108 Papers from the Marine Biological Laboratory at Tortugas. 
end of the worm, and ceases abruptly at the point marked a in figure 1, 
which is the segment separating the narrow middle part of the worm from 
the swollen sexual part of its body. 
The sexual segments are thus twisted off at the point a, and on being 
set free they swim vertically upward to the surface, where the posterior 
end of the worm continues to progress rapidly along, moving backward, 
as is shown in figure 2. 
The male sexual ends are salmon red or dull pink, while the females are 
greenish-gray or drab, so that they can readily be distinguished at a glance. 
If while the sexual end is swimming we cut it into pieces, each sepa- 
rate length continues to swim backward with its characteristic rolling move- 
ment. This shows that the stimulus which produces the twisting movement 
is not localized, but is developed throughout the sexual end of the worm. 
The worms continue to swim in all directions over the surface, and show 
no tendency to congregate in masses, each worm pursuing its own course 
without regard to its fellows of either sex. I have seen them in such 
abundance over the surface above the coral reefs at Tortugas that hardly 
a square foot of the surface was free of a worm. 
When the sun is about to rise, and the first faint rays of light fall 
over the ocean, the worms begin to contract violently, so that the sexual 
products are cast out through rents and tears in the dermo-muscular wall, 
and the torn and shriveled cuticula sinks down to die upon the bottom 
(figure 3). Light is not the sole, but only a contributory, cause of this 
muscular spasm of contraction, for it will take place even in swimming 
worms which have been removed to a dark-room, although in this case the 
“bursting”? of the worm may be delayed for an hour or more after all 
of its fellows in nature have cast forth their genital products and disap- 
peared. Moreover, some few of the worms of the swarm discharge their 
genital products before the rising of the sun. Any mechanical shock will 
bring about an instant bursting of the worm, the females being far more 
sensitive than the males. It is comparatively easy to stupefy the males by 
slowly adding alcohol to the water and killing them without their bursting ; 
but this is more difficult of accomplishment in the case of the females. 
After casting off its posterior sexual segments, the anterior part of the 
worm crawls back into its burrow and regenerates a new sexual end. Only 
the mature worms cast off their posterior ends; the immature worms take 
no part in the swarming reaction. 
This swarming of the Atlantic palolo has been observed for nine years 
at Tortugas, Florida. The principal swarm commonly occurs within three 
days of the day of the last quarter of the June 29 to July 28 moon, although 
smaller swarms may occur upon one or two days preceding or succeeding 
the day of the densest swarm. When the last quarter of the moon falls 
< late in July there may be a response to the first quarter as well as to the last 
