=Reproductive Organs.=—In _Chirocephalus_ the ovaries (Fig. 2, _Ov_) are hollow epithelial tubes, lying one on each side of the alimentary canal, and extending from the sixth abdominal segment forwards to the level of the genital opening; at this point the two ovaries are continuous with ducts, which bend sharply downwards and open into the single uterus contained within the projecting egg-pouch and opening to the exterior at the apex of that organ. Short diverticula of the walls of the uterus receive the ducts of groups of unicellular glands, the bodies of which contain a peculiar opaque secretion, said to form the eggshells. In Apodidae the ovaries are similar in structure, but they are much larger and branch in a complex manner, while each ovary opens to the exterior independently of the other in the eleventh post-cephalic segment; nothing like the median uterus of the Branchipodidae being formed. The epithelium of the ovarian tubes proliferates, and groups of cells are formed; one becoming an ovum, the others being nutrient cells like those which will be more fully described in the Cladocera. In _Chirocephalus_ the testes are tubes similar in shape and position to the ovaries, each communicating in front with a short vas deferens, which dilates into a vesicula seminalis on its way to the eversible penis; an essentially similar arrangement is found in all Branchipodidae, but in Apodidae and Limnadiidae there is no penis. All the Branchiopoda are dioecious,[22] and many are parthenogenetic. Among Branchipodidae _Artemia_ is the only genus known to be parthenogenetic, but parthenogenesis is common in all Apodidae, while the males of several species of _Limnadia_ are still unknown, although the females are sometimes exceedingly common. In _Artemia_, generations in which the males are about as numerous as the females seem to alternate fairly quickly with others which contain only parthenogenetic females; in _Apus_ males are rarely abundant, and often absent for long periods; during five consecutive years von Siebold failed to discover a male in a locality in Bavaria, though he examined many thousands of individuals; near Breslau he found on one occasion about 11 per cent of males (114 in 1026), but in a subsequent year he found less than 1 per cent; the greatest recorded percentage of males is that observed by Lubbock in 1863, when he found 33 males among 72 individuals taken near Rouen. The eggs of most genera can resist prolonged periods of desiccation, and indeed it seems necessary for the development of many species that the eggs should be first dried and afterwards placed in water. Many eggs (_e.g._ of _Chirocephalus diaphanus_ and _Branchipus stagnalis_) float when placed in water after desiccation, the development taking place at the surface of the water.