They are a life stage for most animals of the class Hydrozoa and some hydroids, such as the freshwater hydra, are solitary, with the polyp attached directly to the substrate.
The evidence against this view may be classed under two heads: first, comparative evidence; hydroids very different in their structural characters and widely separate in the systematic classification of these organisms may produce medusae very similar, at least so far as the essential features of me
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