THE ANCHSTRY OF THE CHORDATA. 5d5 
pore, placed on the left side of the body, which in B. Kup fferi 
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 Balanoglossus, 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 
