278 On the Embryogeny of the Bryozoa. 
all the aboral surface unfolds itself in proportion, issuing by 
degrees from the pallial cavity. In other words, there is a 
gradual inrolling of the circlet inwards, accompanied by an 
unrolling of the aboral surface outwards ; it is a peculiar kind 
of reversal of the mantle, which will terminate in the same 
results as the more sudden reversal of the Escharina, but by 
passing through absolutely different stages. 
We see further that, in the Ctenostomata, the portion des- 
tined to break up into globules and formed chiefly by the 
cells of the circlet, will not, as in the Escharina and Cellu- 
larina, have the form of a hollow ring, a torus surrounding the 
whole cavity of the vestibule, but that it consists of two dis- 
tinct parts, an enveloping portion and a portion enveloped, the 
latter alone enclosing the cavity of the vestibule. At first the 
enveloped portion really consists only of the two great lobes 
which bound the fissure of the oral surface ; but later on, and 
in proportion as the enveloping portion increases by the rolling 
inwards of the great cells of the circlet, we see the portion of 
these same cells which has already penetrated to the interior 
and forms the bottom of the peripheral cavity, rolled up 
again, but in the opposite direction, so as to join themselves 
to the lobes of the centre, which in this way receive a great 
increase. Finally, the long ribs which formed the circlet be- 
come divided into two nearly equal parts—an enveloped part 
which is joined to the lobes, and an enveloping part formed 
by the upper parts of the circlet. 
Thus all the cells of the ciliary circlet become folded upon 
themselves two or three times, giving the mass formed by 
their union a very complex aspect, which it is difficult to 
unravel. 
At the close of this period of the metamorphosis the mass 
constituted by the whole of the invaginated circlet is situated 
towards the bottom of the cell, of which it occupies the poste- 
rior and inferior parts (retaining the same orientation as im 
the case of the larva); the entire embryo has the aspect of 
a rounded sac, not at all flattened as in the Escharina, but 
exceedingly uniform and nearly spherical ; we can, however, 
still distinguish the point of closure of the aboral surface 
below, and the point occupied by the extremity of the hood. 
It is in the space left free by the mass of the invaginated 
circlet—that is to say, in the antertor and superior portion of 
the embryo (orientation of the larva)—that the polypide will 
be formed. Unfortunately I have been unable here to inves- 
tigate the important question of the origin of the polypide in 
the same detailed manner as in the Escharina ; nevertheless I 
can establish an important point, namely that there is no 
