Embryogeny of the Bryozoa. 395 
first enclosed in the interior of a cavity (the vestibule) and 
entirely concealed by the aboral surface, gradually encroaches 
more and more upon the outer surface, until it forms by itself 
the external integument, in its turn pushing back the aboral 
surface into the interior of a cavity, the pallial cavity. In 
the most differentiated types of the group of the Chilosto- 
mata and Ctenostomata we have already seen that the aboral 
surface was almost entirely buried in the interior (fig. 3) ; 
nevertheless in this type the apex of that surface, which con- 
stitutes the organ called the hood, never ceases to project ex- 
ternally, and is never concealed by the cells of the circlet. 
In the Cyclostomata it is quite otherwise; the pallial cavity 
attains its maximum of extension, becomes transformed into 
a closed cavity, and the oral surface closes completely over 
the invaginated aboral surface. 
As internal organs we find the sac very well developed and 
quite comparable to what we had in the Escharina; the 
thickening of the hood also exists; but we find that there is 
a complete disappearance of the vitelline mass of the two pre- 
ceding types; the intestine, which is very well developed in 
the larvee of Entoprocta, and reduced to a mass of globules in 
the larve of Chilostomata and Ctenostomata, disappears com- 
pletely in the larve of Cyclostomata. 
In a general way, the larva consists of an entirely ciliated 
sac pierced at each pole by a single aperture, which is very 
dilatable, but scarcely visible when it is closed; this sac is 
formed entirely by the oral surface. The superior (oral) 
aperture leads into the internal sac (1), the inferior (aboral) 
aperture into the pallial cavity. We see that the phenome- 
non of the reversal of the mantle must produce here a trans- 
formation still more complete than in the larvee of Escharina; 
the larve of the Cyclostomata have diverged still more than 
the latter from the original arrangement (Hintoprocta), which 
was nearer that which occurs in the adult. 
5. LopHopopa.—The larvee of the Lophopoda are to the 
larvee of Cyclostomata what those of the Ctenostomata are to 
-the Chilostomata; they resemble larvee of Cyclostomata of 
which the sac has completely disappeared. This disappear- 
ance 1s only more complete than that I have indicated in the 
larvee of Ctenostomata; the sac is suppressed without leaving 
any traces. ‘Thus of the three parts indicated in the interior 
we no longer find more than one, namely no. 3, which 
of itself forms the entire polypide. The rest of the structure 
is comparable to that of the Cyclostomata, as in the latter 
the oral surface forms the whole of the integument and is 
closed up completely over the mvaginated aboral surface. 
