The database for this directory comes from a list of accumulated taxonomic information assembled over the years from the literature of Wim Vervoort and Peter Schuchert (see Schuchert, 1989). The list of the Siphonophorae was provided by Gill Mapstone who also manages the species of this Order. The number of known hydrozoan species is about 3800, although the status of many nominal species is currently unclear. A considerable number of names also represent synonyms.

The list of currently accepted names is at least 90% complete, but the taxonomy of many species still needs revision and newly described species will be added as soon as possible after publication, a task that will be undertaken continuously by the editors. There is currently no broad consensus available for the higher classification of the Hydrozoa and several systems and alternative names are in use. Here, the classification given in Cartwright & Collins (2007) is used, although some minor spelling changes were made. Hence, the classification used in this database is somewhat provisional. The results of ongoing molecular phylogenetic investigations will certainly help to stabilise the taxonomic system in the near future.

No guarantee for the correctness of the data can be given. Please inform Peter Schuchert (the editor responsible for the group) of any omissions, errors or typos you come across, or if you need to confirm some crucial data. If you disagree with a synonym, genus assignment or other taxonomic decision, please, send in your reasoned argument to the editors. We will evaluate each contribution and corrections will be incorporated quickly.