A characteristic of cnidarians is the presence of stinging cells called cnidocytes. These cells contain nematocysts, which are used for both prey capture and defense. Another distinguishing feature of cnidarians is the presence of a gastrovascular canal that functions as both a digestive and circulatory system. Cnidarians have a simple nervous system, which includes a nerve net that coordinates their movements.

Animalia can be categorized into two categories based on whether they have a vertebral column or not.

1. Invertebrates

2. Vertebrates

Invertebrate groups

Invertebrates are organisms that lack a vertebral column. Invertebrates are classified into several phyla based on their characteristics.

  • Porifera (sponges)
  • Cnidaria (jellyfish, coral, anemones)
  • Platyhelminthes (flatworms)
  • Nematoda (roundworms)
  • Annelida (segmented worms)
  • Mollusca (snails, clams, octopuses)
  • Arthropoda (insects, spiders, crustaceans)
  • Echinodermata (starfish, sea urchins)

Examples for Cnidarians

This phylum includes diploblastic species such as Hydra, sea anemones, and jellyfish.


These are perhaps the most well-known cnidarians. They have a bell-shaped body and long, trailing tentacles that they use to capture prey. Some jellyfish can be bioluminescent and glow in the dark.

Sea anemones:

These are sessile cnidarians that attach themselves to rocks or other substrates. They have a cylindrical body and a ring of tentacles around their mouth. Some sea anemones have symbiotic relationships with other animals, such as clownfish.


These are colonial cnidarians that secrete calcium carbonate skeletons. They live in shallow, warm waters and form the basis of coral reefs, which are incredibly diverse ecosystems.


These are colonial cnidarians that are often mistaken for plants. They have a polyp body form and can form branching colonies. Some hydroids have stinging tentacles and are predatory, while others are filter feeders.

Box jellyfish:

Box jellyfish at Bakoven Rock DSC11031.JPG
By <a href=”//commons.wikimedia.org/wiki/User:Pbsouthwood” title=”User:Pbsouthwood”>Peter Southwood</a> – <span class=”int-own-work” lang=”en”>Own work</span>, CC BY-SA 3.0, Link

These are a group of cnidarians that are known for their potent venom. They have a cube-shaped bell and trailing tentacles that can cause severe pain and even death in humans.

Portuguese man o’ war:

This is not a single organism, but a colonial organism made up of several different types of polyps. It has a gas-filled float that allows it to float on the surface of the water, and long, trailing tentacles that can deliver a painful sting.

Sea pens:

Virgularia sp. (Purple sea pen).jpg
By <a href=”//commons.wikimedia.org/wiki/User:Nhobgood” title=”User:Nhobgood”>Nhobgood</a> Nick Hobgood – <span class=”int-own-work” lang=”en”>Own work</span>, CC BY-SA 3.0, Link

These are colonial cnidarians that live on the sea floor. They have a flexible, pen-shaped skeleton and a ring of tentacles around their mouth. Some sea pens can emit bioluminescent light.

Cnidarians characteristics / Features of Cnidarians

All are aquatic, with the majority being marine. Few of these cnidarians live in fresh water.

Two germinal layers make create a multicellular body. As a result, the condition is known as diploblastic.

The digestive tract is a cavity within the body called the coelenteron.

Medusa and Polyp are the two forms. While Polyps attach to the surface, Medusa can move.

They are symmetrical in the radial direction.

All of them are predators. Using Nematocyst / Cnidocyst, paralyze microscopic creatures.

Budding is a method of asexual reproduction. Show sexual reproduction as well.

Polyps in Cnidarians:

Polyps of Cnidaria colony.jpg
By <a href=”//commons.wikimedia.org/w/index.php?title=User:Pinetreella&amp;action=edit&amp;redlink=1″ class=”new” title=”User:Pinetreella (page does not exist)”>Pinetreella</a> – <span class=”int-own-work” lang=”en”>Own work</span>, CC BY-SA 4.0, Link

Polyps are a type of body form found in cnidarians, which include jellyfish, sea anemones, and coral. A polyp is a cylindrical or conical-shaped body with a mouth opening surrounded by tentacles. The polyp body form is usually attached to a substrate, such as a rock or the sea floor. Polyps are typically stationary and do not move around.

Medusa in Cnidarians:

The medusa body form is another type of body form found in cnidarians. Medusae are commonly known as jellyfish and have a bell-shaped body with tentacles hanging down from the edge. Medusae are free-swimming and can move around in the water column.

Similarity between Cnidarians and Ctenophores:

Cnidarians and ctenophores are alike in the way that they both have radial symmetry. This means that their body parts are arranged around a central axis, much like the spokes of a wheel. Both groups also have a simple nervous system and a gastrovascular cavity.

Movement of Cnidarians:

Cnidarians move in different ways depending on their body form. Polyps normally remain immobile and do not move, whereas medusae swim freely and move by contracting their bell-shaped body. Some cnidarians, such as sea anemones, can move slowly by sliding or gliding along a substrate.

Reproduction of Cnidarians:

Cnidarians reproduce in both sexual and asexual ways. Sexual reproduction involves the discharge of eggs and sperm into water, where fertilization happens. The larvae that form settle on a substrate and develop into polyps. Budding, in which a new individual grows from the parent polyp, or fragmentation, in which a piece of the polyp breaks off and grows into a new individual, are two methods of asexual reproduction. Some cnidarians can also reproduce by developing specialized structures known as medusae buds, which eventually develop into adult medusae.

By <a href=”//commons.wikimedia.org/w/index.php?title=User:Mummyna39&amp;action=edit&amp;redlink=1″ class=”new” title=”User:Mummyna39 (page does not exist)”>Hana Janoušková</a> – <span class=”int-own-work” lang=”en”>Own work</span>, CC BY-SA 4.0, Link

Share this post
About Author

Science A Plus

aluminium facts
Previous Post

Sulfur (S) Element, physical properties, chemical properties of sulphur, sulphur reactions and uses, sulphur chemistry,
Next Post