To establish the exact relationship it is necessary not only to breed but to rear the medusa, which cannot always be done in 1 In some cases hydroids have been reared in aquaria from ova of medusae, but these hydroids have not yet been found in the sea (Browne [Io a]).
Is said to produce fully developed gonads, and if kept in aquaria with Tubifex, the number of infected worms steadily increases, a fact pointing to the whole cycle being passed through, without the intermediation of a vertebrate host.
In later times royal persons and great feudal magnates frequently kept menageries of wild animals, aviaries and aquaria, and it is from these that modern public Gardens have taken their origin.
Birds and mammals take the first place; the leading collections devote a good deal of attention to reptiles and batrachians; fishes and aquatic invertebrata are most often to be found only when there are special aquaria, whilst non-aquatic invertebrates are seldom to be seen and at most consist of a few moths and butterflies, spiders, scorpions and centipedes, molluscs and crustaceans.
Paulo in Brazil supplied Sars with representatives of all the three in his Norwegian aquaria, in some of which the little Macrothrix elegans " multiplied to such an extraordinary extent as at last to fill up the water with immense shoals of individuals."