308 
E. W, MACBRIDE. 
represents a, series of five consecutive sections tlirongli tlie 
hinder end of the pharynx to show how the collar-cavities 
and the pharynx cease simultaneously, and that at the 
same time the right and left rudiments of the splanchnocele 
meet in the mid-ventral line. If we examine the hinder end of 
the pharynx in a much older larva a similar set of appearances 
can be made out. Hence we conclude that as new gill-slits 
are added in the extraordinary manner described by Willey 
(37) the pharynx grows in length, and pari passu the collar- 
cavities and their enclosing atrial ridges which flank it. Van 
Wijhe (34) has described in the adult animals cavities in the 
edges of the oral hood and in the sides of the atrial cavity, 
which lie identifies with these inferior portions of the collar- 
cavities. He believes, however, that in the adult the upper 
portions of the collar-cavities become divided into several 
myotonies. His conclusions are very interesting, but owing to 
the lack of material of older stages on which I can depend I 
cannot either confirm or combat his assertions. I think, how- 
ever, that my explanation of the left-sided mouth is simpler 
than his. 
I said above that a plausible explanation of the asymmetry 
of the Amphioxus larvae, which has proved a puzzle to most 
investigators, had been suggested to me. The credit of this 
is due to my assistant, J. Stafford, Esq., Ph.D., but as he is 
unwilling to publish I give it here and take full responsibility 
for it. It is as follows: The pelagic larva of Amphioxus 
represents a pelagic ancestral condition of the race. Whilst 
in this condition the ancestors of Amphioxus were fully 
bilaterally symmetrical, and from this stock the rest of 
Vertebrata are descended. The adult Amphioxus leads a 
burrowing life in sand, a degenerate condition of affairs 
compared with its former free-living condition. Nevertheless, 
in both conditions of life the food and the mode of obtaining 
it are the same. By ciliary action small free-swimming 
organisms are whisked into the mouth, and the surplus water 
escapes by the gill-slits. Now Amphioxus, like the majority 
of fish, is a laterally compi-essed animal, taller than it is 
