DAPHNIADiE. 79 



wheel completely round the pond. Should the mass come 

 near enough the edge to allow the shadow of the observer 

 to fall upon them, or should a dark cloud suddenly 

 obscure the sun, the whole body immediately disappear, 

 rising to the surface again when they have reached beyond 

 the shadow, or as soon as the cloud has passed over. 

 They are very prolific, giving birth to their young a great 

 many times during their lives, and some of the larger 

 species having as many as forty or fifty eggs and upwards 

 in their matrix at once. According to J urine, in June 

 the young ones begin to have eggs, about ten days after 

 their birth, and it is probable they continue to produce 

 all the summer through at frequent intervals. The males 

 are veiy few in number, compared with the females, and 

 are only to be met with at certain seasons, generally, as far 

 as my observation goes, in autumn. From this circum- 

 stance, SchoeflFer and others have considered them as her- 

 maphrodites ; and Sulzer (as quoted by Straus), though 

 he oppugns this opinion, gives a more singular one still, 

 believing that a copulation might take place with the 

 young before they see the light of day ! These authors 

 had never seen the males, nor ever witnessed the act of 

 copulation. Midler and others, however, detected the 

 male, and witnessed the act ; and it is now clearly ascer- 

 tained that one single copulation is sufficient not only to 

 fecundate the mother for her life, but all her female de- 

 scendants for several successive generations. This was 

 observed by Schoeffer, who followed them up to the fourth, 

 by Straus to the fifth, and by Jurine to the sixth ; the latter 

 observing that he thinks it probable it might extend in 

 some species to the fifteenth generation. Extraordinary 

 as this may appear, I have further found that the young 

 produced from the ephij^pial eggs are also fecundated by 

 this one copulation, and have progeny ; and that their 

 young again also produce eggs without the intervention of 

 the male. I have followed up the successive generations, 

 as far as the fourth, in the Daphnia? born in the usual 

 manner, and as far as the third, in those born from ephip- 



