30 BASIC METHODS FOR EXPERIMENTS 



usually in smaller numbers, swimming laboriously, frequently 

 not coming to the surface of the water. Both sexes rapidly 

 increase in numbers during the next fifteen minutes, and in the 

 case of a large swarm, hundreds of males may be in sight at one 

 time. The females are less numerous, though on one night I 

 caught enough to fill a liter jar. A night's swarm lasts for an 

 hour or an hour and a half. Each night from full moon to new 

 moon, with certain exceptions, this scene may be re-enacted. 

 And each night the females swarming are a new crop. 



During the light of the moon, except for the first (June) 

 run, when some animals may in certain seasons appear each night 

 until the next swarming period (July, full moon), no Nereis 

 swarm. And late in September if the nights be cold, they do 

 not swarm throughout the dark of the moon. With these 

 exceptions the swarming of Nereis corresponds to the four lunar 

 cycles during June, July, August and September. Each run 

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

 the succeeding nights, sinks to a low point about the time of 

 the third quarter of the moon, then rises again to fall to extinc- 

 tion at or shortly after new moon. Thus, the curve of nightly 

 numbers during a run is bimodal. 



When a female appears, the males at once surround her, 

 swimming rapidly in ever narrowing circles. In a short time, 

 they shed sperm so that the water appears milky. The female 

 sheds her eggs, shrinking in bulk and in so doing becomes a 

 mere shred of tissue, sinks slowly from view, and dies. 



The earlier work at the Marine Biological Laboratory, 

 Woods Hole, on eggs of Nereis was done at night. The animals 

 shedding when collected were placed in the same vessel, and 

 therefore the eggs were fertilized then or soon after. For the 

 early stages, one was obliged to begin one's observations at once. 



The worms may, however, be kept over night without 

 detriment. The animals should be collected singly and each 

 female placed in a separate finger bowl. Three or four males 

 may be kept in one finger bowl. If the animals are to be kept 

 over night for work the next morning, the sea-water in which 

 they were placed when captured should be renewed in the 

 laboratory. The sexes should never be mixed. The finger bowls 

 are covered and placed on the sea-water table with water flowing 



