io THE PHYSIOLOGY OF REPRODUCTION 



purposes of breeding. In the Atlantic Palolo (Eunice fuc.ata) and 

 the South Pacific Palolo (Eunice viridis) the process invariably takes 

 place twice, upon or near the day of the last quarter of the moon 

 and with the first rays of the sun ; but with the former species it 

 occurs in June and July, and with the latter in October and November. 

 The general conditions of existence for these worms would appear to 

 be remarkably uniform, the temperature variation being from 24 to 

 30 only. In the Japanese Palolo (Ceratocephale osawai) the swarming 

 takes place on nights closely following the new and full moons (i.e. 

 when the spring tides occur), in October and November, the worms 

 swimming out regularly four times a year. Each swarming-period 

 lasts from one to four days. It has been noted further that the swarm 

 is greater after the new moon (when the spring tide is highest) than 

 after the full moon (when the tide is not so high), that each swarm- 

 ing takes place invariably just after the flood in the evening, that it 

 continues for from one to two hours, and is generally larger on warm, 

 cloudy nights than on clear, chilly nights. It would appear also that 

 no individual worm takes part in more than one swarming, in the 

 year. 1 



ARTHROPODA 



Innumerable instances of the periodicity of breeding and its 

 relation to seasonal and environmental changes might be adduced 

 from the great group of Arthropods, but the reason for the variations 

 which occur is not always obvious. Thus, in the common crayfish 

 (Astacus fluviatilis), in France the males are said to approach the 

 females in November. December, arid January, whereas in England 

 they begin to breed as early as the commencement of October, 

 if not earlier. 2 Also, in the Cape species of Peripatus (P. capensis) 

 birth takes place in a fixed season (during April and May), whereas, 

 in the South American species, births .are said to occur probably 

 throughout the entire year. 3 



In the case of the hemipterous insect known as the plant-louse 

 (Aphis), we have evidence that the mode of reproduction is dependent 

 upon temperature. In a favourable summer the females of this 



1 Izuka, "Observations on the Japanese Palolo," Jour, of the College of 

 Science, University of Tokyo, vol. xvii., 1903. This worm is a Phyllodocid and 

 not a true Palolo. Other swarming Polychsetes are Nereis dumerilii, which 

 swarms occasionally ; Nereis limbata, in which each sex secretes a substance 

 which activates the other, the males swarming first and being followed by the 

 females ; and Odontosyllis enopla (a Bermudan syllid), in which the female is 

 phosphorescent intermittently at the hind end of the body, her appearance 

 being followed by the male, the phosphorescence of the female ceasing after 

 spawning is over. I am indebted to Mr. C. F. A. Pantin, of Christ's College, for 

 this information. 



2 Huxley, The Crayfish, London, 1880. 



3 Sedgwick, " Peripatus,''' Comb. Nat. Hist., vol. xii., London, 1901. 



