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Hydroids and medusae are among the most exquisitely beautiful marine animals. Although the marine species vastly outnumber the freshwaters ones, the genus Hydra may nevertheless be the best known hydrozoan. Almost no textbook on invertebrate zoology fails to present Hydra. However, the simple Hydra is not representative at all for the Hydrozoa and it may actually give a wrong impression of the complexity of this animal group.

There are about 3500 known species which cover an immense variety of forms and different life histories, which makes it nearly impossible to devise a generalized picture of this group.
Nevertheless, the original life cycle of the Hydrozoa presumably included a sessile polyp stage that multiplies asexually. 

The polyp stage then buds a free, planktonic medusa stage which will produce the eggs or sperms. More details can be found in the "Information" pages.
This series of internet pages aims at providing a taxonomic overview of the higher taxa. The inspiration for these pages came from the beautiful Echinoid pages. 

However, it is unlikely that individual pages for the lower ranks, like families and genera, can be provided in near future.

Information for some species and genera is given in pdf files. Future versions of these pages will hopefully be created dynamically.

Because its main focus is on taxonomy, aspects of hydrozoan biology and ecology are also not included in these pages. This information can easily be taken from other sources (see also references below).

Although much effort has been made to make this site also useable for casual visitor with no biological training, some basic knowledge on hydrozoan morphology is necessary

Most of the basic terms used here are explained in the "Information" pages and in the subject index. Terms that are restricted to subgroups are on the respective taxon pages.

References to useful introductions to hydrozoans and other aspects that not treated here:

Bouillon, J. 1994. Les Hydrozoaires. In Traité de Zoologie, vol. 3(2) (ed. P. Grassé & J. Doumeng), pp. 29-416. Paris: Masson. 

Brusca, R. C., and G. J. Brusca. Invertebrates. 1990. Sinauer Associates, Sunderland, MA. 

Cornelius, P. F. S. 1995. North-west European thecate hydroids and their medusae. Synopses of the British Fauna New Series 50, 2 volumes.

Gili, J. M. & Hughes, R. G. 1995. The ecology of marine benthic hydroids. Oceanography And Marine Biology An Annual Review 33: 351-426.

Hyman, L. H. 1940. The Invertebrates: Protozoa through Ctenophora. New York and London: McGraw-Hill.

Millard, N. A. H. 1975. Monograph on the Hydroida of southern Africa. Annals of the South African Museum 68: 1-513. 

Werner, B. 1984. 4. Stamm Cnidaria, Nesseltiere. In Wirbellose Tiere (ed. H.-E. Gruner), pp. 10-305. Stuttgart: Gustav Fischer.