ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 63 



tap-water for twenty-four hours. It is suggested that excess of calcium 

 salts in contaminated water, e.g. near paper-mills, may account for the 

 occurrence of tailless trout. 



The author's observations are of great interest in connexion with the 

 migrations of fishes. The migi-ations of anadromous fishes, like the 

 sahnon, are probably correlated with rhythmic changes in metabolism. 

 These alterations in metabolic activity are largely the result of internal 

 changes such as occur with the ripening of the sexual products. 



The fresh water has a low specific gravity and is consistently acid in 

 reaction ; the sea water has a relatively liigh specific gravity and is con- 

 sistently alkaline. In the fresh water the fishes ore positive to currents, 

 and, in a gradient, select water that is just on the acid side of neutrality 

 and of lower density than that of the sea. Salt-water fishes, on the 

 other hand, are probably negative to a fresh- water current, select water 

 on the alkaline side of neutrality, and reject water of low specific gravity 

 for that of higher. The reversal in reaction must occur at least twice 

 in fishes hke the salmon. 



Fishes react to environmental factors long before the adjustment 

 becomes a matter of life and death. Day has shown that the salmon 

 might remain in fresh water without results fatal either to the individual 

 or to the species. " The mechanism, therefore, which is working to 

 preserve the life of the organism is so delicate that it produces beneficial 

 reactions upon the part of the animal far in advance of life and death 

 complications. The working of this mechanism is undoubtedly closely 

 connected with quantitative and perhaps qualitative changes in meta- 

 bolism. These changes in metabolism will have a direct relation to the 

 amount of CO., given off by the organism." 



A slight increase in the carbon dioxide content of an animal's blood 

 results in a marked increase of general irritability, which will result in 

 an increase in the range and vigour of the movements. The state of 

 metabohsm in the sex organs probably furnishes the initial stimulus to 

 breeding migration. The instinct which causes the salmon to leave 

 fresh water for the sea is backed by some very strong stimulus, as is 

 shown by the persistent way in which they will leap from ponds. Yet 

 they are reacting in a way that is not immediately essential. 



Physiology of Migration of Mullets.* — Louis Roule has made 

 interesting observations on the movements of Grey Mullets {3fugU) 

 from littoral ponds and estuaries to the sea. The currents of water 

 from the sea towards the ponds are richer in dissolved oxygen than the 

 currents in the opposite direction. The metabolism consequent on the 

 development of the large gonads involves, as in the salmon, an intensifi- 

 cation of respiration. The mullets therefore make for the water with 

 greater oxygen-content. Roule has previously shown f that the move- 

 ments of the salmon are also affected by the variable oxygenation of 

 the water at the mouths of rivers. Other factors operate, of course, in 

 the migration of fishes, notably those connected with depth, temperature, 

 and salinity. 



* Comptes Rendus, clxi. (1915) pp. 537-9. 

 t Comptes Rendus, clviii. (1914) p. 1364. 



