1880. | Development of Amphioxus lanceolatus. 93 
been generally considered that the mouth cavity of Amphioxus 
was formed by the anterior portions of the side folds overlapping 
and finally enclosing the anterior end of the pharynx, and by this 
method of development making the mouth cavity an anterior 
portion of the cavity surrounding the pharynx. But in none of 
my specimens could any evidences of any such method of forma- 
tion be seen, and there are, moreover, certain facts in connection 
with the formation of the mouth cavity which would seem of 
themselves to corroborate, to a certain extent at least, my view of 
the development. First, the mouth being formed upon the left 
side of the body, has, in the anterior exodermic expansion, a pre- 
viously-formed right wall, and any extension of the right fold 
over this portion of the body would only form a third layer over 
the side and not be of any value in the formation of the cavity, 
or connect it in any manner with the cavity surrounding the 
pharynx. Second, the left fold, in order to form the left side of 
the cavity would have to pass forward over the pharyngeal mouth 
aperture, and in so doing would so affect the inward flow of food 
material that this forward growth could be easily observed, pro- 
vided any such forward growth took place. But the left fold is 
always limited anteriorly, so far as I have observed, by the mouth 
welt, and advances towards the cartilaginous ring only as the 
mouth welt advances towards the edge of the ciliated pit, hence 
my investigations have led me to the conclusion already men- 
tioned, that the mouth cavity is formed by a true introversion of 
the exoderm, and is genetically distinct from the branchial cavity. 
For this reason, combined with the fact that the branchial cavity 
is essentially branchial in its function, and quite different in its | 
formation from the atrium of some at least! of the Tunicates, 
its walls having more of the nature, in their development, of the — 
gill covers or opercula of osseous fishes,? and because it does not 
*The Anatomy of Invertebrated Animals. By T. H. Huxley. London, 1877, 
P- 609. Eo 
: ? To indicate my meaning as to the homology of the branchium, I would say that = 
if those portions of the side folds, which by their union form the median ventral fin, — oo 
should be extended forward to a point along the cesophagus, and by their union or 
Coalescense at this point completely cut off the posterior from the anterior cavity; 
and the ducts of the uriniferous ridges should extend back to the anus, where they — 
could open into a cloaca, and the generative products instead of bursting through the 
lining of the walls into the branchium, be carried back between the lining and the 
muscles to the cloaca and there discharged, or in other words, if the pleuro-] per 
neal cavities of the roof of the branchium should become more enlarged in the } 
