396 Dr. J. Barrois on the 
The following Table, which reduces to three great types 
the different forms of the larvee of Bryozoa, will show a reca- 
pitulation of these different characters :— 
Predominance of the aboral surface; vesti- 
ENTOPROCTA........00- bule at the maximum. 
Intestine well formed. 
CHILOSTOMATA and CrE- 
NOSTOMATA (sac. Te- Predominance of the circlet ; pallial cavity. 
Intestine reduced to a mass of globules. 
duced) aria wasice te 
Predominance of the oral surface. Pallial 
pee eee cavity at the maximum. 
PHOPODA (withoutsac). Intestine has disappeared. 
If we place ourselves, in the first place, at the point of 
view of the larval forms alone, it would appear that we have 
here an essential character based upon the antagonism of the 
two great cavities which occupy the two poles, and, in final 
analysis, upon the greater or less development of the mantle. 
It is in accordance with the extension of the latter that each 
of the two surfaces of which the embryo consists may be by 
turns enclosed in the interior (the oral surface invaginated as 
a vestibule, or the aboral surface as a pallial cavity) or may 
form the totality of the integument of the larva; further, we 
ascertain that when the medium extension of the mantle (2) 
occurs, we also find the state of incomplete disappearance of 
the intestine (replaced only by a mass of globules), while the 
complete disappearance of the intestine corresponds to the 
case of maximum extension. In one word, it would seem, to 
a certain extent, that we have here an essential character, to 
which all the others appear to be subordinated, and which en- 
ables us to arrange all the larval forms in a single series, at 
the same time progressive from the point of view of the ex- 
tension of the mantle, and decrescent as regards the develop- 
ment of the internal organs. 
If we now place ourselves at a more general point of view, 
embracing at one glance the entire development, we shall see 
that this character of development of the mantle, although 
serving to establish the filiation of the larvee, has nevertheless 
no considerable importance as regards the development taken 
as a whole, since all the forms of larve, to whatever type they 
may belong, are invariably brought to the same type by the 
first changes which follow fixation, a common type in which 
all trace of the mantle disappears to give place to a stage in 
which the oral surface is immersed as a vestibule and the 
aboral surface exposed as integument. 
Concluston.—In (1) the development of the mantle in the 
larve of Bryozoa, as well as in (2) the reduction of the inter- 
