Proboscidactyla flavicirrata — Campb ell 
339 
Proboscidactyla metamorphosis resembles that 
of other hydroids, despite the unusual final 
morphology of the polyp. Metamorphosis occurs 
only after settling, which apparently provides 
a trigger for the development of polyp features. 
Settling occurs on the future aboral surface. 
Mouth, tentacles, and the hypostomal dome are 
the first structures to appear, followed by the 
formation of the stolons more proximally on 
the stalk. 
A striking feature of the newly metamor- 
phosed Proboscidactyla is its ability to move 
across the substratum. Adult polyps have this 
same behavior pattern (Campbell, 1967), which 
is essential to colony maintenance. The move- 
ments can be interpreted in terms of stolon 
elongation with the polyp residing near the 
stolon tip. 
While in the mature Proboscidactyla colony 
movement is directed distally along the worm 
tube, thus maintaining the polyp on the edge 
of the tube as it is elongated, the movement 
of the newly metamorphosed larvae was not 
oriented with respect to the tube in the cultures 
observed. Thus, in some instances this move- 
ment carried the polyp father from the tube rim. 
This may be unusual behavior associated with 
laboratory conditions, or it may be a normal 
event which has an unappreciated selective ad- 
vantage. 
The adult Proboscidactyla polyp has two 
tentacles. The newly metamorphosed larvae ob- 
served generally possessed only one. Although 
the identity of the medusoid and polyp genera- 
tion of this species has not been established by 
complete life cycle studies in the laboratory 
(the most complete data are given by Uchida 
and Okuda, 1941:435-439), the identity ap- 
pears very probable. Presumably either these 
young polyps were abnormal, or a second ten- 
tacle arises later. However, another possible ex- 
planation for the single tentacle is that only 
those polyps developing asexually, during 
colonial expansion, have two tentacles; single- 
tentacled polyps may generally start colonies. 
SUMMARY 
Planulae of the hydroid Proboscidactyla 
jiavicirrata are caught in ciliary currents of 
sabellid worm radioles, attach to these radioles 
by means of nematocysts, and are scraped off 
onto the rim of the leathery tube as the worm 
retracts. This explains how Proboscidactyla 
colonies may become initiated on this site, and 
may explain why the colonies are not found 
elsewhere. 
Metamorphosis, apparently triggered by in- 
teraction with the worm, is generally typical for 
hydrozoans except that the final morphology 
is unusual. A young polyp moves across the 
substratum, stretching the proximal end into a 
trailing stolon. 
Most of the newly metamorphosed polyps ob- 
served had only one tentacle, although polyps in 
natural colonies normally have two. 
REFERENCES 
Brinckmann, A., and M. Vannucci. 1965. 
On the life-cycle of Proboscidactyla ornata 
(Hydromedusae, Proboscidactylidae) . Pubbl. 
Staz. Zool. Napoli 34:357-365. 
Campbell, R. D. 1967. Colony growth pattern 
in the commensal hydroid, Proboscidactyla 
flavicirrata. Biol. Bull. (In press.) 
Cazaux, C. 1958. Facteurs de la morphogenese 
chez un Hydraire polymorphe, Hydractinia 
echinata Flem. C. R. Hebd. Seances Acad. 
Sci., Paris 247 (23) :2195-2197. 
Hand, C. 1954. Three Pacific species of "Lar” 
(including a new species), their hosts, 
medusae, and relationships. (Coelenterate, 
Hydrozoa). Pacific Sci. 8 (1) : 5 1-67. 
Schijfsma, K. 1935. Observations on Hy- 
dractinia echinata (Flem.) and Eupagurus 
bernhardus (L.). Arch. neerl. Zool. 
1(3) :26l-3l4. 
Uchida, T., and S. Okuda, 1941. The hydroid 
Lar and the medusa Proboscidactyla. J. Fac. 
Sci. Hokkaido Univ. VI, 7(4) :431-440. 
