546 



CRUSTACEA. 



fertilize not merely tlie ova whicli 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 beginning to propagate; so that, according to the computa- 

 tion of Jurine, founded upon data ascertained by actual observa- 

 tion, a single fertilized female of the common Cyclops quadri- 

 cornis maybe the progenitor in oneyear of 4,442,189,120 young. 



370. 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 there until the young 

 are ready to come forth, so that these animals may be said to be 

 ovo-viviparous. In the Baphnia, 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 coverings 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 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 

 horny casing, enveloped in a capsule which opens like a bivalve 

 shell. The first traces of the "ephippium" are seen after the 

 third moult, as a green matter in the ovaries, which differs both 

 in color and appearance from that of the eggs ; after the fourth 

 moult, this green matter passes from the ovaries into the matrix 

 or open space on the back, and there becomes developed into 

 the " ephippium ;" and at the fifth moult this is thrown off, and 

 the ephippium, with the two eggs enclosed, floats on the water 

 until the next spring, when the young are hatched with the 

 returning warmth of the season. This curious provision is ob- 

 viously destined to afford protection to the eggs which are to 

 endure the severity of winter cold ; and some approach to it 

 may be seen in the remarkable firmness of the envelopes of the 

 "winter eggs" of some of the Rotifera (§279). It has been 

 ascertained by Dr. Baird, that the young produced from the 

 ephippial eggs have the same power of continuing the race by 

 non-sexual reproduction, as the young developed under ordinary 

 circumstances. 



371. In most Entomostraca, the young at the time of their 



