Cnidaria

Cnidaria is derived from a Greek word “cnidos” meaning stinging thread. These entities are characterized by the presence of cnidae. Cnidaria is a phylum under Animalia Kingdom, a classification comprising 10,000 described species, some of them are corals, sea anemones, jellyfishes and hydras. These aquatic entities are seen in both marine and freshwater habitats.

It is yet another ancient group comprising many fossil representatives. These are the simplest living entities, having true tissues. These were said to be the first animals in evolutionary history to have a definite shape. Members of this group are incredibly diverse in their form, as specified by massive medusae and corals, colonial siphonophores, feathery hydroids and box jellyfish with complicated eyes. Even so, such animals possess stinging cells referred to as nematocytes.

Formerly, Cnidarians were classified with ctenophores in the phylum Coelenterata. However, with an increasing awareness of its differences, they were placed in a separate phyla.

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Phylum Cnidaria Examples

A few examples of members belonging to Phylum Cnidaria are –

• Jellyfish – Phyllorhiza punctata (Scyphozoa)
• Box jellyfish – Carybdea branchi (Cubozoa)
• Siphonophore – Physalia physalis (Hydrozoa)
• Polypodium hydriforme (Polypodiozoa)
• Cerianthus filiformis (Ceriantharia)
• Sea anemones (Actinaria, part of Hexacorallia)
• Sea fan Gorgonia ventalina (Alcyonacea, part of Octocorallia)
• Coral Acropora muricata (Scleractinia, part of Hexacorallia)

Cnidaria – Common Names

Cnidarians are radially symmetrical, soft-bodied animals found in aquatic habitats. Their common names are sea anemones, jellyfish, corals and hydras. Some other cnidarians include Portuguese men-of-war, sea fans, sea pens and sea whips.

Characteristics of Cnidarians

• Mostly marine entities while few such as hydra are found in freshwater
• While some are solitary (sea anemone) some others are colonial (Corals)
• Depict a tissue grade of organization and are diploblastic
• Members exhibit radial symmetry, however sea anemones exhibit biradial symmetry
• Body wall comprises an outer epithelium referred to as epidermis and inner epithelium referred to as gastrodermis. There is a gelatinous mesoglea found between the inner and outer epidermis
• There are two different forms of cnidarians – Polyp and medusa. Polyp is hydroid form, sessile with mouth-up orientation. The medusa is bell or umbrella shaped with mouth down alignment
• Mesoglea comprises amoeboid cells which come from the ectoderm. The mesoglea in polyps is thin and thick in medusa, essential for buoyancy
• Body wall comprises stinging cells referred to as cnidocytes. Each of the cnidocyte cells comprises fluid filled membranous capsules – cnida. Cnidocytes are functional in defending and capturing prey
• They possess a blind sac-like central cavity referred to as gastrovascular cavity or coelenterons which opens out by the mouth surrounded by tentacles. Mouth is functional in ingestion and egestion. Coelenterons helps in digestion and circulation
• Through the process of diffusion across the body wall, there is exchange of respiratory gases and elimination of excretory wastes takes place
• Neurons are connected, forming a pair of nerve nets – one in the gastrodermis and the other in the epidermis. Nerve impulses travel in any direction. Along with nerve nets, the medusae have ganglia and nerve rings surrounding the margin of the bell
• In the medusoid form, sensory structures (statocysts) occur
• Asexual reproduction is by fission, budding and fragmentation
• Generally unisexual however some are bisexual. External fertilization occurs, and cleavage is holoblastic. Indirect development and includes a free swimming ciliated larval stage referred to as planula
• The alternation of asexually reproducing polyp form in species having polyp and medusa phases and in sexually reproducing medusa form is referred to as metagenesis
• They exhibit regeneration

Body Structure of Cnidarians

Cnidarians exhibit a specific characteristic – tentacles with stinging nematocytes which serve as small harpoons which react to stimuli by giving out tiny stinging cells which can poison and hook potential prey.

Cnidarians do not show bones and a central nervous system, rather have a nerve net. Members of this group have only two body layers – ectoderm and the endoderm. Mesoglea is found between these two body layers. The mesoglea (jelly-like) serves as a little more than a glue in some of these members while in a majority of the cnidarian animals, as seen in jellyfish, it makes for the most of the animal.

The main body cavity has one main opening, the mouth, which is engirdled by tentacles. In the non-mobile or sessile forms, the mouth is pointing up. In the mobile medusa form, the mouth is pointing downwards. Muscles of the body wall aids the medusa in swimming, while the tentacles in anemones and the corals move with the help of hydrostatic action.

Body Symmetry of Cnidarians

Most of the cnidarians exhibit radial symmetry, wherein the symmetry around a center point, such that any line drawn through the center of the entity divides the body into mirror images. Several cnidarians also show a second axis of bilateral symmetry, some others only show bilateral symmetry. Bilateral symmetry is when a single plane is drawn through the middle of the entity, showing mirror images across the plane.

Members of this class contain members with radial symmetry and the members with bilateral symmetry, and the members exhibit both kinds of symmetries.

Classification of Cnidarians

The classification of Cnidarians is into 4 main categories –

• Anthozoa – almost completely sessile. Example – corals, sea anemones, sea pens
• Scyphozoa – swimming (true jellyfish)
• Cubozoa – box jellies, possess complex eyes and potent toxins
• Hydrozoa – most diverse group with hydroids, siphonophores, several medusae, fire corals

This was a brief on Cnidarians. Explore related NEET concepts, at BYJU’S.

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