204 EDMUND B, WILSON, 
line just behind the circlet of the arms. This thickening 
is apparently produced by an infolding of the body wall; 
at any rate, it soon becomes hollow and pouch-like, and 
communicates with the exterior by means of a small opening, 
while fibrous or muscular connections between it and the 
stomach walls may be observed. The pouch grows rapidly, 
and at length comes to occupy a large part of the peri- 
visceral cavity, doubling back and forth, and becoming 
transversely folded on its inner wall. 
When the pouch has attained its complete development 
the larva becomes sluggish, sinks to the bottom, and the 
body usually contracts forcibly, so that it becomes shortened 
and rounded, and the walls become tense from the internal 
pressure thus produced. Suddenly, at what Metschnikoff 
appropriately terms the “critical stage” of the metamor- 
phosis, the pouch turns itself outward through its external 
opening, unrolling like the finger of a glove or the eye ten- 
tacle of a snail. As it unrolls, the middle part of the diges- 
tive canal is drawn out into its cavity, thus forming a long 
U-shaped loop. At the same time the larval body shrinks 
together, and is doubled up toward the dorsal side, so that 
the mouth and anus are brought close together. The hood 
is withdrawn into the esophagus, leaving only a small rem- 
nant, which overhangs the mouth and persists as an epis- 
tome. The greater part of each swimming arm undergoes a 
kind of granular disintegration and drops off, leaving only a 
small thickened basal portion, which becomes directed for- 
wards, and subsequently grows into the corresponding ten- 
tacle of the adult Phoronis. The larval body fuses com- 
pletely with the everted pouch, which now constitutes the 
greater part of the body of the worm. The subsequent 
external changes consist merely in the elongation of the 
tentacles and a great increase in their number, and the 
elongation of the body. The worm secretes for itself a 
membranous tube, in which it dwells, protruding only the 
extremity with its crown of tentacles. The resemblance of 
the latter to the lophophore of a hippocrepian Polyzo6n is 
wonderfully close, though this resemblance is undoubtedly 
adaptive and secondary. A clear idea of the metamorphosis 
may be gained from the diagrammatic figs. 15 to 19. 
Figure 1 represents a lateral view of the first stage figured 
by Metschnikoff. Ath. is the hood; m.is the mouth lead- 
ing by a short cesophagus to the stomach (s¢.). The latter 
communicates by a short intestine with the anus (a). Rudi- 
ments of two swimming arms are seen at a’, a.” Fig. 2 
gives a ventral view of the same; rudiments of the second 
