EEPEODUCTION. 417 



confounded together by many authors. The female Entomostraca fre- 

 quently carry their ova in two transparent sacculi attached to the hinder 

 part of the body ; and it is in these egg-bags that the oviducts terminate ; 

 so that the ova, as they are formed, are expelled into the singular 

 receptacles thus provided. Without such a provision, indeed, it would 

 be difficult to conceive how the ova could possibly remain attached to 

 the parent, as they far surpass in their aggregate bulk the size of her 

 entire body, and therefore could not by any contrivance be developed 

 internally without bursting the crustaceous covering that invests the 

 mother. Jurine*, Ramdohrf, and other authors have carefully watched 

 the generative process in several genera, and brought to light many 

 important and curious facts connected therewith. In Cyclops, a species 

 to be met with in every ditch, the impregnation of the ova is undoubt- 

 edly effected in the body of the parent ; and the eggs, when formed, are 

 expelled into two oval sacs placed on each side of the tail, which Jurine 

 calls external ovaries. The number of eggs contained in these sacs 

 gradually increases ; and they exhibit a brown or deep-red colour until 

 a short period before the growth of the embryo is completed, when they 

 become more transparent. In about ten days the eggs are hatched and 

 the young escape; and such is the prodigious fertility of these little beings 

 that a single female will, in the course of three months, produce ten suc- 

 cessive families, each consisting of from thirty to forty young ones. 



(1070.) In the genus Apus, another plan is resorted to for the 

 protection of the ova: the eleventh pair of legs, called by Schaffer 

 " womb-legs," have their first joints expanded into two circular valves, 

 which shut together like a bivalve shell, and thus form a receptacle in 

 which the eggs are contained until they arrive at maturity. 



(1071.) In Daphnia (fig. 212), the ovaria are easily distinguished 

 through the exquisitely transparent shell, especially when in a gravid 

 state ; and the eggs, after extrusion, are lodged in a cavity situated 

 between the shell and the exterior of the body, where they remain until 

 the embryo attains its full growth. 



(1072.) One fact connected with the reproduction of the Entomos- 

 traca is so remarkable, that, had we not already had an instance of 

 the occurrence of a similar phenomenon in the insect world (Aphides), 

 the enunciation of it would cause no little surprise to the reader ; and 

 had its reality been less firmly substantiated by the concurrent testi- 

 mony of numerous observers who have witnessed it in many different 

 genera (Cyclops, Daphnia^ &c.), it might still be admitted with suspi- 

 cion. In the genera above mentioned, it has been ascertained, by careful 

 experiments, that a single intercourse between the sexes is sufficient to 

 render fertile the eggs of several (at least six, according to Jurine) 

 distinct and successive generations. 



* Histoire des Monocles. 1 vol. 4to. Geneve, 1820. 



f Materiaux pour 1'Histoire de quelques Monocles allemands. 4to. 1805. 



2E 



