THE ANCESTEY OF THE CHOBDATA. 87 



pore^ placed on the left side of the body, which in B. Kupfferi 

 is stated to be paired. 



From the middle body cavities open a pair of pores into the 

 atrial cavity, which is partly enclosed by 



(11) A rudimentary operculum. 



Having these facts in view, and having set aside the pre- 

 liminary objection that no high degree of segmentation is pre- 

 sent in Balauoglossus, we may consider their bearing on 

 theories as to the ancestry of the Chordata. 



Previous Suggestions as to the Ancestry of the 



Chordata. 



Setting aside the possibility of Annelids having been geneti- 

 cally connected with the Chordata, the most notable alternative 

 suggestion is that of Balfour, that the Nemertines might be 

 thus regarded. This view has been supported and extended by 

 Hubrecht. It has thus been thought that the Chordate nervous 

 system might have arisen by the longitudinal coalescence of 

 two such cords as are present in Nemertines. But even the 

 facts of other Chordate developments almost preclude the 

 view that their nervous system is a double structure ; the 

 medullary plate of Amphioxus is distinctly single, and it is only 

 in the medullary folds of higher and more complex forms that 

 even an appearance of a double structure is produced, while no 

 really double origin occurs. This being so, the mode of origin 

 in Balanoglossus is practically conclusive against the theory of 

 double origin. It is possible, and even likely, that Nemertines 

 bear some distant relation to Chordata, as will be further dis- 

 cussed subsequently, but if this is so it can no longer be sup- 

 posed that their nervous system is other than a special develop- 

 ment within the group. 



In most speculations as to the origin of Vertebrata, it is 

 assumed that all the lower forms of Chordata are degenerate. 

 The supporters of the Annelid theory especially are compelled 

 to resort to this view severally in the case of the Ascidians 

 Amphioxus, and the Marsipobranchs. These, with the excep- 

 tion of the Enteropneusta, are the only forms which could have 



