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THE POPULAR SCIENCE MONTHLY 



centration of salts is maintained, after evaporation below the original 

 water level, by regularly adding fresh water from a vessel standing 

 near, so that the temperature will not be altered. The aquarium bot- 

 tom is varied according to the nature and habitat of the animal being 

 studied. Coarse mussel-shell sand is used for AmpJiioxus and fine 

 slime for marine annelids like Capitella. The burrowing animals are 

 protected from direct sunshine. Where the proper bottom is not pro- 

 vided many burrowing animals lose the habit in confinement. Crus- 

 taceans are very sensitive to temperatures higher than 22° C. or lower 

 than 6° C. Some kinds of living plants and animals can be trans- 

 ported from the sea simply in moist water-plants. Those of other 

 species must be placed in individual glass containers, each covered with 

 wet parchment paper and packed in a straw basket and all carried in a 



Fig. 16. Pump foe the Sea-water Circulation. 



large wooden tub. For especially sensitive animals sometimes a steel 

 flask of compressed oxygen, the " hydrobion," is attached to the trans- 

 port tank. By means of a reducing valve opened at the beginning of 

 the journey, the flow of oxygen is properly regulated. On account of 

 injury in travelling to the apparatus, or to the animals in striking 

 against the outflow tube, it is found more satisfactory to saturate the 

 sea water with oxygen before the journey and so dispense with the 

 " hydrobion/' 



The aquarial and circulatory system for fresh water is less compli- 

 cated than that for sea water since it is not so necessary to use the non- 

 corrosive materials in its construction. The water supply, from a 

 height of 2,000 meters, is free from the excess of lime and phosphorus 

 found in the ground water which comes through the Danube alluvium. 

 The rain water, collected in subterranean cisterns, is also used. The 

 more active animals, especially the carnivorous species which chase their 

 prey, require the larger aquaria. An animal 10 cm. long should be 



