PHYLUM CNIDARIA

SYSTEMATIC BIOLOGY					KINGDOM ANIMALIA
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PHYLUM CNIDARIA

INTRODUCTION TO THE CNIDARIA

Cnidaria (ni-DA-re-a) is the Latinized form of a Greek word (κνιδοσ) that means sting. The reference is to the stinging cells called cnidae or nematocysts.

The Cnidaria is a natural group of diploblastic organisms with a mostly acellular mesogloea that is derived from the ectoderm. They contain specialized cells, the cnidocytes, which produce a variety of adhesive and stinging structures collectively called cnidae, most of which are the stinging structures called nematocysts. The phylum includes hydroids (A), jellyfish (B-C), hydroids and corals (D). The calcareous corals are responsible for the occurrences of tropical reefs and whole tropical islands. These frequently form the basis of very productive tropical and subtropical marine ecosystems. Hinde (2001) says that the Cnidaria have a fossil history that goes back to the late Pre-Cambrian and are members of the Ediacaran fauna. Corals and coral-like animals appear in the fossil record of the Ordovician Period and form reef systems like the modern corals.

A. Hydra

B. Cassiopeia, the mangrove jellyfish.

C. Chironex, the sea wasp.

D. Living corals from the Great Barrier Reef.

Image A. http://www.ucmp.berkeley.edu/cnidaria/hydrozoa.html
Image C. http://www.aims.gov.au/pages/research/project-net/dma/pages/seawasp-01.html
Images B & D from the Systematics biodiversity image collection.

SYNOPTIC DESCRIPTION OF THE CNIDARIA

The following information came from Margulis and Schwartz (1998), Buchsbaum (1938), Barnes (1980), Barnes (1984), Brusca and Brusca (2003), Hickman (1973), Hinde (2001), Storer and Usinger (1965), and Tudge (2000).

I. SYNONYMS: Coelenterates

II. NUMBER: >10,000 species known.

III. PHYLUM CHARACTERISTICS:

A. Structure

Symmetry: Radial.

Body Cavity: Not present.

Body Covering: Epidermis.

Support: Hydrostatic "skeleton" some with external or internal skeletons of calcium carbonate or organic material.

Digestive System: Digestive or gastrovascular cavity open at one end. Animal is a carnivore. It captures animals with nematocysts on tentacles.

Circulatory System: None.

Locomotion: Medusoid adults swim by undulations of bell-like body. Hydroids and corals are sessile. Some with planula larvae that move by ciliated epithelium.

Excretory System: None.

Nervous System: Simple nerve net.

Endocrine System: None.

B. Reproduction:

Reproductive System: Specialized gonads. Eggs and sperm are produced. Asexual reproduction occurs.

Development: Some with planktonic planula larvae.

C. Ecology: Found mainly in marine environments. Some freshwater taxa.

SYSTEMATICS OF THE CNIDARIA

A recent review of metazoan relationships by Collins et al. (2005) suggests that the Cnidaria are sisters to the Bilateria and more recently derived than the Ctenophora (see also Martindale et al. 2002; and Aleshin and Petrov 2002). Furthermore, the basal nature of the Anthozoa (no medusae) is called into question such that both the Anthozoa and the medusoids (Scyphozoa + Cubozoa) are basal in the Cnidaria. All seem to agree on the more derived position of the Hydrozoa (e.g. Schuchert 1996; and Collins et al. 2005).

HIERARCHICAL CLASSIFICATION OF THE CNIDARIA

TAXONOMY OF THE CNIDARIA. Taxonomy of the Phylum after the system of of Brusca and Brusca (2003). Descriptions of the following taxa were taken from Margulis and Schwartz (1998), Buchsbaum (1938), Barnes (1980), Barnes (1984), Brusca and Brusca (2003), Hickman (1973), Hinde (2001), Storer and Usinger (1965), and Tudge (2000).

CLASS HYDROZOA (5 ORDERS)

Polyp and medusoid stages; either might be reduced. Mesogloea acellular; nematocysts only in epidermis; gametes develop in epidermis. Polyp radially symmetrical. Coelenteron not separated by septa. Typically colonial with individuals connected; often individual polyps with different functions. Medusoid form small, usually with mouth on hollow stalk. Shelf or velum extending inward from the margin of the bell. >3,000 extant species.

Bouganvilla, Calycopsis, Eleutheria, Eudendrium, Hydra, Hydractina, Hydrocoryne, Janaria, Lar, Pennaria, Polyorchis, Sarsia, Staurocladia, Stylactis,Tubularia, Abietinaria, Aequoria, Aglaophenia, Bonneviella, Campanularia, Cuvieria, Obelia, Gonionemus, Gonothyrea, Lovenella, Plumularia, Sertularia, Aegina, Botrynema, Craspedacusta, Cunina, Hydroctena, Liriope, Polypodium, Rhopalonema, Solmissus, Agalma, Apolemia, Eudoxoides, Nectocarmen, Physalia, Rhyzophysa, Sphaeronectes, Porpita, Velella, Halammohydra, Otohydra.

CLASS SCYPHOZOA (4 ORDERS)

Medusa dominant; polyp small or absent. Medusa large and free living. Gastrodermal gonads. Mesogloea cellular. No velum. Coelenteron partitioned by four septa. ~200 extant species.

Aurelia, Cyanea, Haliclystus, Lucernaria, Atolla, Linuche, Naustihoe, Periphylla, Stephanoscyphus, Tetraplatia, Chrysaora, Pelagia, Sanderia, Stygiomedusa, Cassiopeia, Cephea, Eupilema, Mastigias, Rhizostoma, Stomolophus.

CLASS CUBOZOA (1 ORDER)

Medusa square with four flat sides. Margin of bell drawn in to form velum-like organ. Tentacle (or cluster) at each corner. Nematocysts very painful. ~15 extant species.

Tripedalia, Carybdea, Chironex, Tamoya.

CLASS ANTHOZOA

No medusoid stage. Polyp large with thick, cellular mesogloea. Coelenteron partitioned by mesentaries with glands, filaments and nematocysts. Stomodaeum extends from mouth down into enteron. One or two ciliated grooves directing water into coelenteron. Oral disc surrounded by tentacles. Epidermal and gastrodermal nematocysts. No operculum. Often colonial. ~6,000 extant species.

SUBCLASS CERIANIPATHARIA (2 ORDERS)

Unspecialized musculature. Mesentaries (single, not paired and complete) and tentacles simple. Six primary mesenteries.

Cerianthus, Antipathes, Arachnanthus, Botruanthus, Ceriantheomorphe, Ceriantheopsis, Pachycerianthus.

SUBCLASS OCTOCORALLIA (=ALCYONARIA; 8 ORDERS)

Colonial with internal skeleton of calcium carbonate or horny material. Polyps with eight tentacles and coelenteron divided into eight compartments by mesenteries.

Alcyonium, Anthomastus, Ceratocaulon, Gersemia, Parerythropodium, Pseudogorgia, Acanthogorgia, Biareum, Corallium, Eugorgia, Eunicella, Isis, Leptogorgia, Muricea, Parisis, Psamogorgia, Swiftia, Epiphaxum, Heliopora, Anthoptilum, Balticina, Cavernularia, Funicularia, Ptilosarcus, Stylatula, Umbellula, Virgularia, Haimea, Hartea, Monoxenia, Psuchastes, Taiaroa, Clavularia, Cornularia, Sarcodictyon, Tubipora, Gorgonia, Renilla, Pennatula, Coelogorgia, Paratelesto, Telesto, Telestula.

SUBCLASS HEXACORALLIA (=ZOANTHARIA; 4 ORDERS)

Solitary or colonial anthozoans, most with secreted supporting material that is calcareous and external to the polyp. Polyps with six (or multiples thereof) simple tentacles. Coelenteron divided into many compartments by paired septa, also in multiples of six and with retractor muscles.

Adamsia, Aiptasia, Alicia, Anthopleura, Anthothoe, Bartholomea, Bunodactis, Calliactus, Condylanthus, Diadumene, Edwardsia, Epiactus, Halcampa, Haliplanella, Heteractis (Radianthus), Liponema, Metridium, Peachia, Phyllodiscus, Ptychodactis, Stichodactyla, Stomphia, Triactis, Agaricia, Astrangia, Balanophyllia, Dendrogyra, Flabellum, Actinia, Fungia, Goniopora, Letepsammia, Meandrina, Montipora, Oculina, Pachyseris, Porites, Psammocora, Siderastraea, Stylophora, Epizoanthus, Isaurus, Isozoanthus, Palythoa, Parazoanthus, Thoracactus, Zoanthus, Amplexidiscus, Corynactis, Rhodactis, Ricordea.

This page is maintained by Jack R. Holt. Last revised 03/14/2008.