200 Mr Potts, The Swarnung of Odontosyllis. 
eggs laid by females on the borders of the swarm are certainly not 
developed. 
How, then, does it come about that a species like V. dumerilii, 
the heteronereids of which become sexually mature at any time 
within a widely extended period, and in which the swarming habit 
is very indefinitely developed, is able to maintain its great numbers — 
and wide distribution. Not only is there no arrangement to ensure 
the simultaneous swarming of the sexes, but it has frequently been — 
observed in this and related species that large swarms consist of a 
single sex*. Under these circumstances there must be an enormous” 
waste. 
There is, however, one obvious explanation. NV. dumerilis is a 
polymorphic species, and according to the results of von Wisting-— 
hausen and Hempelmann all individuals have more than one 
period of sexual maturity. When they first reach a certain 
length the female lays eggs, within the tube she normally m- 
habits, to the number of 1000 or more. These are generally 
fertilised in the most economical manner by the male, who creeps 
into the tube and spreads his sperm over the eggs. It would seem” 
probable that this is the method most responsible for the main- 
tenance of the species, the production of sexually ripe heteronereids 
at a later period of life being a subsidiary (possibly incipient or 
degenerate) phenomenon. 
These comments on swarming in Odontosyllis and Nereis are 
only intended to illustrate the diversity in the habit existing 
among related forms. I have not attempted to discuss the thorny 
question of the part played by external stimuli or the possibility 
of an inherited rhythm in the organism. 
Note. Since writing the above I have had access to a paper 
by Lillie and Just (Biol. Bull., Feb. 1918) on the breeding habits — 
of Nereis lumbata. In this species swarming takes place quite | 
regularly in four runs during the summer corresponding to the — 
lunar cycles in the months June, July, August, September, occur- 
ring on many successive nights shortly after sunset and lasting — 
little longer than an hour. This is, then, an advanced case of the 
swarming habit, but the great interest lies in the fact that the 
female produces a substance which acts on the male causing the ~ 
emission of sperm. It will be of great interest to see whether the © 
distribution of this phenomenon is at all general or whether it 
% 
: 
) 
is a peculiar development of the swarming habit to ensure fertili- 
sation, like the mating relations in Odontosyllis enopla. 
* Cf. Sorby, Hempelmann, loc. cit. 
