches of Hubrecht on the anatomy of the Nemertines 2 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 an attempt to deal with the whole subject ; while the 



1 Monograph on the development of Elasmobranch Fishes, pp. 170 173. 



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

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

 Jonrn. of Micr. Science, 1880. 



