G. GENERAL NATURAL HISTORY AND ZOOLOGY. 227 

 and Asellus resist the action of sea water for several hours 



) 



while others perish in a few minutes. The fresh-water artic- 

 ulates that can live with impunity in sea water are those in 

 which no absorption of salt takes place by the skin ; those 

 which die in it in a comparatively short time having absorbed 

 chlorides of sodium and magnesium, which the experimenter 

 found to be the most injurious salts, the sulphates having no 

 special effect. When the fresh- water articulates pass, by a 

 slow transition, from fresh to sea water, and reproduction has 

 taken place during this transition, the new generation resist 

 the action of the sea water longer than the ordinary individ- 

 uals of the species. 



In the investigations upon the marine Crustacea of the 

 Belgian coast the conclusions arrived at were, first, that the 

 commonest species die in fresh water after the lapse of a va- 

 riable time, which, however, does not exceed nine hours; 

 second, that the marine Crustacea, when immersed in fresh 

 watei'j give up to this the salts, especially the chloride of 

 sodium, with which their tissues were impregnated. The 

 converse of this observation was also true, that the fresh- 

 water articulates immersed in sea water absorb these salts ; 

 third, that in most cases the presence of chloride of sodium 

 forms one of the indispensable conditions of resistance for the 

 marine Crustacea; but this salt appears to be the only one 

 necessary; fourth, the smaller individuals, and those which, 

 having just moulted, have the integuments delicate, present 

 less resistance than the others to the influence of liquids of 

 exceptional composition ; fifth, the difference between the 

 densities of sea water and fresh water can not be regarded 

 as the cause of the death of marine Crustacea in fresh water. 



As a general conclusion, applicable to both groups, Profess- 

 or Plateau states that the idea of endosmose enables us to 

 explain the absorption of salts by the delicate skin or the 

 branchial surfaces of fresh-water articulates when immersed 

 in sea water. The fact that diffusion and dialysis take place 

 with more energy in the case of the chlorides of sodium and 

 magnesium than in that of sulphate of magnesia explains 

 why it is that the chlorides of sea water are alone absorbed. 

 Dialysis explains why marine Crustacea, when placed in fresh 

 water, lose the salts with which they were impregnated. 10 

 A, May, 1871, 362 : from Mem. Acad, de Belgique. 



