Actinozoan: A Group of Radially Symmetrical Animals

Actinozoan: A Group of Radially Symmetrical Animals

Actinozoan is an old term that was used to describe animals that have their organs arranged radially around a central axis. The word comes from the Greek aktis, meaning ray, and zoon, meaning animal. Actinozoan was first used by Henri Marie Ducrotay de Blainville in 1834, who included many different kinds of animals in this group, such as sea anemones, corals, jellyfish, hydroids, echinoderms, bryozoans, and rotifers.

However, later zoologists realized that actinozoan was not a natural group of animals, but rather a mixture of unrelated forms that had convergently evolved radial symmetry. Thomas Huxley, in 1875, proposed a new classification of animals based on their embryonic development. He divided the actinozoan into two groups: Coelentera and Actinozoa. Coelentera were animals that had two layers of cells (ectoderm and endoderm) and a central cavity (coelenteron). Actinozoa were a subgroup of coelentera that had their embryos developing inside their body and released through the mouth. Huxley’s Actinozoa included sea anemones, corals, sea pens, and comb jellies.

Modern biology has further refined the classification of animals and rejected Huxley’s Actinozoa and Coelentera as valid groups. It is now known that comb jellies are not closely related to the other actinozoan animals, but belong to a separate phylum called Ctenophora. The remaining actinozoan animals are now placed in the phylum Cnidaria, which also includes jellyfish and hydroids. Cnidaria are characterized by having specialized stinging cells called cnidocytes, which they use for defense and prey capture. Cnidaria are divided into four classes: Anthozoa (sea anemones, corals, and sea pens), Hydrozoa (hydroids, fire corals, and siphonophores), Scyphozoa (true jellyfish), and Cubozoa (box jellyfish).

Actinozoan is therefore an obsolete term that is no longer used in modern zoology. However, it is still occasionally applied to the class Anthozoa, which are the most diverse and abundant group of cnidarians. Anthozoa are exclusively marine animals that live as solitary or colonial polyps. They have a cylindrical body with a mouth surrounded by tentacles at one end and a basal disc at the other end. They lack a medusa stage in their life cycle. Anthozoa are important components of coral reefs and other marine ecosystems.


  1. Actinozoa – Wikipedia
  2. Actinozoan Definition & Meaning – Merriam-Webster
  3. Actinozoan – Definition, Meaning & Synonyms |

Anthozoan Diversity and Ecology

Anthozoans are the most diverse and abundant group of cnidarians, with over 6,000 species described. They are found in all oceans and at all depths, from the intertidal zone to the deep sea. Anthozoans can be divided into two subclasses: Hexacorallia and Octocorallia. Hexacorallia have six or multiples of six tentacles and mesenteries (internal partitions), while Octocorallia have eight tentacles and mesenteries. Hexacorallia include sea anemones, stony corals, black corals, and tube anemones. Octocorallia include soft corals, sea pens, sea fans, and sea whips.

Anthozoans have a variety of forms and colors, ranging from simple solitary polyps to complex colonial structures. Some anthozoans are sessile and attach to hard substrates, while others are free-living and can move around. Some anthozoans are symbiotic with algae called zooxanthellae, which live inside their tissues and provide them with organic nutrients through photosynthesis. These anthozoans are usually found in shallow and clear waters, where they form coral reefs that support a high biodiversity of marine life. Other anthozoans are azooxanthellate and do not depend on algae for nutrition. These anthozoans are usually found in deeper and darker waters, where they feed on plankton or detritus.

Anthozoan Reproduction and Development

Anthozoans can reproduce both sexually and asexually. Sexual reproduction involves the production of gametes (eggs and sperm) that are released into the water column or retained within the polyp. Fertilization can be external or internal, depending on the species. The resulting zygote develops into a free-swimming larva called a planula, which settles on a suitable substrate and metamorphoses into a new polyp. Asexual reproduction involves the formation of new polyps by budding, fission, or fragmentation. These new polyps can remain attached to the parent polyp and form a colony, or detach and become independent individuals.

Anthozoans have a simple body plan that consists of two layers of cells: an outer ectoderm and an inner endoderm. Between these two layers is a gelatinous matrix called the mesoglea, which provides support and flexibility. The mouth is located at the center of the oral disc, which is surrounded by tentacles that bear cnidocytes. The mouth leads to a tubular pharynx that connects to the gastrovascular cavity, which is divided by mesenteries into chambers. The mesenteries also support the gonads (reproductive organs) and the filaments (digestive organs). The anthozoan body lacks a true nervous system, but has a network of nerve cells that coordinate its movements and responses to stimuli.

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