Anthozoa

With about 6,000 species, the class Anthozoa includes about two-thirds of extant cnidarian phylum. But unlike other cnidarians, anthozoans do not have a medusa stage in their development. Some of species harbor a type of algae (dinoflagellates called zooxanthellae), with which they form a symbiotic relationship.

Anthozoans are also known as flower animals of the sea for their lustrous, rainbow-like colors. Subclass Alcyonaria includes animals such as soft coral, sea fan, sea pen, and blue coral. Each polyp has eight tentacles that are pinnately branched. Subclass Zoantharia includes animals such as sea anemone, stony coral, and black coral. Even though the name of the subclass means it has six tentacles, most of members don't have six tentacles. Rather, zoantharians typically have a multiple of six mesenteries and tentacles. It was created on the 5th day of the creation.

Anatomy
Anthozoans have two mainly related structures, the actinopharynx and the mesenteries, which are unique among cnidarian polyps. The actinopharynx, or stomodeum, is a tubular gullet extending all the way from mouth to the coelenteron. The actinopharynx of most species contains at least one siphonoglyph, flagellated longitudinal channel, that drives water into the coelenteron. Most of corals and sea anemones have two siphonoglyphs situated directly opposite on one another in the actinopharynx. Siphonoglyphs and their associated structures make the polyps bilateral or a biradial symmetry. Mesenteries, Longitudinal sheets of tissue, increase surface area for respiration and especially for absorption of food, they also provide support. Perhaps because mesenteries increase, some polyps of anthozoa can grow much larger than any other polyps in other class of Cnidaria. 

Reproduction
All anthozoans can reproduce sexually and some can also reproduce asexually.


 * Sexual - Organisms in the Anthozoa class can reproduce sexually by forming eggs and sperm, which fertilize like the other organism classes in Cnidaria.
 * Asexual - Anthozoans may reproduce asexually by budding or by dividing themselves in half.

Anthozoans completely lack a medusa stage. They live only as polyps, which is unique from other cnidarians. 

Ecology


Anthozoans can feed on large prey or particulate food by using their nematocysts, or stinging cells. A number of anthozoans supplement their diet by growing symbiotic algae in their tissues. Hermatypic, or reef-buildings, scleractinian corals can live successfuly by having symbiotic dinoflagellate algae in the genus Symbiodinium living inside their tissues. Even though Coral polyps use their tentacles and nematocysts to feed, Symbiodinium may actually produce most of a coral's food. For this reason, reef-building corals have to be with light or they can't survive (although solitary corals, octocorals, and anemones can live in much cooler and deeper waters). Therefore, one of the reason of global warming and rising sea levels will be the death of coral reefs.