Embryogeny of the Bryozoa. 397 
nal organs which accompanies it, and even in (3) the very 
existence of this mantle, we must see only a series of purely 
adaptive characters, which, although very important in the 
distribution of the larvee of Bryozoa in a natural series, do 
not belong at all to the normal development, and are only pro- 
ducts of life in a free state, between the hatching of the larva 
and its fixation. 
All this is demonstrated :—1, by the important fact that 
the larvee of Entoprocta (which, as every thing indicates, must 
be regarded as the ancestral form) are of all the least different 
from the adult form; 2, by the fact that, starting from this 
first form, we can now follow step by step the modifications 
of the different other forms; lastly, 3, by the fact that with 
all these other forms the first phenomena of the metamor- 
phosis consist in a return, not to the state of a cystide, but to 
an arrangement analogous to that of the larve of Entoprocta, 
and nearer to that of the adult form ; for this is the significa- 
tion that must be ascribed to the important phenomenon of 
the reversal of the mantle, with disappearance of the pallial 
cavity, and reconstruction of the cavity of the vestibule. 
The regular course of development would consist in the 
direct transformation into the adult of an organism resembling 
a larva of Entoprocta; the rest (that is to say, all that serves 
to change the Entoproct larval type to give origin to the 
other larval types that I have described) only constitutes purely 
adaptive phenomena, perturbatory of the regular course of the 
embryogeny, and acquired during the course of life in the 
free state. 
MECHANISM OF THE T'RANSFORMATION.—CHARACTERS OF 
THE ADULT, 
Let us now endeavour to form a general idea of the manner 
in which the adult state is fashioned, starting from the common 
stage which follows fixation; by this means alone we shall 
find the answer tothe problem that we set before us at the 
commencement—namely, to unite the two successive forms 
(larva, adult) of the embryogeny, and especially to appreciate 
the structure of the adult according to the data of embryogeny. 
It is not difficult to refer to the same type the two principal 
modes that we have described of the passage from the larva 
to the adult in the Emtoprocta and Hctoprocta. The first 
phenomenon in either case consists in the immersion of the 
oral surface within the embryo, accompanied by an extension 
of the aboral surface, which spreads out so as to form the 
whole of the integument which will give origin to the defini- 
tive cell. 
