418 CRUSTACEA. 



(1073.) In many species there is a double mode of reproduction, the 

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

 only, the males disappearing entirely at other times ; while the latter 

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

 supplied, and is repeated many times, so as to give origin to many 

 successive broods. Further, a single act of impregnation serves to 

 fertilize not merely the ova which are then mature, or nearly so, but 

 all those subsequently deposited by the same female, even at consider- 

 able 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 beginning 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 might be 

 the progenitor of 4,442,189,120 young. 



(1074.) The eggs of some Entomostraca are deposited free in the 

 water, or are carefully affixed in clusters to aquatic plants ; but they are 

 more frequently carried for some time in special receptacles developed 

 from the posterior part of the body, and in many instances they are 

 retained there until the young are ready to come forth. In Daphnia 

 the eggs are received into a large cavity between the back of the animal 

 and the shell, and there the young undergo almost their whole deve- 

 lopment. Soon after their birth a moult or exuviation takes place, and 

 the egg-coverings are got rid of with the cast shell. In a very short 

 time afterwards another brood of eggs is seen in the cavity, and the 

 same process is repeated. At certain times, however, the Daphnia may 

 be seen with a dark opake substance within the back of the shell, which 

 has been called the ephippium, from its resemblance to a saddle. This, 

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

 composed of a mass of hexagonal cells ; and it contains two oval bodies, 

 each consisting of an ovum covered with a dense horny casing, enve- 

 loped in a capsule, which opens like a bivalve shell. Prom the recent 

 observations of Mr. Lubbock*, it appears that the ephippium is really 

 only an altered part of the carapace, its outer walls being a part of 

 the outer layer of the epidermis, and its inner valve the correspond- 

 ing part of the inner layer. The development of the ephippial eggs 

 takes place at the posterior part of the ovaries, and is accompanied by 

 the formation of a greenish-brown mass of granules ; from this situation 

 the eggs pass into the receptacle formed by the new carapace, where 

 they become included between the two layers of the ephippium. This 

 is cast, in process of time, with the rest of the skin, from which, how- 

 ever, it soon becomes detached, and continues to envelope the eggs, 

 generally floating on the surface of the water until they are hatched 

 with the returning warmth of spring. This curious provision is ob- 

 viously destined to afford protection to the eggs which are to endure 

 * Proc. of Roy. Soc., Jan. 29, 1857. 



