Image from World Hydrozoa Database.

Fewer than 1% of cnidarian species occur in freshwater. The phylum is perhaps best known for marine jellyfish, corals, and sea anemone. In freshwater, the Cnidaria is represented by the Class Hydrozoa. However, only a few dozen species are known from inland habitats, and probably only about 10 occur in North America (Jankowski et al. 2008).

The cnidarians represent a zoological advance in complexity over the Porifera in that they go through the developmental process of gastrulation and possess true tissues -- like other groups of metazoans. However, members of the Cnidaria only have two embryonic tissue layers. A unique characteristic of this phylum is alternation of generation between an asexual polyp stage and a sexual medusa stage (although one or the other may be suppressed in various taxa). The body plan is radially symmetrical with only a diffuse nerve net rather than a central nervous system.

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Bibliography

• Pechenik, J.A. 2015. Biology of the Invertebrates, 7th edition. McGraw-Hill, NewYork.. 606 pp.
• Zhang, Z.-Q. 2011. Animal biodiversity: An introduction to higher-level classification and taxonomic richness. Zootaxa 3148: 7-12. [Available online]
• Slobodkin, L.B. & P.E. Bossert. 2010. Cnidaria. [in] J.H. Thorp & A.P. Covich (eds.). Ecology and Classification of North American Freshwater Invertebrates, 3rd edition: 125-142.
• Jankowski, T., A.G. Collins & R. Campbell. 2008. Global diversity of inland water cnidarians. Hydrobiologia 595: 35-40. [Available online]
• Daly, M., M.R. Brugler, P. Cartwright, A.G. Collins, M.N. Dawson, D.G. Fautin, S.C. France, C.S. McFadden, D.M. Opresko, E. Rodriguez, S.L. Romano & J.L. Stake. 2007. The phylum Cnidaria: A review of phylogenetic patterns and diversity 300 years after Linnaeus. Zootaxa 1668: 127-182. [Available online]
• Brusca, R.C. & G.J. Brusca. 2003. Invertebrates, 2nd edition. Sinauer Associates, Inc., Sunderland, Massachusetts. 936 pp.
• Smith, D.G. 2001. Cnidaria (Hydroids, jellyfish). Pennak's Freshwater Invertebrates of the United States, 4th edition: 61-76.
• Jankowski, T. 2001. The freshwater medusae of the world — a taxonomic and systematic literature study with some remarks on other inland water jellyfish. Hydrobiologia 462: 91-113. [Available online]
• Dumont, H.J. 1994. The distribution and ecology of the fresh- and brackish-water medusae of the world. Hydrobiologia 272: 1-12. [Available online]
• Underhill, J.C., W.D. Schmid & D.E. Gilbertson. 1982. Taxonomic Keys to the Common Animals of the North Central States, exclusive of the parasitic worms, terrestrial insects, and birds, 4th edition. 205 pp.
• Batha, J.V. 1974. The distribution and ecology of the genus Hydra in the Milwaukee area of Lake Michigan. Ph.D. Dissertation, University of Wisconsin, Madison. 172 pp.
• Hyman, L.H. 1959. Coelenterata. [in] W.T. Edmondson (ed.). Ward & Whipple Fresh-Water Biology, 2nd edition: 313-322.
• Miller, D.E. 1936. A limnological study of Pelmatohydra with special reference to their quantitative seasonal distribution. Transactions of the American Microscopical Society 55(2): 123-193. [Available online]
• Welch, P.S. & H.A. Loomis. 1924. A limnological study of Hydra oligactis in Douglas Lake, Michigan. Transactions of the American Microscopical Society 43: 203-235. [Available online]

Student Essays

• "Alternation of Generation and Exceptions in Cnidaria" by Margaret Armstrong (BIOL/WATR 361, Fall 2012 )
• "Ecological and Economic Impacts of Freshwater Cnidarians" by Eric Brandt (BIOL/WATR 361, Fall 2013 )
• "Life Cycle of Freshwater Jellyfish: Craspedacusta sowerbyi" by Sarah Francart (BIOL/WATR 361, Fall 2012 )

Web Pages

• Animal Diversity Web
• Biology Browser
• Freshwater Animal Diversity Assessment
• Integrated Taxonomic Information System
• Systema Naturae 2000
• Tree of Life
• WikiSpecies
• Wikipedia
• World Hydrozoa Database
• World Register of Marine Species