BECDAKT'S EXPERIMENTS. 153 



because the Cordylophora is a quite soft aiiimal of the Polyp 

 group, and yet it could quickly become accustomed to a 

 diminution of salt in the water which would, beyond a doubt, 

 entirely destroy many apparently stronger animals. It would 

 probably be of much assistance and interest to compare ex- 

 amples of Cordylophora from different localities, to see whether, 

 perhaps, the variations in the mode of life have not given rise 

 to some variation in the structure of the animals living under 

 different conditions. This point has not, so far as I know, 

 hitherto been closely investigated. 



Onlv three series of experiments are known to me, which were 

 made under artificial conditions, with the express purpose of 

 determining what animals could bear a transfer from salt to 

 fresh water and vice versa. The experiments made long ago by 

 Beudant have never hitherto been repeated. He found that 

 various fresh-water molluscs were quickly killed if they were 

 suddenly transferred from fresh water to the concentrated salt 

 water of the Mediterranean : but when he increased the amount 

 of salt very gradually he obtained very different results. He 

 began in April by putting animals into water which contained 

 only one per cent, of salt, and by September, by gradual addi- 

 tions of salt, he had brought it to a solution of about four per 

 cent. Species of the genera Lymncea, Physa, Planorbis, and 

 Ancylus, lived in this salt water as well as in pure fresh water, 

 wliile of Paludina vivipara, Bythinia tentaculata, and Neritina 

 fluvialilis, a much greater number of individuals had died in 

 the salt water than in the fresh water. Of bivalves Unio, 

 Anodonta, Cyclas every specimen had perished before the 

 water had reached its highest strength of four per cent. He 

 subsequently conducted the experiments in the inverse order 

 ;at Marseilles, placing true marine animals in fresh water. He 

 then found that a sudden transfer killed almost every species, 

 while gradual additions of fresh water to the salt were borne 

 by many species, till in the course of a few months it had become 

 perfectly fresh, so that finally true marine animals were living 

 with Lymntea and Plauorbis. The edible mussel seemed par- 

 ticularly resistent, for not one single specimen perished through- 

 out the whole duration of the experiments. Of 610 icdivi- 



