Ill] OSMOTIC FACTORS IN GROWTH 127 



their young alive ; males disappear altogether in the more con- 

 centrated 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 "brine-shrimp," 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. ynilhausenii, 

 inhabits the natron-lakes of Egypt and Arabia, where, under the 

 name of "loul," or "Fezzan-worm," it is eaten by the Arabs*. 

 This fact is interesting, because it indicates (and investigation 

 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 

 them t), are no more salt than are those of any ordinary crus- 

 tacean or other animal, but contain only some 0-8 per cent, of 

 NaClt, 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 "semi-permeable 

 membrane," through which the dissolved salts are not permitted 

 to diffuse, though water passes through freely : until a statical 

 equilibrium (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 corresponding to what have been 



* These " Fezzan-worms," when first described, were supposed to be "insects' 

 eggs"; cf. Humboldt, Personal Narrative, vi, i, 8, note; Kirby and Spence. 

 Letter x. 



t Cf. Introd. a VeUide de la medecine experimentile, 1885, p. 110. 



t Cf. Abonyi, Z. f. w. Z. cxiv, p. 134, 1915. But Fredericq has shewn that 

 the amount of NaCl in the blood of Crustacea (Carcinus moenas) varies, -and 

 all but corr s onds, with the density of the water in which the creature 

 has been kept {Arch, de Zool. Exp. et Gen. (2), in, p. xxxv, 1885); and 

 other results of Fredericq's, and various data given or quoted by Bottazzi 

 (Osmotischer Druck und elektrische Leitungsfahigkeit der Fliissigkeiten der 

 Organismen, in Asher-Spiro's Ergebn. d. Physiologie, vii, pp. 160-402, 1908) suggest 

 that the case of the brine-shrimps must be looked upon as an extreme or exceptional 

 one. 



