50 HENRY BURY. 
tions on each side unite first (fig. 5), then the two sides unite 
ventrally, and at a later period their union on the dorsal side 
completes the ring. 
Mouth and Atrium.—Before describing the formation of 
the three anterior rings, we must turn for a moment to the 
behaviour of the mouth, and the formation of the atrial cavity. 
Quite early in the metamorphosis four pieces of the ciliated 
band group themselves round the mouth and there form a ring 
(figs. 3 and 4; and 21, pl. ii, figs. 14—16). The fate of 
the “nerve bands” (fig. 1) I have never been able to deter- 
mine. 
In fig. 4. it will be noticed that the mouth is pushed over to 
the left side of the larva, while the apex of the latter is turned 
considerably to the right. The latter change is partly visible 
in fig. 3, but varies considerably in different larve, being 
always most marked in spirit specimens in which some 
shrinkage has occurred. 
In the next stage the mouth retreats into the interior, and 
an atrial cavity is formed, the external opening of which narrows 
rapidly and passes over to the left side (fig. 5); the apex of 
the larva has meantime nearly regained its original position, 
though it is still much to the right of a line passing through 
the longitudinal axis of the stomach. 
The portions of the ciliated band which, as above described, 
encircle the mouth, now lie at the bottom of the atrial cavity, 
where they form, as Metschnikoff described, the epithelium 
of the tentacles. Though they form a ring, I have not included 
it among the ciliated rings of the pupa, of which there are five 
outside the atrium. 
Figs. 4.and 5 represent such well-marked stages in the meta- 
morphosis that it will be convenient to refer to them in future 
as marking, respectively, stage A (before the atrium is formed) 
and stage B) with the aperture of the atrium not yet terminal 
and the five ciliated rings still very incomplete). We will now 
return to these ciliated rings, to assist the study of which I 
have given in fig. 6 a diagrammatic view of the anterior 
pole of Auricularia, constructed on the same lines as fig. 3. 
