M. F. Plateau on the Aquatic Articulata. 363 



Freshwater Articulata. 



1. Sea-water has, if anj, only a veiy slight influence upon 

 the aquatic Coleoptera and Hemiptera in the perfect state ; 

 this influence may be a little greater upon the larvse. 



2. Sea-water produces injurious effects upon the freshwater 

 Articulata with a delicate skin or furnished with branchiae ; 

 and these effects are, in general, the more marked in propor- 

 tion as the delicate surface is considerable. 



Thus larvae of Agrion appear to live indefinitely in sea- 

 water, whilst those of Gloeon die in it on the average in two 

 hours and three minutes. Among Crustacea Gammariis Roe- 

 selii and Asellus aquaticus resist the action of sea-water for 

 several hours ; whilst the Cladocera, Ostracoda, and Copepoda 

 perish in a few minutes. A special table shows the influence 

 of the thickness of the integuments and of the presence or 

 absence of branchia3. 



3. The freshwater Articulata which 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 have absorbed chlorides of sodium and magnesium. 



The direct experiments which I have been able to make 

 upon the aquatic Articulata had, as their starting point, a very 

 important experiment of M. Claude Bernard's, which has 

 lately been referred to and developed by M. H. Emery. M. 

 Emery placed a frog in water containing about 25 per cent, of 

 common salt. The frog at first moves about rapidly ; in 

 from three to five minutes it becomes insensible and motion- 

 less ; it is then washed carefully and placed in pure distilled 

 water, when the animal soon resumes its activity, and the dis- 

 tilled water is found to furnish an abundant precipitate with 

 nitrate of silver. 



I simply transcribe the description of a single one of my 

 experiments, in order to show clearly how I operated in all 

 those relating to the absorption by the skin or to the excretion 

 of the salts of sea-water. 



After ascertaining that the distilled water of which I was 

 going to make use gave no precipitate with nitrate of silver, 

 and carefully washing with this same water the glass tubes 

 necessary for my experiments, I placed nine individuals of 

 Asellus aquaticus in a solution of common salt containing 

 (by weight) 6-092 of salt and 96-954 of water— that is to say, 

 a quantity of salt exactly double that contained in sea-water. 



The Aselli remained in this solution for eighty-seven 

 minutes, at the end of which they manifested uneasiness ; 

 they were then taken out, placed for a moment upon bibulous 



