Ionic regulation in Carcinus maenas

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Abstract
Analyses have been made, with respect to the principal inorganic constituents, of the blood and urine of Carcinus, when living in normal sea water, diluted sea water, and sea water modified by the addition of magnesium sulphate. The composition of the blood of individuals living in normal sea water is as follows (the concentration of each ion being expressed as a percentage of the concentration that would be expected if the blood were in dialysis equilibrium with the external medium): Na 110%, K 118%, Ca 108%, Mg 34%, Cl 104%, SO4 61%. This ionic regulation is the resultant of the following processes: active absorption by the gills of sodium, potassium, calcium and chloride at a rate greater than that at which they are lost by diffusion ; differential excretion by the antennary gland, which tends to conserve potassium and eliminate magnesium and sulphate ; inward diffusion across the gills of magnesium and sulphate in accordance with the concentration gradient. In normal conditions there is active absorption of water. In dilute media, when osmoregulation begins, this is suspended, and possibly there is a fall in the passive permeability of the gills to water. In other respects osmoregulation is brought about by an intensification of the processes responsible for ionic regulation. The permeability of the cuticle under physiological conditions is such that it does not affect the salt and water exchange of the animal, which is controlled by the branchial epithelium. The structure of the gills of four species of Decapoda is described, and correlated with their powers of osmotic and ionic regulation.

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