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.
Habitat.—All the Phyllopoda, except Artemia, are confined to stagnant shallow waters, especially to such ponds as are formed during spring rains, and dry up during the summer. In waters of this kind the species of Branchipus, Apus, etc., develop rapidly, and produce great numbers of eggs, which are left in the dried mud at the bottom after evaporation of the water, where they remain quiescent until a fresh rainy season. The mud from the beds of such temporary pools often contains large numbers of eggs, which may be carried by wind, on the legs of birds, and by other means, to considerable distances. Many exotic species have been made known to European naturalists by their power of hatching out when mud brought home by travellers is placed in water. The water of stagnant pools quickly dissolves a certain quantity of solid matter from the soil, and often receives dissolved solids through surface drainage from the neighbouring land; such salts may remain as the water evaporates, so that the water which remains after evaporation has proceeded for some time may be very sensibly denser than that in which the Branchiopods were hatched; these creatures must therefore be able to endure a considerable increase in the salinity of the surrounding waters during the course of their lives. My friend Mr. W. W. Fisher points out that the plants present in such a pond would often precipitate the carbonate of lime, so that this might be removed as evaporation went on, but that chlorides would probably remain in solution; from analyses which Mr. Fisher has been kind enough to make for me, it is seen that this happened in a small aquarium in my laboratory, in which Chirocephalus diaphanus lived for four months. In April, mud from the dry bed of a pond, known to contain eggs of Chirocephalus, was placed in this aquarium in Oxford, and water was added from the tap. Oxford tap-water contains about 0·3 grm. salts per litre, the chlorine being equivalent to 0·023 grm. NaCl. Water was added from time to time during May and June, but in July evaporation was allowed to proceed unchecked. At the end of July there was about half the original volume of water, the Chirocephalus being still active; the residue contained 0·96 grm. dissolved solids per litre, with chlorine equal to 0·19 grm. NaCl, so that the percentage of chlorides was about eight times the initial percentage, but there were only three and a fifth times the original amount of total solid matter in solution, the carbonate of lime having precipitated as a visible film.
Some species of Branchipus (e.g. B. spinosus, M. Edw.) and of Estheria (E. macgillivrayi, Baird, E. gubernator, Klutzinger) occur in salt pools, but Artemia flourishes in waters beside whose salinity that endured by any other Branchiopod is insignificant. In the South of Europe, Artemia salina may be found in swarms, as it used to be found in Dorsetshire, in the shallow brine-pans from which salt is commercially prepared; Rathke quotes an analysis showing that a pool in the Crimea contained living Artemia when the salts in solution were 271 grms. per litre, and the water was said to have the colour and consistency of beer.
The behaviour of the animals in the water differs a little; in normal feeding all the species swim with the back downwards, as has already been said; the Branchipodidae rarely settle on the ground, or on foreign objects, but the Apodidae occasionally wriggle along the bottom on their ventral surface, and Estheria burrows in mud.
The greater number of species are found in pools in flat, low-lying regions, and many appear to be especially abundant near the sea; Apus cancriformis has, however, been found in Armenia at 10,000 feet above sea level.
Wells and underground waters do not generally contain Phyllopods; but a species of Branchipus and one of Limnetis, both blind, have been described from the caves of Carniola.
One of the many puzzles presented by these creatures is the erratic way in which they are scattered through the regions they inhabit; a single small pond, a few yards or less in diameter, may be the only place within many miles in which a given species can be found; in this pond it may, however, appear regularly season after season for some time, and then suddenly vanish.