Phylum Coelenterata Examples and Types

Animals belonging to the phylum Coelenterate belong to the class of Invertebrata. They are often referred to as coelenterates. Coelenterates are marine animals, either existing in groups or solitarily. They are usually found residing at the bottom of water bodies, mostly appended to rocks. They can likewise be found swimming freely or sedentary.

Coelenterates are characterized by the presence of a radially symmetrical body and the sensory tentacles present on the mouth. A few examples of animals belonging to the Coelenterata phylum include sea anemones, hydra, sea pens, comb jellies, coral animals, obelia, aurelia, xenia, metridium, and many more.

Characteristics of coelenterates

Every phylum has certain specific characteristics based on which animals are grouped. Below are the characteristics specific to the Coelenterata phylum.

• Coelenterates are either marine or freshwater organisms.
  - Example of marine coelenterate - Sea Anemones.
  - Example of freshwater coelenterate - Hydra.
• Animals belonging to this phylum exist independently and in colonies as well.
  - Example of animals existing solitarily - Hydra
  - Example of animals existing in colonies - obelia
• A few coelenterates are free-floating while the others are sedentary.
  - Example of sedentary coelenterate - coral animals
  - Example of free-floating coelenterata - aurelia
• A few coelenterates consist of a skeleton composed of calcium carbonate. Example - Coral animals.
• All Coelenterates are multicellular organisms consisting of true tissues.
• Coelenterates are either flat, cylindrical, or cup-shaped.
• They show a tissue level of organisation where cells performing a similar function are grouped into a tissue.
• Coelenterates are characterised by the presence of stinging capsules called nematocytes. These nematocytes are present on the whole body and tentacles as well. Hence, they are also referred to as cnidocytes or cnidoblasts. The key function of nematocytes is to defend the organism from predators or attackers. Nematocytes release certain toxic chemicals which paralyse the predator and protect the organism.
• Most coelenterates are radially symmetrical consisting of a central gastrovascular cavity. This gastrovascular cavity consists of only one opening i.e. mouth present on the hypostome. The anus is absent.
• Digestion may take place extracellularly or intracellularly. The tentacles present on the mouth help the organism capture, swallow, and digest its food.
• These animals are acoelomates and are diploblastic. The two layers of the body include the outer ectoderm layer and the inner endoderm layer.
• Coelenterates do not possess specific organs for circulation, excretion, and respiration. The outer body surface aids in respiration. Excretion is via simple diffusion.
• The primary function of tentacles is to aid them in locomotion. Nevertheless, Coelenterates such as coral animals are fixed on the ground.
• Coelenterates do not possess the brain and a definite nervous system. The diffuse nerve cells present in their body act as a nervous system.
• Coelenterate animals exist in two basic forms - polyp and medusa. The polyp form of a coelenterate consists of an endoskeleton and exoskeleton. The polyp form is cylindrical and sessile whereas medusa is free-swimming and umbrella-shaped.
  Examples of polyp Coelenterata - Adamsia and Hydra
  Examples of medusa coelenterates - Jellyfish
• A few coelenterates exist in both forms and show alteration in generations. This phenomenon where one generation produces another is called metagenesis.
  - Polyps, upon asexual reproduction, produce medusae forms.
  - Medusae, upon sexual reproduction, produce polyp forms. 
  - An example of a coelenterate existing in both polyp and medusa forms is obelia.
• The larva of coelenterates consists of cilia and swims about freely.

Classification of the phylum Coelenterata

The animals belonging to the phylum Coelenterata are classified into three main types.
Hydrozoa
Scyphozoa
Anthozoa

Hydrozoa

• Hydrozoan coelenterates are marine and found mostly in freshwater.
• Animals belonging to this class may live independently or in groups.
• Asexual polyps dominate this class.
• The mesoglea layer of hydrozoans is acellular.
• The body of a hydra consists of cnidocytes. Cnidocytes, also called stinging cells or nematocytes, release hypnotoxin-class chemicals. These chemicals are known to protect the organism from prey animals.

Examples of hydrozoa: Hydra, Physalia, Obelia, and Tubularia.

Scyphozoa

• Coelenterates belonging to Scyphozoa are exclusively marine.
• Animals belonging to this class are mostly free-swimming and live solitarily.
• Sexual medusae forms dominate this class and are umbrella-shaped.
• Polyp forms do not exist in this class.
• Scyphozoa have cellular mesoglea.

Examples of Scyphozoa: Rhizostome and Aurelia (Jellyfish).

Anthozoa

• Similar to the anthozoans, Coelenterates belonging to Anthozoa are also exclusively marine.
• Anthozoans have mesoglea that are composed of amoeboid cells and fibrous connective tissues.
• Medusae forms do not exist in this class.
• Animals of this class possess a gastrovascular cavity.
• A few Anthozoa coelenterates are hermaphrodites whereas the others have separate sexes.

Examples of Anthozoa: Xenia, tubipora, metridium, and Telesto.