Pro boscidactyla flavicirrata — Cam p b e l l 
scrape the planulae off onto the rim. This trans- 
fer was also made possible by a stickiness of the 
planulae, which developed after the contact 
with the worm. At least 20 minutes elapsed be- 
tween attachment to the pinnules and transfer 
to the tube rim, and most retractions of the 
worm were unsuccessful in achieving this 
transfer. Probably the elapsed time represents 
the time required for secretion of an adhesive 
material. 
Accidental contacts between the swimming 
planulae and the worm tube were refractile, and 
had no effect on the activity of the planulae. 
However, after a planula had been transferred 
normally to the tube it would undergo metamor- 
phosis even if removed from the tube. 
The visible onset of metamorphosis occurs 
about 6 hours after settling. Figure 2 shows the 
mature planula before attachment. Nematocysts 
are more abundant at the anterior end. The 
planula is characteristically spindle shaped. 
Figure 3 shows a planula several hours after 
attachment, as the first visible signs of metamor- 
phosis become apparent. There is a loss of 
refractile quality of the endoderm at the future 
oral pole. This reflects the formation of a well 
delineated high columnar epithelium which is 
characteristic of the hypostome of the adult 
polyp. Also at this time the mouth has begun 
to form. The animal in Figure 4 shows a pro- 
trusion in the body wall below the hypostomal 
region. This evagination becomes a single ten- 
tacle, which elongates rapidly (Fig. 5). 
Within several hours another protrusion is 
seen developing on the body wall (Fig. 5). It 
always develops after the tentacle has been initi- 
ated, and usually after the tentacle has elongated. 
It is always on the same side of the body as the 
tentacle, and is generally situated in the middle 
of the body. This protuberance develops into 
the "foot” (Campbell, 1967) which may be 
homologous with the stolon tip in other hy- 
droids. The segment of polyp posterior to the 
foot adheres to the substratum, secretes a very 
fine perisarc, and becomes the stolon. The foot 
marks the anterior extent of the attachment to 
the substratum, and the distal portion of the 
polyp remains erect from the substratum. 
During the next 24 hours, the observed, 
single-tentacled polyps began to glide along the 
substratum. The polyp column posterior to the 
337 
"foot” elongates during polyp movement, be- 
coming a stolon. Figure 6 shows a polyp just 
beginning its movement. The stolon elongates 
not by terminal extension, for the aboral end 
remains fixed with respect to the substratum (or 
it may actually move in the direction of the 
polyp). Elongation is apparently due to stretch- 
ing by the advancing polyp. The mechanism of 
this advancement was not determined, but the 
movement appeared similar or identical to that 
of mature polyps (Campbell, 1967). It was al- 
ways oriented in the direction of the tentacle 
and foot. 
About 30 young polyps were raised for more 
than a week during which time they were fed 
sabellid eggs. None of them during this time 
developed a second tentacle. Of about 250 
metamorphosing polyps studied, however, 2 
possessed two tentacles just after metamor- 
phosis (Fig. 7). The formation of these paired 
tentacles was not observed. 
After metamorphosis, the young polyps on 
the worm tube were not oriented with respect 
to the axis of the tube, although the majority 
were right at the edge of the tube. Movement 
carried some further from the rim. The behavior 
of these young polyps was not observed for a 
longer time, and so it is not known whether 
all of them were capable of forming colonies. 
DISCUSSION 
Interaction between the worm and the planula 
appears to be a prerequisite to settling in 
Proboscidactyla flavicirrata. Since planulae de- 
prived of contact with worms did not undergo 
metamorphosis during the more than 2 weeks of 
observation, the contact itself is probably a 
stimulus for metamorphosis. It is possible that 
nematocyst discharge is a direct part of this 
stimulation. These conclusions explain how the 
close association between the hydroid colony and 
worm tube, and the polyp’s initial position on 
the rim of the tube, may arise. It would be in- 
teresting to know if other species of worms or 
other animals, which must also draw Probosci- 
dactyla planulae into contact with themselves, 
similarly stimulate the planulae to settle and 
undergo metamorphosis. If there were a species 
specificity involved, this intricate interaction 
could present an explanation of the specificity 
