Hydrozoa Encrusting a Small Shell
Hydrozoa (hydrozoans) are a taxonomic class of very small, predatory animals which can be solitary or colonial and which mostly live in saltwater. A few genera within this class live in freshwater. Hydrozoans are related to jellyfish and corals and belong to the phylum Cnidaria.
Benson John Lossing, ed. The New Popular Educator (London, England: Cassell & Company Limited, 1891)