REPRODUCTION OF ENTOMOSTRACA. 643 



their usual ' habitats.' It does not appear, however, that the adult 

 Animals can bear a complete desiccation, although they will pre- 

 serve their vitality in mud that holds the smallest quantity of 

 moisture ; but their Eggs are more tenacious of life, and there is 

 ample evidence that these will become fertile on being moistened, 

 after having continued for a long time in the condition of fine dust. 

 Most Entomostraca, too, are killed by severe Cold, and thus the whole 

 race of adults perishes every winter ; but their Eggs seem unaffected 

 by the lowest temperature, and thus continue the species, which would 

 be otherwise exterminated. — Again, we frequently meet in this 

 group with that Agamic Reproduction, which we have seen to pre- 

 vail so extensively among the lower Radiata and Mollusca. In 

 many species there is a double mode of multiplication, the Sexual 

 and the Non-sexual. The former takes-place at certain seasons 

 only ; the Males (which are often so different in conformation from 

 the females, that they would not be supposed to belong to the 

 same species, if they were not seen in actual congress) disappearing 1 

 entirely at other times. The latter, on the other hand, continues 

 at all periods of the year, so long as warmth and food are supplied ; 

 and is repeated many times (as in the Hydra), so as to give origin 

 to as many successive ' broods. ' Further, a single act of impreg- 

 nation serves to fertilize not merely the Ova which are then mature 

 or nearly so, but all those subsequently produced by the same 

 female, which are deposited at considerable intervals. In these 

 two modes, the multiplication of these little creatures is carried on 

 with great rapidity, the young animal speedily coming to maturity 

 and bsginning to propagate ; so that according to the computation 

 of Jurine, founded upon data ascertained by actual observation, a 

 single fertilized Female of the common Cyclops quadricornis may 

 be the progenitor in one year of 4,442,189,120 young. 



503. The Eggs of some Entomostraca are deposited freely in the 

 water, or are carefully attached in clusters to Aquatic Plants ; but 

 they are more frequently carried for some time by the parent in 

 special receptacles developed from the posterior part of the body ; 

 and in many cases they are retained thereuntil the young are ready 

 to come-forth, so that these animals may be said to be Ovo-Yivi- 

 parous. In the Daphnia, the eggs are received into a large cavity 

 between the back of the animal and its shell, and there the young 

 undergo almost their whole development, so as to come-forth in a 

 form nearly resembling that of their parent. Soon after their birth, 

 a Moult or exuviation of the Shell takes-place ; and the egg-cover- 

 ings are cast-off with it. In a very short time afterwards, another 

 brood of eggs is seen in the cavity, and the same process is repeated, 

 the shell being again exuviated after the young have been brought 

 to maturity. At certain times, however, the Daphnia may be seen 

 with a dark opaque substance within the back of the shell, which 

 has been called the Ephippiitm from its resemblance to a saddle. 

 This, when carefully examined, is found to be of dense texture, and 



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