Mr Potts, The Swarming of Odontosyllis. 199 
(1) that these great swarms of nereids are only seen rarely ; 
(2) that they occur at almost any time of the day (at early 
‘morning, at midday* or in the evening) and nearly any period 
of the summer ; 
(3) that the date of swarming has no definite relation to the 
full moon. 
On the whole I think we are justified in stating that in J. 
dumerilu and probably other species the swarming habit is not 
fixed. Hempelmann has noticed a slight correlation between the 
appearance of the heteronereids and the phases of the moon, but 
this is by no means marked. There is no doubt from Hempelmann’s 
observations that the ascent of the sexually mature worms is due 
to a combination of causes which act throughout a long period and 
whose efficacy fluctuates considerably. 
Enough has been said to indicate the irregularity in period and 
time of the swarming of Nereis dumerili and other associated 
forms. ‘There is, however, at least one good case in the genus 
where an absolute periodicity has been established. I refer to 
NV. (Ceratocephale) osawat of Japant, the heteronereids of which 
regularly issue forth four times in the year in the months October 
and November, in 3—4 day periods. Their date of appearance is 
absolutely fixed for the days following the new moon. Their 
presence on the surface is limited to from one to two hours in 
the evening, but the time of appearance is by no means so 
definitely fixed as in the case of Odontosyllis phosphorea and 
enopla (sunset). They appear in fact well after sunset and often 
after moonset, so that the immediate stimulus would appear to 
be independent of the action of light. 
Other cases might be quoted, but I think my main contention, 
the diversity of swarming habits in Nerevs, is sufficiently proved. 
The phenomenon of swarming, at least in its final form, does 
appear to be of a definitely adaptive nature. The object is the 
fertilisation of the maximum number of eggs, and this is gained 
by the simultaneous emission of eggs and spermatozoa from a 
crowded mass of male and female individuals. Galloway and 
Welch found in O. enopla that of eggs collected in connection 
with the swarming worms from 45—80 °/, were already fertilised. 
In the case of the Atlantic Palolo worms, which turn the clear 
blue waters of the Tortugas into a thick milky mass with their 
eggs and spermatozoa, it is difficult to imagine how any of the 
eggs escape fertilisation. Yet Professor Mayer tells me that the 
* Mr William Brockett in the month of June, 1910, collected a rather large 
number of heteronereids off Mersea Island on the Essex coast between midday and 
two o’clock in the afternoon. 
+ Izuka, ‘‘ Observations on the Japanese Palolo Ceratocephale osawai,” Journ. 
Coll. Sc., Tokio, T. xxxvz. 1903. 
