Phylum Coelenterata: Characteristics, Classification, and Examples

• These are the simplest multicellular invertebrate species, which can be found in colonies or singly.
• They can also be found sitting or swimming freely.
• The Phylum Coelenterata is made up of marine organisms with radially symmetrical bodies and sensory tentacles that aid in the capture of prey.
• Hydra, comb jellies, true jellies, sea pens, coral animals, sea anemones, and other animals are members of the Phylum Coelenterata.

Key Terms: Phylum Coelenterata, Invertebrate, Aquatic species, Multicellular organisms, Intracellular digestion, Extracellular digestion, Method of reproduction

Characteristics of Phylum Coelenterata

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The following are the general characteristics of Phylum Coelenterata

• Most of these organisms live in marine or aquatic environments.
• The mouth is enclosed by thin, short tentacles.
• These species show tissue-level organization.
• They are diploblastic organisms, with a body composed of two layers of cells.
  • The first layer known as the Ectoderm makes up the cells outside the body.
  • The second layer known as the Endoderm is the inner lining of the body.
• They have cavities in their bodies.
• The digestion occurs both intracellular and extracellular.
• The body is radially symmetrical.
• They excrete and breathe by simple diffusion.
• The nervous system and circulatory systems are not available.
• The method of reproduction is asexual, through budding.
• Only a small number of Coelenterates reproduce sexually. For example, Rhizostoma, Hydra, Xenia, etc.

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Classification of Coelenterata 

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Coelenterates are divided into three categories:

• Hydrozoa
• Scyphozoa
• Anthozoa

Hydrozoa

Hydrozoa is a Coelenterate from Class I. It gets its name from the Greek words hydro (water) and zoon (animal).

• Hydrozoa can be found in all three layers of the marine ecosystem.
• Some, like Physalia and Velella, live on the surface and have large sail-like structures to keep them afloat above water.
• These are also beneficial in terms of locomotion.
• Long tentacles with nematocysts, also known as stinging cells, aid Hydrozoa in catching food that is frequently found below the surface.
• The second group of Hydrozoa is found in the pelagic region, also known as the middle water zone.
• This is where organisms such as Bougainvillea and Chelia can be found.
• These organisms have long tentacles that allow them to jet propel themselves forward while also sinking rapidly when necessary.
• The third group of Hydrozoa lives on the seafloor, where they usually anchor themselves to rocks.
• Sea Corals are a good example of this.

Hydrozoa

Characteristics of Hydrozoa

The following characteristics are shared by these organisms.

• Some hydrozoa are found to live in colonies, while others live in solidarities.
• These organisms are mostly marine and freshwater in nature.
• Asexual polyps are the most common type.
• In them, the mesoglea is acellular and exists in its most basic form.
• The Medusa possesses true value.
• Hydra, Tubularia, Obelia, and Physalia Physalis are examples of hydrozoa.
• Sexual reproduction results in the production of planula larvae.
• They have two body types: medusae and polyps.
• Cnidocytes are found in the bodies of Hydrozoa. These are also referred to as stinging cells.
• Hydrozoa is classified into 2700 species and 7 orders.

Scyphozoa

Scyphozoa, also known as Scyphomedusae, is a Coelenterate Class II organism. Scyphozoa as a species is frequently a source of concern when it washes up on beaches or comes into contact with humans.

• Jellyfish stings have frequently been known to kill or seriously injure humans.
• They also cause problems for large-scale fishing by frequently clogging nets and straying into communities. 

Characteristics of Scyphozoa

These organisms have the following characteristics.

• They are only found in the sea.
• These are mostly solitary and free-swimming organisms.
• Scyphozoa's dominant characteristic is the medusa, which is shaped like an umbrella or a large bell.
• Polyps are either non-existent or extremely short-lived in these organisms.
• The mesoglea is typically cellular in nature.
• Scyphozoa species include Rhizostoma, Jellyfish, and Aurelia aurita.

Anthozoa

Anthozoa and Actinozoa are Coelenterate Class II organisms. These get their names from the Greek words anthos (flower) and zoios (animal).

Characteristics of Anthozoa

The characteristics of Anthozoa include:

• These creatures can only be found in the sea.
• Their way of life is either colonial or solitary.
• These organisms lack a medusa.
• Anthozoa's mesoglea is made up of ameboid cells and fibrous connective tissues.
• Anthozoa species include Xenia, Telesto, Metridium, Tubipora, and others.
• The gastrovascular cavity is present in these organisms.
• Anthozoa protects and supports its soft body tissues by secreting nonliving substances around and outside its body.
• Not all Anthozoa are of the same genus. Some are hermaphroditic, while others have separate sexes.
• Anthozoa can have a variety of appearances. They can range in size from a half-inch to nearly two feet in large clusters of sea pens. Colors can range from red to purple.

Examples of Coelenterates

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The following are examples of Coelenterates

• Portuguese man-of-war (Physalia)
• Brain coral (Meandrina)
• Sea-pen (Pennatula)
• Sea-fan (Gorgonia)
• Sea anemone (Adamsia)

Things to Remember

• Phylum Coelenterata is the simplest group of animals and belongs to the category of invertebrates.
• They are usually found at the bottom of the sea, often attached to rocks.
• Coelenterates are multicellular organisms that can either be found living solitarily or in groups. 
• Moreover, they can also be found sedentary or swimming about freely.
• Coelenterate organisms have a multicellular body organization with tissue grade.
• They have an inner endoderm layer called the gastrodermis and an outer ectoderm layer called the epidermis. Their skin is diploblastic.
• Coelenterates are divided into three categories: Hydrozoa, Scyphozoa, and Anthozoa.

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