Phylum Cnidaria: Definition, Characteristics & Examples

Phylum Cnidaria: The term “cnidaria” is derived from the Greek word “cnidos,” which means “stinging thread.” These organisms are distinguished by the existence of cnidae. Cnidaria is a phylum in the Animalia Kingdom with 10,000 documented species, including corals, sea anemones, jellyfishes, and hydras. These aquatic creatures can be found in both marine and freshwater environments. These are the most basic biological things, with real tissues. These were thought to be the earliest animals in evolutionary history with a distinct form. 

Massive medusae and corals, colonial siphonophores, feathery hydroids, and box jellyfish with sophisticated eyes are examples of members of this category. Cnidarians were previously grouped with ctenophores in the phylum Coelenterata. However, when people became more conscious of their peculiarities, they were classified as a separate phylum. In this article, we will explore cnidarians examples, cnidaria characteristics and their importance. 

Phylum Cnidaria Definition

Phylum Cnidaria is also known as Phylum Coelenterate. This phylum consists of radially or radially symmetrical aquatic invertebrates having unique stinging structures in the tentacles surrounding the mouth. The organisms of this species mainly belong to marine life, and hardly a few live in freshwater. This phylum includes animals like Jellyfish, Hydra, Corals, etc.
The word Cnidaria originated from the Greek word “Cnidos”, which means “Stinging nettle”.

Fig: Examples of Phylum Cnidaria

What is the History of Phylum Cnidaria?

The origin of Cnidaria is very ancient, more than \(580\) million years ago. In some fossils, it is indicated that corals existed \(490\) million years ago. It is believed that Phylum Cnidaria originated during the Cryogenian period.

Fig: Fossil of Cnidaria

Cnidarians Body Forms

1) They are characterized by stinging cells called Cnidoblast and a cavity called coelenterates, justifying the name Cnidaria or Coelenterata.
2) They are exclusively aquatic and marine.
3) They are radially symmetrical and diploblastic animals.
4) The ectoderm of Coelenterates is provided with a special type of cells called cnidoblasts or stinging cells. These cells have to hypnotize, used for defense and paralyzing the prey. They are found in large numbers around the mouth and over the tentacles.
5) They show both intracellular and extracellular digestion in the gastrovascular cavity.
6) The circulatory, respiratory and excretory systems are absent in Cnidarians. Gaseous exchange and excretion take place through the body surface.
7) To maintain equilibrium, the body of Cnidaria possesses single or complicated sensory organs like statocysts and tentaculocyst.
8) They have a poorly developed nervous system and it is present in the form of a nerve net in the body walls and the tentacles.
9) They show polymorphism. Two body forms are present: asexual polyp and sexual medusa stage.
10) The Polyp is a cylindrical shape in Hydra.
11) The Medusae is umbrella-shaped in Jellyfish.
12) Asexual reproduction occurs by the method of budding or spores.
13) Sexual reproduction occurs by the production of gametes. Sexes are not separate.
14) Embryo shows indirect development.
15) They show external fertilization.
16) Some exhibit alternation of generation, i.e., metagenesis, a phenomenon where the asexual generation (polyp) alternates with the sexual generation(medusa).
17) Some common examples are Hydra, Aurelia (JellyFish- Medusae form), Adamsia (Sea anemone- Polyp form), Pennatula (Sea pen), Gorgonia (Sea Fan). Meandrina (Brain coral), Physalia (Portuguese man-of-war), Obelia (Both polyp and medusa form).

Fig: Polyp and Medusa

Classification of Cnidarians

The phylum Cnidaria is also called as Colenterata and it is divided into three classes which is mentioned below:

I. Hydrozoa: The Greek word “Hydros” means water and “Zoon” means animal.
a) They are mostly marine, and some are freshwater animals.
b) They are mainly colonial, and some are solitary.
c) They show both asexual polyp and sexual medusa stage.
d) Polyp stage is the dominant, medusa stage is reduced or absent.
e) Presence of true velum.
f) Acellular mesoglea is present.
g) Gonads are epidermal in origin.
h) To maintain equilibrium, they possess statocysts.
i) Examples: Hydra, Obelia, Physalia, Tubularia, etc.

II. Scyphozoa: The Greek word “Skyphos” means cup and “Zoon” means animal.
a) They are entirely marine animals.
b) Medusa form is dominant, and it is umbrella shaped.
c) Cellular Mesoglea is present.
d) The polyp form is rudimentary.
e) The pseudo velum is present.
f) Gonads are endodermal in origin.
g) To maintain equilibrium, they possess tentaculocyst.
h) Examples: Jellyfish, Aurelia, Rhizostoma, etc.

III. Anthozoa: The Greek word “Anthos” means flower and “Zoon” means animal.
a) All the animals are marine and show colonial forms.
b) The polyp stage is present.
c) The medusa stage is absent.
d) Absence of velum.
e) Cnidocytes are present in epidermal as well as gastrodermal region
f) The mouth is oval, and it is surrounded by a whorl of tentacles that resembles the structure of a flower.
g) Gonads are endodermal in origin.
h) Example: Monactis (Sea anemone), Telesto, Tubipora, Xenia, etc.

Phylum

Cnidarians examples

The examples of Phylum Annelida are mentioned below:

Hydra

1. The systematic position of Hydra is –
Kingdom: Animalia
Phylum: Cnidaria
Class: Hydrozoa
Order: Anthoathecata
Family: Hydridae
Genus: Hydra
2. They are diploblastic, radially symmetrical, small, multicellular, cylindrical, and freshwater organisms.
3. They possess two types of tissue layers: the ectoderm and the endoderm.
4. They are found attached to solid objects in water such as leaves, stones, weeds, etc.
5. The body of a Hydra is organized like a tube (polyp-like) with tentacles arranged around the head pole of the organism.
6. They are hermaphrodites.
7. They reproduce asexually by a process known as budding.

Fig: Hydra

Physalia (Portuguese Man of War)

1. The systematic position of Physalia is –
Kingdom: Animalia
Phylum: Cnidaria
Class: Hydrozoa
Order: Siphonophorae
Family: Physaliidae
Genus: Physalia
2. They show polymorphism.
3. They are colonial sponges. The colony carries a transparent bluish or pinkish balloon-like large float or pneumatophore at the top.
4. The pneumatophore of Jellyfish floats above the surface and the whole colony remains inside water.
5. The up and down movements of the body in water takes place due to the secretion and absorption of gases like \({N_2}\) and \(CO\).
6. These gases are produced from the gas glands.
7. The sexual reproduction takes place by a method known as broadcast spawning.

Fig: Physalia

Jellyfish

1. The systematic position of Jellyfish is –
Kingdom: Animalia
Phylum: Cnidaria
Class: Scyphozoa
Order: Semaeostomeae
Family: Ulmaridae
Genus: Aurelia
2. They possess two types of tissue layers: the ectoderm and the endoderm.
3. Their mouth is found in the centre of its body.
4. The body of a jellyfish shows radial symmetry and is divided into three main parts: the umbrella, the oral arms (around the mouth) and the tentacles armed with tiny, stinging cells.
5. The tentacles serve as defense organs and as a powerful weapon for capturing prey.
6. They have no brain, heart, bones, or eyes.
7. They show bioluminescent that means they produce their own light.

Fig: Jellyfish

Sea Anemone

1. The systematic position of Sea Anemone is –
Kingdom: Animalia
Phylum: Cnidaria
Class: Anthozoa
Order: Actiniaria
Family: Hormathiidae
Genus: Monactis
2. They look like flowers, but they are meat eater’s animals.
3. They have a soft, simple polyp-style body with two tissue layers: the ectoderm and the endoderm and a central gut cavity.
4. The mouth is enclosed by stinging tentacles.
5. Their body lacks external or internal calcareous skeletons.
6. They show symbiotic relationships with other marine organisms.
7. They show both sexual and asexual type of reproduction.

Fig: Sea Anemone

Phylum Cnidaria Importance

The economic and ecological importance of Phylum Cnidaria are mentioned below:

Economic Importance

1. Tourists go to the Coral reefs for diving and snorkelling activities.
2. Corals are used in large quantities in aquariums to give a marine look.
3. Corals are used in the jewel industry.
4. Some compounds from Cnidarians are used for treating cancer and other illnesses.
5. Coral skeletons are used in making cement.

Ecological Importance

1. Corals provide habitat to many species like fishes, algae, seaweed, etc.
2. They provide food to many marine animals, and some Cnidaria are used as food by humans, such as sea anemones.
3. Coral reefs protect the land from ocean waves.

Summary

Most animals in Phylum Cnidaria are marine, and very few of them are found in freshwater. Organisms of this phylum show radial, biradial symmetry and are diploblastic. Though the organisms of this phylum are not so familiar, still they are a significant part of the animal kingdom and play a vital role, especially in the aquatic world. Cnidarians have an ancient history; Cnidarians include jellyfish, corals, and hydra as the most known animals in this phylum.

FAQs

Q.1. What organisms are present in the Phylum Cnidaria?
Ans: The organisms present in the Phylum Cnidaria are Hydra, Sea Anemone, Jellyfish, Corals, Physalia, Obelia, Tubularia, etc.

Q.2. What are the different classes of the Phylum Cnidaria?
Ans: The different classes of Phylum Cnidaria are Hydrozoa, Scyphozoa and Anthozoa.

Q.3. What is the meaning of the term Cnidaria?
Ans: The word Cnidaria originated from the Greek word “Cnidos” which means “Stinging Nettle”.

Q.4. What are \(5\) characteristics of Cnidarians?
Ans: 1) They are characterized by the presence of stinging cells called Cnidoblast and a cavity called coelenterates, justifying the name Cnidaria and Coelenterata.
2) They are exclusively aquatic and marine.
3) They are radially symmetrical and diploblastic animals.
4) The ectoderm of Coelenterates is provided with a special type of cells called cnidoblasts or stinging cells. These cells have hypnotoxin which are used for defense and paralyzing the prey. They are found in large numbers around the mouth and over the tentacles.
5) Two types of body forms are present: asexual polyp and sexual medusa stage.

Q.5. What are stinging cells?
Ans: Stinging cells are also known as Cnidocytes. They are specialized neural cells. These cells contain an organelle called cnidocyst, which is the product of extensive Golgi secretions.