74 MORGAN. [Von. IX. 
My main point of contention is this: the gill-baskets of 
Balanoglossus and Amphioxus are so similar that we must 
believe them to have had a common origin. On this evidence 
we may safely rest our conclusion of the relationship of the 
two forms. Admitting this, I have tried to force the issue 
to its legitimate conclusion, wishing to avoid the introduction 
of extreme phylogenetic speculation and to stick to observed 
facts as closely as possible. The gill-basket in Balanoglossus 
is formed on the dorso-lateral walls of a single metamere, but 
in Amphioxus the similar structure extends through a series 
of metameres. The conclusion seems to follow that the last 
metamere of Balanoglossus must correspond to the series of 
metameres of Amphioxus. 
We find in Balanoglossus that only a single metamere 
intervenes between the first true! gill-slit and the mouth, that 
lies between the first and second metameres. Similarly in 
Amphioxus the mouth lies in the young animal approximately 
between the first and second metameres, and the first pair 
of gill-slits is found in the region of the second and third 
metameres. 
Further, we see in Balanoglossus that the zxvaginated dorsal 
nerve cord can correspond only to the anterior end of the 
nerve cord of Amphioxus, and that the sawferfictal dorsal nerve- 
path, stretching through the gill region thence to the end 
of the body, must be the homologue of the remainder of the 
nerve cord of Amphioxus. 
1] leave out of account the possibility that the collar-pores may be modified 
gill-slits. 
