148 FRANK R. LILLIE AND E. E. JUST. 



in an hour, or an hour and a half, all have disappeared for the 

 night. Next night the scene may be repeated. The females are 

 a fresh crop each night, as will be seen from the description 

 below, though the same males may presumably appear on several 

 successive nights, a circumstance which, if true, will partly 

 explain the usual great numerical preponderance of males. ^ 



In any given night the males invariably appear first. As the 

 females appear in increasing numbers, the males gradually grow 

 fewer. During the first nights of a run, as well as towards the 

 end, the males are relatively more abundant. On nights of the 

 greatest swarms, when the females appear in great numbers, the 

 disproportion is perhaps not as noticeable. It seems, however, 

 that there is no definite place in the run at which this sex ratio 

 changes, although there are some nights throughout the summer 

 when more females than males are caught. 



During the light of the moon, with exceptions noted beyond, 

 no Nereis are to be found. They therefore occur in four periods 

 or "runs," during the summer, corresponding to the lunar cycles 

 in the months of June, July, August and September. Each 

 run begins near the time of the full moon, increases to a maximum 

 during successive nights and sinks to a low point about the time 

 of the third quarter, then again rises and falls to extinction 

 shortly after the new moon. Each run is therefore typically 

 divided in two sub-runs, or the curve of nightly numbers during 

 a run is bimodal, with a deep depression about the time of the 

 -third quarter. 



When a female appears she is soon surrounded by several 

 males, if the latter are abundant, which swim rapidly in narrow 

 circles about her. In a little while they begin to shed sperm, 

 rendering the water milky, and soon she begins to shed her eggs, 

 shrinking in bulk as she does so, until, a mere shadow of her 

 former self, she sinks slowly in the water to die. We have never 

 succeeded in keeping females alive for more than a few days 

 after the eggs are shed. 



2 The worms are eaten voraciously by certain fish, as we ascertained in 191 1 when 



Nereis were kept in aquaria with Fundidus. During August, 1911, it was often 



noted that fish attracted by the Hght seemed to keep down the number of Nereis 



•caught. The slowly moving females were always more easily caught by the fish. 



This may therefore be another cause that influences the sex ratio. 



