394 Dr. J. Barrois on the 
rangement of these parts in the larve of Entoprocta. It is 
here that they affect an arrangement nearest to that they will 
have in the adult. The aboral surface already forms the 
integument of the larva, and the oral surface is retractile and 
capable of closing to form a vestibule ; it only needs a rotation 
of the organs (1) and (2), the incubatory pouch and intestine, 
‘these quitting the vestibule to place themselves in connexion 
with the organ (3), the labial thickening of the larvee of Ento- 
procta, to pass from the larval to the adult form. All the 
five parts are well represented, and occupy an arrangement as 
near as possibte to that which they are to present in the adult, 
2. CurLostomaTA.—In figure 2 it is otherwise; the cirelet 
has grown towards the aboral pole, raising itself above the 
aboral surface, and thus giving origin to a new cavity, the 
pallial cavity. At the same time the oral surface has lost the 
power of retracting itself into the interior in the form of a 
vestibule. From all this we can foresee that there will be a 
necessity for greater changes in the metamorphosis. in the 
Entoprocta, in order to pass to the first stage following fixa- 
tion, it suffices for the larva to rest upon the margin of the 
circlet ; here a greater change is necessary ; before arriving at 
the corresponding stage a preliminary phenomenon must take 
place; and this consists in the reversal of the mantle. 
The three internal parts (sac, intestine, and hood) are cer- 
tainly present ; but the second has become degenerescent,, and 
now only consists of a mass of globules in which one can no 
longer distinguish any organization. 
3. CTENOSTOMATA (fig. 3).—The type represented by the 
larve of Escharina has attained its full development among 
the Ctenostomata. Here the pallial cavity has become enor- 
mous, and the cells of the circlet form long ribs which occupy 
nearly the whole integument of the larva; the latter now 
merely represents a sort of little cask formed entirely by the ribs 
of the circlet, and at each end of which appears what remains 
of the primitive oral and aboral surfaces. ‘The three internal 
parts are as in the larve of Escharina; nevertheless we find 
that the sac is reduced to quite rudimentary dimensions, and 
is evidently in course of disappearance. 
4, CYCLOSTOMATA.—Figure 41s a Cyclostome larva. The 
circlet is deficient all through this larval type; but the oral 
surface, of which it is only the boundary, has continued to 
grow towards the aboral pole, until at last it closes altogether 
above the apex of the aboral surface. Here we find carried to 
its highest expression the process which we have seen be- 
coming more and more strongly marked starting from the 
Entoprocta, and in consequence of which the oral surface, at 
