468 
PRESIDENT’S address. 
Aurelia aurita, whose body contains more than 90 per cent, of 
water, the amount and proportion of the salts contained in its 
internal fluid may be quite different from that of the surrounding 
sea water. (Macallum, 1903.) 
It seems that herein lies an important clue to the origin of 
fresh water animals, those species in which the selective contrast 
is at its maximum, being those best able to live in altered con- 
ditions. This is notably the case with vertebrates, where the 
serum salinity is not greatly different for fresh water and 
marine species. In the Eel, for instance, transference from 
fresh to salt water causes a depression of the freezing point of 
the serum from '570 to '745 only, whereas that of sea water 
would be about 1'9. In the case of fishes, it is probably only 
the gill membranes which are permeable, and it is suggested by 
Siedlecki 1 that the branchial mucus reduces the permeability. 
Again, the Phvllopod Crustacean Artemia is accustumed to live 
in the Great Salt Lake, in water containing extraordinary 
amounts of salt (up to 14,623 grains per gallon), and yet can 
live in water which is almost fresh. Specimens taken from 
such strong brine and washed do not taste salt, so that probably 
the animal’s tissues are largely impermeable to it. 2 Fresh 
water invertebrates retain a relatively high saline concentration 
in spite of living in pure water. But in the majority of marine 
invertebrates examined hitherto, a very short sojourn in altered 
sea water leads to equilibrium of salt concentration, and one 
Avould expect that the groups which have been most able to 
invade fresh water would be those which could offer the greatest 
resistance to diffusion. It would be of the greatest interest to 
investigate the blood of estuarine species of Crustacea, for 
example, from this point of view. Carcinus maenas, for 
instance, allows very rapid diffusion, and cannot live in fresh 
water. It may be that species like Neomysis vulgaris and 
Leander squilla, which can live in water which is almost fresh, 
are able to keep their serum concentration almost constant. 
Many experiments have been made with regard to the capacity 
1. Siedlecki. C. R. Acad, de Paris, CXXXVII., 1903. 
2. Talmage. Amer. Monthly Mic. Journ., XXI., 1900, p. 217. 
