Zoanthids in Cnidarian Classification:

Bothersome hydras, jellyfish, sea anemones, tube anemones, reef and non-reef-building corals, soft and hard, black, horny and stony, sea pens, sea pansies , sea wasps, Portuguese men of war and sea fans; these are the stinging-celled animals, the Cnidaria ("Nigh-dare-ee-yah"); in some older texts as the Phylum Coelenterata

These organisms are radially symmetrical (hence inclusion in a sub-Animal Kingdom, the Radiata), are made up of two basic tissue layers and possess salient "stinging" cells. These are grouped as thread-like (volvent) barbed/spined with and without toxins (penetrant), and "sticky" for anchoring (glutinant).

Their body shapes come in two basic formats; a sessile polyp-like and free-swimming medusa-like. The basic features of these body-types are evident; most species are either one or the other, medusoid or polypoid, others pass through both in their life cycle. The body cavity formed by the tissue layers bears series of tentacles around a single body/mouth/"anus" opening/mouth (the coelenteron).

Palythoa caribaeorum, the White Encrusting Zoanthid. Grown to white polyps of 1/4-1/2" diameter with two rows of tentacles that touch when fully open. Cozumel and Bonaire pix.

Parazoanthus catenularis, Brown Sponge Zoanthid. Polyps of about 1/8" diameter bear 20 brown to yellow pointed tentacles, with oral disks that are darkened in their centers. Cozumel pic by Di.

Parazoanthus parasiticus, the Sponge Zoanthid. Small (1/8" diameter) greenish tentacled polyps embedded in a sponge... in Cozumel on/in a Red Boring Sponge, Cliona delatrix.

Parazoanthus parasiticus, The Sponge Zoanthid, at right on a Niphates digitalis, the Pink Vase Sponge in Cozumel. Below in St. Thomas, U.S.V.I.

Parazoanthus swiftii, the Golden Zoanthid. 1/4" diameter polyps. Encrust several species of sponges. Here on a Brown Tube Sponge, Agelas conifera in the Bahamas and Cozumel.

Parazoanthus tunicans, the Hydroid Zoanthid. 1/4 " in size. Tropical West Atlantic. Grow on Feather Tree Hydroids... not toxic themselves... but don't touch the hydroid! Cozumel pix by Di.

Zoanthus pulchellus, the Mat Zoanthid. Green, brown flattened discs of up to dime size with tan tentacles. Also from the Caribbean, the every-now-and-then offered. Below, in a back reef setting in Tobago and St. Thomas, U.S.V.I.