250 THE RATE OF GROWTH [ch. 



of self-regulation; and these physiological differences help to explain, 

 in both cases, the limitation of this species or that to more or less 

 brackish, or more or less saline, waters. In deep-sea crabs {Hyas, 

 for instance) the osnjotic pressure of the blood keeps nearly to that 

 of the milieu exteme, and falls quickly and dangerously with any 

 dilution of the latter; but the httle shore-crab (Cardnus moenas) 

 can hve for many days in sea-water diluted down to one-quarter of 

 its usual sahnity. Meanwhile its own fluids dilute slowly, but not 

 near so far; in other words, this crab combines great powers of 

 osmotic regulation with , a large capacity for tolerating osmotic 

 gradients which are beyond its power to regulate. How the unequal 

 balance is maintained is yet but httle understood. But we do know 

 that certain organs or tissues, especially the gills and the antennary 

 gland, absorb, retain or ehminate certain elements, or certain ions, 

 faster than others, and faster than mere diffusion accounts for; in 

 other words, "ionic*' regulation goes hand in hand with "osmotic" 

 regulation, as a distinct and even more fundamental phenomenon*. 

 This at least seems generally true — and only natural — that quickened 

 respiration and increased oxygen-consumption accompany all such 

 one-sided conditions: in other words, the "steady state" is only 

 maintained by the doing of work and the expenditure of energyf. 



To the dependence of growth on the uptake of water, and to the 

 phenomena of osmotic balance and its regulation, HoberJ and also 

 Loeb were inclined to refer the modifications of form which certain 

 phyllopod Crustacea undergo when the highly sahne waters which 

 they inhabit are further concentrated, or are abnormally diluted. 

 Their growth is retarded by increased concentration, so that 

 individuals from the more saline waters appear stunted and dwarfish ; 

 and they become altered or transformed in other ways, suggestive 

 of "degeneration," or a failure to attain full and perfect develop- 



* See especially D. A. Webb, Ionic regulation in Cardnus moenas, Proc. R.S. (B), 

 cxxix, pp. 107-136, 1940. 



t In general the fresh- water Crustacea have a larger oxygen -consumption than 

 the marine. Stenohaline and euryhaline are terms applied nowadays to species 

 which are. confined to a narrow range of salinity, or are tolerant of a wide one. 

 An extreme case of toleration, or adaptability, is that of the Chinese woolly-handed 

 crab, Eriockeir, which has not only acclimatised itself in the North Sea but has 

 ascended the Elbe as far as Dresden. 



X R. Hober, Bedeutung der Theorie der Losungen fiir Physiologic und Medizin, 

 Biol. Centralbl. xix, p. 272, 1899. 



