2O2 E. E. JUST. 



experimental study. Strict watch, however, was kept on these 

 worms throughout the summers named. This is especially true 

 for the summer of 1913; during June, July, and August I went 

 out every night, giving attention wholly to Platynereis swarming. 



The animals on swarming nights swim near the surface of the 

 sea, the males invariably appearing first, the females later. The 

 females rarely exceed fifteen, and indeed on some nights no fe- 

 males swim, while the number of males may be very large. 

 Verrill ('73) says that the worms swim at noon. I have never 

 noted this. 1 Hempelmann ('n) might lead one to think that 

 Nereis dumerilii swarms early in the morning. I looked for this 

 during August, 1913, but did not find Platynereis swarming 

 before or at sunrise. The evening swarm may last two hours. 



The small reddish males swim with great rapidity in an ever 

 more narrowing circle within the patch of light thrown by the 

 observer's lantern until the swarm is at its height. Here and 

 there often at a greater depth than the males swims with slow 

 and even laborious movements, the larger female, pale yellow in 

 color with a thin dorsal line of green the remnant of the empty 

 gut. One cannot but suspect that the sex ratio in some way 

 depends on the rate of movement : the females are easy prey for 

 fish, the males must easily escape their enemies. The sex ratio 

 of the captured animals must be also influenced by the fact that 

 the females tend to keep further below the surface than the males. 

 This is true of Autolytus to a marked degree as I have repeatedly 

 observed. (So too, Andrews, '92, and Mensch.) Verrill, how- 

 ever, says of Nereis limbata that in their burrows "there are few 

 males in proportion to the females" as in the case of Platynereis, 

 the reverse is true of these worms during swarming. 



As the male comes in the vicinity of female he swims very 

 rapidly in spirals tangential to the surface. They swim together 

 and after copulation and egg-laying, the female slowly sinks from 

 view. 



The swarming occurs nightly throughout the months of July 

 and August during the dark of the moon. From new moon to 

 full moon, whether there be moonlight or not the animals do not 

 swarm. Only mature animals swarm. 



1 In July, 1914, I found spent males swimming during the day. 



