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 bottom is varied according to the nature and habitat of the animal being studied. Coarse mussel-shell sand is used for Amphioxus and fine slime for marine annelids like Capitella. The burrowing animals are protected from direct sunshine. Where the proper bottom is not provided many burrowing animals lose the habit in confinement. Crustaceans are very sensitive to temperatures higher than 22° Cor lower than 6° C. Some kinds of living plants and animals can be transported 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
large wooden tub. For especially sensitive animals sometimes a steel flask of compressed oxygen, the "hydrobion," is attached to the transport 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 complicated 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