Embryogeny of the Bryozoa. 277 
Metamorphosis (fig. 8). 
This latter character has a very great influence upon the 
metamorphosis of the larve of the Ctenostomata, inasmuch as 
the devagination of the sac observed in the Escharina and 
Cellularina is no longer met with, the whole being reduced to 
the reversal of the mantle. 
Moreover the length of the cells of the circlet, which had 
no effect in the metamorphosis of the Cellularina, here has a 
great influence upon the course of the phenomena, and pro- 
duces a kind of reversal of the mantle absolutely different 
from that we have seen, each of the large cells which consti- 
tute the circlet having to fold several times upon itself before 
penetrating to the interior of the embryo. 
The phenomena in general occur as follows :— 
The whole of the elongated band which, in the larva, 
forms the whole of the oral surface reduced by the circlet (that 
is to say, the free and rounded part which immediately 
surrounds the oral pole, together with the portion enclosed in 
the circlet), sinks into the interior of the embryo, producing 
a long fissure, over which the neighbouring parts close. 
These parts are here formed exclusively by the two sym- 
metrical portions of the circlet which were included between 
the two free and enclosed divisions of the oral surface. When 
this last has buried itself in the interior, these two symmetri- 
cal prolongations of the circlet become inflated into two large 
lobes which bound the fissure and which are formed by the 
antero-inferior part of the cells of the circlet, the extremities 
of which are folded back upon themselves. 
Here, in the absence of a special organ, it would seem that 
the rather sudden drawing back of the oral surface acts in the 
manner of a piston to produce fixation, which is effected by 
the projecting lobes which bound the median fissure above, 
and generally by their posterior part. ‘The cavity V (fig. 8) 
to which this drawing back gives origin, and which is bounded 
by the two lobes in question, constitutes a first and important 
cavity concealed by the lobes L (fig. 8); it represents the 
vestibule here. A second cavity, peripheral to the lobes, is 
soon formed around the former one, and results from the re- 
versal of the ciliary circlet which closes over the two lobes. 
This reversal does not take place at all in the same manner 
as in the larvee of the Escharina and Cellularina: the cells of 
the circlet, instead of turning bodily, taking as their turning- 
point the line of junction with the oral surtace, roll themselves 
up in their lower portion, in this way penetrating gradually 
into the embryo; while at the same time, at the upper part, 
