Anthozoans are solitary or colonial organisms with a
ciliated free larval stage and a polyp stage corresponding to an adult sexual
stage. They never have a medusa stage. The polyps have an ectodermic pharyngeal
tube (stomodeum) which allows for communication between the mouth and gastric
cavity. The radius of this cavity is divided into partitions, the mesenteries,
extending from the wall towards the axis of the polyp and attached or not
to the stomodeum. The polyps all have gonads in the mesoglea of the mesenteries.
All anthozoans are marine.
Present-day Anthozoa consist of two sub-classes:
The Octocorallia, The Hexacorallia.
Subclass Octocorallia
The big majority of the Octocorallia are sedentary colonies,
whose individuals communicate between themselves through endodermic canals,
the solenia. As well as a bilateral symmetry, the polyps have a radial symmetry
of eight parts. They have eight hollow and pinnate tentacles. Unlike the
Hexacorallia, the stomodeum has one or two furrows called siphonoglyphs.
This subclass includes the pennatulacea and the alcyonacea which are commonly
known as the soft corals. Among the Octocorallia, the best known is undoubtedly
the red coral, Corallium rubrum, greatly favoured by jewellers.
Subclass Hexacorallia
The Hexacorallia are solitary or in colonies. The polyps
have a radial symmetry of six or a multiple of six. The mesenteries mark
the compartments of the polyp's cavity. The hollow tentacles in multiples
of six, are placed in radial series or on alternate cycles depending strictly
on the disposition of the mesenteries.
This subclass has seven orders one of which is Scleractinaria,
where the main reef-building corals belong.
During the early stages of development in Scleractinaria, a primitive bilateral
symmetry appears, followed by a hexaradial symmetry, apparent on the polyp
and on the skeleton. However the hexaradial symmetry changes frequently
in adults. The organisms are mainly sessile, consisting of one or several
individuals known as polyps. Polyp division, asexual budding results in
several polyps bound to each other to form a colony. The polyp secretes
an aragonitic calcareous skeleton which forms part of the coral reefs. Most
of the Scleractinia have endosymbiotic dinoflagellates, known as zooxanthellae,
autotrophs or mixotrophs with regard to carbon.
There are terms allowing for the definition of organisms with regard to
the presence of zooxanthellae and also with their capacity to build reefs:
A zooxanthellate
organism has endosymbiotic dinoflagellates, the zooxanthellae. An azooxanthellate does not have zooxanthellae. Certain species can have both zooxanthellae and azooxanthellae individuals. aposymbiotic organism is temporally without its zooxanthellae. A hermatypic organism takes part in the building of the coral reef. An ahermatypic
organism does not take part in the building of the coral reef.
The limit between hermatypic and ahermatypic is often difficult to establish.
The hermatypic Scleractinaria take part in building the coral reef. Together
with the calcareous algae, they are the only organisms that synthesize their
calcium carbonate skeleton intensely enough to be able to compensate for
the natural destruction of the reef.
The Octocorallia,