964 CKUSTACEA 



single fertilised female of the common Cyclops quadricornis may be 

 the progenitor in one year of 4,442,189,120 young. 1 



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 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-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, w r hich has been called 

 the ephippium, from its resemblance to a saddle. This, when care- 

 fully 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. From the observations of 

 Sir J. Lubbock, 2 it appears that the ephippium is really only an 

 altered portion of the carapace, its outer valve being a part of the 

 outer layer of the epidermis, and its inner valve the corresponding 

 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 ; and from this 

 situation the eggs pass into the receptacle formed by the new cara- 

 pace, where they become included between the two layers of the 

 ephippium. This is cast off, in process of time, with the rest of the 

 skin, from which, however, it soon becomes detached ; and it con- 

 tinues 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 obviously affords protection to 

 the eggs which are to endure the severity of winter cold ; and an 

 approach to it may be seen in the remarkable firmness of the 

 envelopes of the * winter eggs ' of some Rotifera. There seems a 

 strong probability, from the observations of Sir J. Lubbock (now 

 Lord Avebury), that the ' ephippial ' eggs are true sexual products, 

 since males are to be found at the time when the ephippia are de- 

 veloped ; whilst it is certain that the ordinary eggs can be produced 

 non-sexually, and that the young which spring from them can multi- 

 ply the race in like manner. The young which are produced from 

 the ephippial eggs seem to have the same power of continuing the 



1 For an interesting account of the parthenogenetic development of Apus and its 

 allies see the sixth of Von Siebold's Beitrdge zur Parthenogenesis der Arthropoden 

 (Leipzig, 1871). 



2 ' An account of the two Methods of Reproduction in Daplmia, and of the 

 Structure of the Ephippium,' in Phil. Trans. 1857, p. 79. On the ' summer-egg ' of 

 Daphnia see Lebedinsky, Zool. Anzeig. xiv. p. 149. 



