380 LEFEVRE. [VoL. XIV. 
lost at an early stage, namely, ‘at a time when the two peri- 
branchial pouches have merely begun to envelop the branchial 
sac.” I find that it persists for a very much longer time, and 
is still present, although greatly constricted, at a stage when 
some of the gill slits have been formed and the peribranchial 
cavity has been wholly separated from the branchial sac. PI. 
XXXI, Fig. 24, shows the connection (7s¢.c.) at such a stage. 
The first indication of the right peribranchial sac is a slight 
longitudinal folding-in of the wall of the inner vesicle some 
distance up on the right side, which appears after the shifting 
of the vesicle has begun. This furrow starts a little in front of 
the anterior termination of the left peribranchial fold, and as it 
deepens and extends posteriorly, it is gradually carried down 
towards the ventral side, in the same way as the pericardial 
rudiment. It is already present at the stage represented in 
Pl. XXX, Fig. 11, but has not yet reached back far enough to 
appear in a section which shows the left fold. In Fig. 12, 
which is taken from the same series of sections, but a little 
further forward, it is well marked (f17.f0.s.). 
As the shifting continues, the inner vesicle tends more and 
more to assume a symmetrical position. The two peribranchial 
furrows, which deepen rapidly and run in obliquely to meet 
each other, do not come together on the dorsal surface of the 
vesicle, but some distance below it. The result of this is that 
when the right and left peribranchial sacs are separated from 
the inner vesicle a median dorsal portion connecting them is 
cut off at the same time. This median piece, hence, does not 
arise, as Kowalewsky (12 and 13) describes, from the fusion of 
the lateral sacs dorsally, but the three portions are formed by 
one and the same process. We now find a median vesicle, the 
later pharynx, surrounded dorsally and laterally by a saddle- 
shaped bag which consists of the dorsal or cloacal and the lat- 
eral divisions of the peribranchial cavity. This is essentially 
the same process as that which Pizon (22) and Hjort (8) have 
described for Botryllus. 
In Perophora the folds which separate the peribranchial cavity 
from the inner vesicle do not involve the entire length of the 
latter, but leave nearly the whole of the anterior half undivided, 
