Limbryogeny of the Bryozoa. 391 
in the Cellularina, to a lamina with two mamille, which, how- 
ever, soon becomes flattened, and then only forms a single 
rounded plate, by means of which fixation is effected. 
2. At the same time the issue of the aboral surface from 
the pallial cavity occurs, a phenomenon which takes place, 
like the devagination of the sac, with some rapidity ; we see 
the devaginations (devagination of the sac and devagination 
of the aboral surface) effected at once at the two poles of the 
larva, while the oral surface which unites these two poles at 
first remains nearly of the same length. 
3. Finally, the oral surface rolls up on itself so as to give 
origin to a torus nearly resembling that of the Escharina. 
By this process the margin of the aboral surface is brought 
into contact with the adhesive plate, and the cell is completely 
closed. The calcareous inerustation of its surface is preceded, 
as in the Eischarina, by the secretion of a cuticular envelope, 
the formation of which commences here very early, and im- 
mediately after the devagination of the aboral surface. 
4, The torus constituted by the oral surface and the upper 
portion of the adhesive plate is destined, as in the Escharina, 
to fall subsequently into degenerescence, forming a mass of 
opaque globules; further, the essential part of the polypide 
is also formed by the invagination of the apex of the aboral 
surface (representing the hood). 
5. The aspect of a flattened disk, which is at first assumed 
by the young cell of the Cyclostomata, makes its appearance 
very rapidly after the margin of the aboral surface has come 
into contact with the adhesive plate. It long retains its cir- 
cular form, with the rudiment of the polypide occupying ex- 
actly its centre. Soon after the invagination of the hood 
(which is effected in a straight line and not obliquely, as in 
the Escharina) this rudiment presents its two distinct layers ; 
but I cannot say what is the origin of the external muscular 
layer. A remarkable fact, and one that shows that the Cyclo- 
stomata separated earlier from the ancestral type and are of 
more ancient existence than the Chilostomata*, is that the 
complete rudiment of the polypide retains for a considerable 
time (during the whole of the stage of the discoidal cell) its 
* Tt is no doubt through this that their larvee present a greater reduc- 
tion of the internal organs than those of the Chilostomata (see the filia- 
tion of the larve in the conclusions); the two facts confirm one another 
and furnish a mutual support. Every one knows also that the Cyclo- 
stomata are much more widely distributed in geological formations than 
the Chilostomata, and that they especially predominate in the most 
ancient strata. The study of the structure of the larva and of the for- 
mation of the cell, and paleontology furnish us with perfectly concordant 
results, and are conclusive as to the antiquity of the Cyclostomata. 
