The Annual Breeding-Swarm of the Atlantic Palolo. 



1 1 1 



overcast or cloudy weather, so that even diffuse moonlight appears to be 

 capable of calling forth the breeding-swarm. 



In the Atlantic palolo the annual breeding-season is only of I to 6 days' 

 duration, and the males outnumber the females in the ratio of about 3 to 2, 

 whereas in Nereis, where the breeding-season is fully 100 days long, the 

 males greatly outnumber the females. It is evident that a shortening of 

 the breeding-season would cause a greater concentration of breeding indi- 

 viduals, and would therefore permit of a relative decrease in the number 

 of males and a corresponding increase in the number of females; for when- 

 ever a female swarms it is important for the preservation of the species that 

 there should be a male near her to fertilize her eggs. If the breeding-season 

 be of long duration, the males must greatly outnumber the females to secure 

 this fortuitous proximity, but if all of the females swarm within a few 

 days very much fewer males will suffice to accomplish this purpose. 



We have advanced beyond the period in the history of biology when one 

 had but to discover an advantage to determine a cause ; but that some such 

 cause may have contributed to shorten the breeding-season in such animals 

 as the Atlantic, Pacific, and Japanese palolo worms is shown by the fact 

 that more eggs are fertilized when males are near the female than when 

 they are far away. For example, I took a female Atlantic palolo from the 

 midst of the swarm and placed her in sea-water 200 meters away from the 

 nearest swarming males. Of the eggs which were laid by this female in 

 the water removed from the place of the swarm only an occasional one 

 developed, whereas practically every egg developed in the sea-water where 

 males were near. 



The polar bodies are given forth as soon as the eggs are cast out from 

 the female, and fertilization occurs in the water; but the egg does not 

 mature if it be cast out at any time other than that of the normal breeding- 

 swarm. When about 10 to 15 hours old the larvae are nearly all negatively 

 phototactic either in diffuse light or in sunlight. When about 28 hours old, 

 however, they mainly become positively phototactic and remain thus, even 

 after the eighth day, when they will have ceased to swim through the water 

 and have sunken to the bottom. Within 24 hours after sinking to the 

 bottom, however, they become indifferent to light in so far as their move- 

 ment is concerned. 



The segmentation closely resembles that of Nereis and the larva is 

 telotrochal. 



Further accounts of the Atlantic palolo will be found in Bulletin of 

 the Museum of Comparative Zoology at Harvard College, 1900, vol. 36, 

 pp. 1-14, plates 1-3, and in the Science Bulletin of the Museum of the 

 Brooklyn Institute of Arts and Sciences, 1902, vol. i, No. 3, pp. 93-103, 

 i plate. 



The Japanese palolo is treated of in detail by A. Izuka, 1903, Journal 



