448 



Entomostraca (Water-Fleas)* 



well supplied with growing weeds, are the best " hunting 

 grounds." It is a mistake to suppose that a dirty, foul- smelling, 

 stagnant pool is the likeliest to afford a rich harvest to the 

 collector in any branch of natural history ; a pond, to be good, 

 should be of tolerable size, long-established, and stocked with 

 aquatic plants. These, by the constant liberation of oxygen, 

 tend to keep the water pure, besides affording food and a home 

 among their branches to countless animals which could not 

 thrive without them. There are, however, several species of 

 Entomostraca, whose congenial habitat is the muddy bottom of 

 ponds and ditches. 



The rate of increase amongst these creatures is prodigious. 

 " Specimens of the Cyclops quadricornis are often found carry- 

 ing thirty or forty eggs on each side (fig. 4) ; and though the 

 other species, which have only one external ovary, do not carry 

 so many, still the number is very considerable. Jurine has, 

 with great fidelity, watched the hatching and increase of the 

 Cyclops quadricornis in particular, and has given a calculation 

 which shows the amazing fertility of the species. He has seen 

 one female isolated lay ten times successively ; but in order to 

 speak within bounds he supposes her to lay eight times within 

 three months, and each time only forty eggs. At the end of 

 one year, this female would have been the progenitor of 

 4,442,189,120 young!! The first mother lays forty eggs, 

 which at the end of three months, at eight layings during that 

 time, would give 320 young. Out of this number he calculates 

 eighty as males (there being in every laying a great proportion 

 of females) the remaining 240 as females. The following table 

 will show the prodigious extent of their fecundity* : — 





No. of 





layings. 



1st mother . 



8 



1st family of 

 females . 



8 



2nd family of 

 females . 



8 



3rd family of 

 females . 



8 



Time employed for 

 these eight layings. 



From 1st Jan. to 



end of March, 

 From 1st April to 



end of June, 

 From 1st July to 



end of Sept. 

 From 1st Oct. to 



end of Dec. 



Total . . . 



Each laying sup- 

 posed to be of 

 forty young. 



320 



76,S00 



18,432,000 



4,423,680,000 



1,442,189,120 



Subtract for 

 males. 



80 



■ 19,200 



4,608,000 



1,105,920,000 



1,110,547,280 



Females 

 remaining. 



240 



57,600 



13,824,000 



3,317,760,000 



3,331,641,810 



Among the most remai'kable phenomena connected with the 

 natural history of the Crustacea, are the transformations which 



* Eaird, British Entomostraca, p. 189 et sea. 



