348 CRUSTACEA. 



In Daphnia (Jig. 155) the ovariaare easily distinguished through 

 the exquisitely transparent shell, especially when in a gravid state ; 

 and the eggs after extrusion are lodged in a cavity situated be- 

 tween the shell and the exterior of the body, where they remain 

 until the embryo attains its full growth. 



(385.) 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 concur- 

 rent testimony of numerous observers who have witnessed it in 

 many different genera (Cyclops, Daphnia, &c.), it might still be 

 admitted with suspicion. 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 gene- 

 rations. 



Some authors have supposed, from the circumstance of all the 

 individuals which have been met with belonging to some genera 

 being females, that some of these little beings were hermaphrodite, 

 or self-impregnating ; but such an opinion rests on very doubtful 

 grounds, especially as there seems good reason to believe that in 

 many instances the forms of the male and female of the same 

 species are so different that they might easily be mistaken for 

 totally distinct animals. 



(386.) The last point which we have to notice, in connection with 

 the history of the Crustacea, is, the progress of their developement 

 from the embryo condition to their mature state. This is a sub- 

 ject which has given rise to considerable discussion, especially as 

 relates to the changes which occur during the growth of the more 

 highly organized forms ; some authors contending that they leave 

 the egg complete in all their parts, and presenting their adult con- 

 figuration, while others assert that they undergo changes so import- 

 ant as only to be comparable with the metamorphosis of insects. 



Among the Entomostraca such changes have been again and 

 again witnessed, and the appearances observed during their growth 

 carefully recorded. From these observations very important results 

 have been obtained, inasmuch as many forms previously described 

 as distinct species have been found to be merely the same animal 

 in different stages of developement. In Cyclops, for example, 

 the newly hatched embryo possesses only four legs, and its body 



