DEVELOPMENT OF BALANOGLOSSUS KOWALEVSKII. 109 
it may be desirable to discuss briefly the new light which these 
facts throw upon the affinities of the Enteropneusta. 
In 1881 Metschnikoff published a detailed comparison of 
Balanoglossus with the Echinoderms, comparing Tornaria with 
Bipinnaria, showing that the resemblance is close, and con- 
cluding with the suggestion that Balanoglossus should be in- 
cluded among the Echinodermata in a separate division, 
“ Bilateralia.”” The branchial structures he compared to the 
openings from the body cavities of Echinoderms. This view, 
as thus expressed, receives no support from further observa- 
tions, and would now appear to be untenable. 
As mentioned above, all the Enteropneusta possess a sup- 
porting structure which is comparable with the notochord in 
every way, except in extent and in the persistence of its con- 
nection with the alimentary canal. Its resemblance to that of 
Amphioxus is especially striking, for in Amphioxus the noto- 
chord projects a long way in front of the mouth. It moreover 
possesses gill-slits which are not only without parallel, except 
among the Chordata, but also in structure, position, and de- 
velopment, agree exactly with those of Amphioxus, in which 
the slits acquire the same U-shaped form. 
The agreement in the position of the blood-vessels and 
skeleton of the gill bars is also very close. The fact of their 
gradual increase in number from before backwards throughout 
life is another common feature. 
The position and mode of origin of the central nervous 
system is also similar in both forms; the invagination of the 
dorsal cord in Balanoglossus being, however, only partial, while 
that of Amphioxus is complete. 
The mesoblastic pouches suggest the same resemblance, 
differing oniy from those of Amphioxus in number, being one 
median and four lateral, while those of Amphioxus are one 
median and twenty-eight lateral. As I have already pointed 
out, the fate of this anterior pouch is in the two animals closely 
similar. In both it is divided into two as the notochord grows 
forward. In Amphioxus the division is complete, while in 
Balanoglossus it is partial. In both, the backwardly-projecting 
