more concentrated brines, and then the females lay eggs, which, however, only begin to develop when the salinity is somewhat reduced.
The best-known case is the little ãbrine-shrimp,ã Artemia salina, found, in one form or another, all the world over, and first discovered more than a century and a half ago in the salt-pans at Lymington. Among many allied forms, one, A. milhausenii, inhabits the natron-lakes of Egypt and Arabia, where, under the name of ãloul,ã or ãFezzan-worm,ã it is eaten by the Arabs81. This fact is interesting, because it indicates (and inôÙvesôÙtiôÙgaôÙtion has apparently confirmed) that the tissues of the creature are not impregnated with salt, as is the medium in which it lives. The fluids of the body, the milieu interne (as Claude Bernard called them82), are no more salt than are those of any ordinary crustacean or other animal, but contain only some 0ôñ8 per cent. of NaCl83, while the milieu externe may contain 10, 20, or more per cent. of this and other salts; which is as much as to say that the skin, or body-wall, of the creature acts as a ãsemi-permeable membrane,ã through which the dissolved salts are not permitted to diffuse, though water passes through freely: until a statical equiôÙlibôÙrium (doubtless of a complex kind) is at length attained.
Among the structural changes which result from increased concentration of the brine (partly during the life-time of the individual, but more markedly during the short season which suffices for the development of three or four, or perhaps more, successive generations), it is found that the tail comes to bear fewer and fewer bristles, and the tail-fins themselves tend at last to disappear; these changes corôÙreôÙsponôÙding to what have been {128} described as the specific characters of A. milhausenii, and of a still more extreme form, A. kûÑppeniana; while on the other hand, progressive dilution of the water tends to precisely opposite conditions, resulting in forms which have also been described as separate species, and even referred to a separate genus, Callaonella, closely akin