Coelenterata Overview Structure and Classification

The term ‘ Coelenterata ’ comes from ancient Greek ‘hollow’ and ‘intestine’.  Belonging to the animal kingdom, Coelenterata, or more commonly known as the Phylum Coelenterata are the simplest group of animals that fall under the category of invertebrates. They are usually found living at the bottom of the sea, often attached to rocks. Coelenterates are multicellular organisms and can be found living solitarily or also in groups. They can also be found sedentary or swimming about freely.

The Phylum Coelenterata consists of marine organisms that have a body that is radially symmetrical and a mouth that has sensory tentacles that help in capturing prey easily. Other animals belonging to the Phylum Coelenterata are hydra, comb jellies, true jellies, sea pens, coral animals, sea anemones and more.

Characteristics of Coelenterata

Usually known to be the simplest of all animal groups, Coelenterates have true tissues and the gastrovascular cavity, which is the characteristic coelenteron.

However, these organisms also have a few other characteristic features that make them highly different from other organisms in the animal kingdom, as listed below. 

• These organisms are mostly aquatic. Some live in marine water, for example, Sea Anemones, while some live in freshwater, for example, Hydra. 

• They may live in colonies like obelia or may live solitarily like Hydra. 

• Some Coelenterates are free-floating or are known to swim about freely, like Aurelia. Others are sedentary like Corals. 

• Their bodies are mostly cylindrical, flat or resemble the shape of a cup. 

• Their bodies can be radially symmetrical with a central gastrovascular cavity or maybe completely bilaterally symmetrical like Sea Anemones. 

• Body organisation of organisms belonging to the Coelenterates are multicellular with tissue grade.

• They have an inner layer of endoderm called the gastrodermis, and outer ectoderm called the epidermis. Their body wall is diploblastic.  

• Coelenterates are acoelomate animals.

• These organisms have a blind body sac plan. 

• A non-cellular layer of mesoglea is present. This layer is gelatinous in nature. 

• The function of an intestine in Coelenterates is played by an internal hollow cavity called coelenteron, located inside the body. 

• Coelenterates have a holozoic form of nutrition.

• The mouths of these organisms are encircled with tentacles which have nematocysts. 

• There is no presence of an anus in these organisms; the mouth is responsible for both ingestion and elimination. 

• Coelenterates have tentacles that help them capture their prey, eat and digest it. These tentacles are also used for defence purposes. 

• Modes of digestion in these organisms are either intracellular or extracellular. 

• There is no presence of a skeletal structure of any sort, however, certain coral polyps secrete calcareous skeleton, which is made of up minerals like calcium carbonate. 

• There are no circulatory, respiratory or excretory organs in Coelenterates. The outer body surface is responsible for respiratory and excretory functions. 

• These organisms use tentacles and their smooth muscle fibres for locomotion or general movement. Organisms like Corals are fixed on the substratum though. 

• Diffuse nerve cells make up the nervous system in Coelenterates; these organisms, however, do not have a brain.  

• Reproduction is seen in both sexual and asexual modes. While sexual reproduction takes place by the method of fusion of gametes, asexual reproduction takes place by fission and budding. 

• These organisms exhibit the behaviour of polymorphism, which means during their life cycle, they occur in the form of medusa- sexual form and polyp- asexual form. 

• Polyps have an exo-skeleton and endo-skeleton. 

• They have interstitial cells due to which the process of regeneration is well developed in Coelenterates. 

• The bodies of these organisms consist of cnidoblasts, which are knob-like structures responsible for the secretion of hypnotoxin chemicals. These chemicals are useful in self-defence or attacking and paralyzing the prey.  

• The larva is ciliated and free-swimming. 

Common examples include Obelia, Hydra, Metridium, Rhizostoma, Xenia, Aurelia and more. 

Classification of Coelenterata

Coelenterates can be classified into the following three categories:

• Hydrozoa

• Scyphozoa

• Anthozoa

Here is a more detailed explanation of the functions of each of the categories. 

1. Hydrozoa

Hydrozoa belongs to Class I of the Coelenterates. It derives its name from Hydra: water and Zoon: animal. These organisms have the following characteristic features. 

• Some hydrozoa are found to exist in colonies while few live solidarily.  

• These organisms are mostly of the marine and freshwater kind. 

• The dominant form among them is asexual polyps. 

• The mesoglea is acellular in them and exists in the simplest form.

• The medusa has true valum. 

• Examples of hydrozoa are Hydra, Tubularia, Obelia, Physalia physalis.

• The planula larvae are produced as a result of sexual reproduction.

• They have two distinct body forms- a medusa and a polyp. 

• Hydrozoa has Cnidocytes present in their bodies. These are also known as stinging cells. 

• Hydrozoa exists in 2700 specimens and 7 orders. 

As marine organisms, Hydrozoa exists in all three layers of the marine ecosystem. Some live on the surface and have large sail-like structures to keep them afloat above water, for example, Physalia and Velella. These also help in locomotion. Long tentacles that also have nematocysts or alternatively known as stinging cells that help Hydrozoa catch food that’s often below the surface. 

The second group of Hydrozoa live in the pelagic region, or more commonly known as the middle water zone. This is where organisms like Bougainvillea and Chelia exist. These organisms have long tentacles that help them jet propel themselves forward and sinking down rapidly when required.  

The third group of Hydrozoa exists in the bottom, that is the sea bed, where they usually anchor themselves to the rocks. A good example of these is Sea Corals. 

2. Scyphozoa

Scyphozoa or Scyphomedusae belongs to Class II of the Coelenterates. These organisms have the following characteristics.

• They exclusively belong to marine habitats.

• These are mostly solitary and free-swimming organisms. 

• The medusa in Scyphozoa resembles an umbrella or a large bell in shape and is of dominant characteristic. 

• Polyps are either completely absent in these organisms or extremely short-lived. 

• The mesoglea is usually cellular in nature. 

• Examples of Scyphozoa include Rhizostoma, Jellyfish or Aurelia aurita.

As a species, Scyphozoa can often be a source of trouble when they wash up on the beaches, or when they come in direct contact with humans. Jellyfish stings have often been known to cause deaths or serious damage to humans. They are also sources of trouble when it comes to large scale fishing by often clogging the fishing nets or straying into communities. 

3. Anthozoa

Anthozoa or Actinozoa belongs to Class II of the Coelenterates. These organisms have the following characteristics. These derive their name from Anthos: flower and zoios: animal. 

• These animals exclusively belong to the marine habitat. 

• Their living habits are either colonial or solitary. 

• These organisms do not have a medusa. 

• The mesoglea of Anthozoa consist of ameboid cells and fibrous connective tissues. 

• Examples of Anthozoa include Xenia, Telesto, Metridium, Tubipora and more. 

• These organisms have a gastrovascular cavity. 

• Anthozoa protects its soft body tissues and lends support to its body by secreting nonliving substances around and outside its body. 

• Not all Anthozoa belong to the same sex. Some are hermaphroditic while others are segregated into separate sexes. 

Anthozoa can be of varying appearances. They can range anywhere from a half-inch to almost a two feet expanse of large clusters of sea pens. Colours can be as varied as red, purple, white, yellow, blue, violet and more.