390 Dr. J. Barrois on the 
surface projects into the interior of the pallial cavity, so as to 
form the homologue of the hood; but this hood, which, how- 
ever, is not so well indicated as in the Chilostomata, never 
projects out of the cavity. 
Moreover the bottom of the sac rises here, as in the Cellu- 
larina, into a sort of papilla, which more or less fills the whole 
of the cavity. 
The larvee of the Discopore are remarkable in that their 
general aspect is the same as that of the larve: of the Chilo- 
stomata. heir general form is more or less discoidal ; and their 
posterior part, inflated by the sac, is much thinner than their 
anterior part: the latter, which is flattened, forms in the 
middle a very slight depression in the form of an elongated 
fissure, which is very visible, especially after the fixation. We 
have here therefore as it were a last trace, very feeble indeed, 
but still perceptible, of the pyriform organ of the larvee of the 
Chilostomata. 
On the other hand, we do not see in the larve of the Dis- 
copore (any more than in the other types of Cyclostomata) 
any trace of a ciliary circlet; but this difference is not funda- 
mental. We may regard the cells of the circlet as produced 
by the increase of the last row of the cells of the margin of 
the oral surface, an increase which may or may not exist 
without affecting the great structural features. ‘The segmen- 
tation shows that these cells are formed at the expense of the 
aboral half of the ovum ; but the study of the phenomena of 
the metamorphosis proves that they manifestly belong to the 
oral surface, of which we have every right to regard the ~ 
circlet as forming the boundary. If we pass from the larve 
of the Discoporee to those of the ordinary types, such as those 
of the Frondipore, Crisie, and 'Tubulipore, we shall see that 
a change has been effected analogous to that which takes 
place between the Escharina and bugula; that is to say, the 
anterior part of the oral surface has quitted the oral pole to 
become vertical (adapting itself to the enclosure of the ante- 
rior part of the oral surface in the circlet). In consequence of 
this change the larva loses its discoidal aspect to acquire that 
of a small cylinder, entirely constituted by the oral surface, 
and pierced at each extremity by an aperture; the superior 
aperture gives access to the sac, the inferior one to the pallial 
cavity. This is the type that I have figured in my first 
memoir. 
Metamorphosis. 
This greatly resembles that of the Escharina. 
1. The sac becomes devaginated to give origin, at first, as 
