Hydra is a member of the animal phylum Cnidaria (formerly Coelenterata). It is classified as follows:

Phylum Cnidaria

Class Hydrozoa

Subclass Hydroidolina

Order Anthoathecata

Suborder Aplanulata

Family Hydridae

Genus Hydra

The genus Hydra was established by Linnaeus in 1758 (Linnaeus, 1758). In 1914, Paul Schulze (Schulze, 1914) added two additional hydra genera – Pelmatohydra, and Chlorohydra. However Pelmatohydra and Chlorohydra are no longer valid taxa. Pelmatohydra is now Hydra oligactis, and Chlorohydra is now Hydra viridissima. You will sometimes see Hydra viridissima referred to as Hydra viridis. viridis is not a valid Hydra species name. The most recent change to the taxonomy of Hydra was the establishment of the suborder Aplanulata by Collins et al. in 2005 (Collins et al., 2005). Aplanulata is a clade of hydrozoans, including Hydra, which have lost the planula larval stage.

REFERENCES

Collins, A.G., Winkelmann, S., Hadrys, H., and Schierwter, B. (2005). Phylogeny of Capitata and Corynidae (Cnidaria, Hydrozoa) in light of mitochondrial 16S rDNA data. Zool. Scripta 34, 91-99.

Linnaeus, C. (1758). Systema Naturae, 10th Edition.

Schulze, P. (1914). Bestimmungstabelle der deutschen Hydraarten. Sber. Ges. naturf. Freunde Berl. 9, 395-398.