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U. S. BUREAU OF FISHERIES 



specimens are needed for exhibition, and in northern latitudes 

 enough collecting must be done in summer to provide against acci- 

 dents that may occur in winter. Fresh-water forms can not be had 

 when lakes and streams are frozen, and the winter season is unfavor- 

 able for the transportation of collections from the Tropics. 



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Exchanges of specimens with other aquariums are helpful only in 

 varying the exhibits, as each must do its own collecting. Aquariums 

 in the United States are few in number and so located as to be subject 

 to certain geographic and climatic limitations. 



The collecting of aquatic animals involves their transportation in 

 weighty tanks of water, which, moreover, must be kept pure in 



