402 Dr. J. Barrois on the 
metamorphosis that we have observed. The larva, in creep- 
ing, must, by means of its cilia, give rise to a current des- 
tined ‘to carry towards its mouth the alimentary particles upon 
which the animal feeds; now we may suppose that this 
current, passing beyond the mouth, comes to strike against the 
anal extremity of the vestibule, pushing back the latter gra- 
dually towards the superior extremity of the larva, and finally 
producing the movement of rotation of all this part of the 
oral surface, carrying with it the digestive tube. We thus 
obtain a mechanical explanation of the fundamental pheno- 
menon of the metamorphosis. 
To conclude, we may put forward the hypothesis of the 
very ancient existence of a group of Probryozoa, composed of 
swimming organisms, free, and possibly analogous to the 
Rotifera (at least as regards the aspect and general arrange- 
ment of the body), and of which the few larve of Entoprocta 
that we know now-a-days represent the sole survivors; from 
this group the existing Bryozoa are derived by adaptation to 
a new mode of life: certain larvee have accustomed themselves 
to creep, as indicated by fig. 9, upon their oral surface, in- 
stead of swimming freely through the water; and hence the 
changes already described, which produce the Bryozoan form, 
and the efficient cause of which may be ascribed to the current 
which flows into the mouth of the animal. 
EXPLANATION OF PLATE XIV. 
Explanation of the letters. 
1, Rudiment of the polypide (hood). 
2. Intestine or vitelline mass, 
3. Internal sac or cloaca. 
Fr, Frontal surface of adult. 
Tg. Tergal surface, 
Pd, Inferior surface or foot. 
ant, Subbuccal (Entoproct) or prebuccal (Ectoproct) organ indicating the 
anterior part of the larva. 
9. Vestibule. 
a. Pallial cavity. 
fun, Attenuated portion of the oral surface, uniting the half which forms 
part of the polypide to that which forms the rest of the invagi- 
nated vestibule. 
Explanation of the figures. 
The parts in black represent the oral surface, the parts in grey the ab- 
oral surface ; the internal organs are left white, 
Figs. 1-5 represent the five principal larval types, showing the variable 
extent of the oral (black) and aboral (grey) surfaces in each of them, as 
well as the extent attained by the three principal internal organs (1, 2, 8). 
