392 - Dr. J. Barrois on the 
rounded structure, composed of two concentric hemispheres, 
suspended from the place where the hood was situated. It is 
only later, at the period when the cell loses its radiate sym- 
metry and acquires a bilateral structure, that the rudiment 
becomes converted into a small closed sac with a double wall, 
as in the Chilostomata. The epoch of this change coincides 
with that in which the internal torus becomes emarginate on 
one side to acquire a horseshoe-like shape—a change which 
is itself connected with the appearance of the first indication 
of the vertical tube of the definitive cell. The formation of 
this tube commences by an elevation of the anterior part of 
the discoidal cell. In conclusion it may be mentioned that 
the transparent collar with which the base of the discoidal 
cell is furnished is produced at the expense of the adhesive 
plate, of which it represents the cuticular part. 
VI. LopHopopa. 
The facts which I have described for the preceding species 
lead us to give a new interpretation to the facts already ac- 
quired as to the freshwater Bryozoa (embryogeny of Alcyo- 
nella), facts which suffice to enable us, notwithstanding their 
still incomplete state, to correlate them closely with the 
marine forms and to assign them a well-defined position. 
Larva.—The known development is very simple :—(1) 
a blastula is formed, in the midst of which appears (2) a fold 
which rises above one of the two halves, so as to envelop it 
completely ; lastly (3) a young polypide buds forth internally 
upon the part thus enveloped. 
There seems to me to be no doubt from what precedes 
(especially if we refer back to the development of the Cyclo- 
stomata) that this blastula is a pseudoblastula, and the an- 
nular fold the mantle of the larva. ‘The covered half, together 
with the internal lamina of the mantle, represents the aboral 
surface; the second half, in conjunction with the external 
lamina of the mantle, represents the oral surface. . Lastly, 
the apex of the covered half, which gives origin to the poly- 
pide by its internal budding, represents the hood. 
Metamorphosis.—The study of the metamorphosis confirms 
these homologies. The larva, in fact, becomes fixed by the 
oral pole—that is to say, by the extremity opposite to the 
aperture which leads into the pallial cavity. 
Afterwards the reversal of the mantle takes place, in con- 
sequence of which the whole aboral surface issues from the 
pallial cavity to form the integument of the cell ; while the oral 
surface is entirely covered so as to form a thick internal mass, 
which soon becomes degenerescent, forming a mass of globules 
