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Phylum Coelenterata (Cnidaria): Cnidoblasts and Nematocysts

Biology · Animal Kingdom · NEET

Coelenterata (Cnidaria) animals have special stinging cells called cnidoblasts (also called cnidocytes) on their tentacles and body. Each cnidoblast holds a tiny stinging capsule called a nematocyst, which they use for defense, anchorage, and catching prey. Memory hook: "Cnida = a needle that stings" — the phylum is literally named after these stinging cells.
Cnidoblast (stinging cell) with Nematocyst insideCNIDOBLAST = the cellNematocyst(capsule)NnucleusfiresHydra (polyp)dots = cnidoblasts on tentacles
A cnidoblast (cnidocyte) is the stinging cell; the nematocyst is the capsule inside it that fires a thread to sting or trap prey. These cells stud the tentacles of cnidarians like Hydra.

Your doubts, answered

What is the difference between a cnidoblast and a nematocyst?

They are not the same thing, and NEET loves to test this. A cnidoblast (also called cnidocyte) is the whole stinging CELL. A nematocyst is the stinging CAPSULE or organelle INSIDE that cell. So the nematocyst is a part of the cnidoblast. Simple way to remember: cnidoblast = the cell, nematocyst = the weapon (capsule) it carries.

Why is Coelenterata also called Cnidaria?

The name 'Cnidaria' comes from the cnidoblasts (cnidocytes), the stinging cells found on the tentacles and body of these animals. Because this stinging cell is the most special and defining feature of the phylum, the group was named after it. NCERT gives both names together: Phylum – Coelenterata (Cnidaria).

What are cnidoblasts used for?

NCERT lists three jobs: anchorage (holding on to a surface), defense (protecting themselves), and capture of prey (catching food). The nematocyst inside the cnidoblast shoots out a thread to sting or trap. This is why touching a jellyfish can hurt you — the nematocysts fire into your skin.

Is Coelenterata diploblastic or triploblastic?

Coelenterata is DIPLOBLASTIC — the body is built from only two germ layers (ectoderm and endoderm), with a jelly-like mesoglea in between (not a true third layer). They show tissue level of organisation. This is a common NEET point: sponges are cellular level, coelenterates are tissue level and diploblastic.

Where is the mouth in a cnidarian and how do they digest food?

They have a single opening called the mouth, which sits on a raised part called the hypostome. This mouth leads into a central gastro-vascular cavity (coelenteron). Digestion is BOTH extracellular (in the cavity) and intracellular (inside cells). Remember: one opening only — the same hole is used for taking in food and throwing out waste.

What are the two body forms of cnidarians?

They show two body forms: polyp and medusa. The polyp is sessile (fixed) and cylindrical, like Hydra and Adamsia. The medusa is umbrella-shaped and free-swimming, like Aurelia (jellyfish). Animals that have both forms show alternation of generations, called metagenesis (e.g., Obelia). You will study this in detail in the next concept.

⚠️ The NEET trap
✗ Cnidoblast and nematocyst are the same structure, so either word can be matched to Coelenterata.
✓ The cnidoblast (cnidocyte) is the stinging CELL; the nematocyst is the capsule INSIDE it. In NEET 2021, 'Cnidoblasts' had to be matched to Coelenterata — the stinging cell is the diagnostic feature of the phylum.
🧠 Cnidoblast = the cell, Nematocyst = the capsule inside. Cell holds the capsule, never the other way around.

Real NEET questions

NEET 2021

Match List I with List II. List I: (a) Metamerism (b) Canal system (c) Comb plates (d) Cnidoblasts List II: (i) Coelenterata (ii) Ctenophora (iii) Annelida (iv) Porifera Choose the correct option.

A · (a)-(iii); (b)-(iv); (c)-(ii); (d)-(i) ✓
B · (a)-(iv); (b)-(i); (c)-(ii); (d)-(iii)
C · (a)-(iv); (b)-(iii); (c)-(i); (d)-(ii)
D · (a)-(iii); (b)-(iv); (c)-(i); (d)-(ii)
Solution: Cnidoblasts (stinging cells) are the diagnostic feature of Coelenterata, so (d)-(i). Metamerism (segmentation) is in Annelida (a)-(iii). Canal system is in Porifera (sponges) (b)-(iv). Comb plates are in Ctenophora (c)-(ii). This gives option A: (a)-(iii); (b)-(iv); (c)-(ii); (d)-(i).
NEET 2019 (Odisha)

Match the genera with their phylum: (a) Ophiura (b) Physalia (c) Pinctada (d) Planaria — with (i) Mollusca (ii) Platyhelminthes (iii) Echinodermata (iv) Coelenterata.

A · (a)-(iv), (b)-(i), (c)-(iii), (d)-(ii)
B · (a)-(iii), (b)-(iv), (c)-(i), (d)-(ii) ✓
C · (a)-(i), (b)-(iii), (c)-(iv), (d)-(ii)
D · (a)-(iii), (b)-(iv), (c)-(ii), (d)-(i)
Solution: Physalia (Portuguese man-of-war) is a coelenterate, so (b)-(iv). Ophiura (brittle star) is an echinoderm (a)-(iii). Pinctada (pearl oyster) is a mollusc (c)-(i). Planaria is a flatworm, Platyhelminthes (d)-(ii). This is option B. Physalia is the key cnidarian example to memorise.

Solved Animal Kingdom NEET PYQs

Try the real previous-year questions from this chapter — each with the answer and a full solution.

See all 41 Animal Kingdom NEET PYQs ›
Next concept: Polyp, Medusa and Metagenesis in Cnidarians →Keep learning — 2 minFeeling ready? Solve the Animal Kingdom NEET PYQs ›Or practice on your phone — get the free MedicNEET app ›

Frequently asked

Are cnidoblast and cnidocyte the same thing?

Yes. Cnidoblast and cnidocyte are two names for the same stinging cell. NCERT writes them together: 'cnidoblasts or cnidocytes'. The nematocyst is the capsule inside this cell, so it is different from both.

What level of organisation does Coelenterata show?

Coelenterata shows tissue level of organisation. Cells that do the same job are arranged into tissues. This is one step higher than sponges (Porifera), which show only cellular level.

Give some examples of Phylum Coelenterata for NEET.

Common NCERT examples are Hydra, Adamsia (sea anemone), Physalia (Portuguese man-of-war), Aurelia (jellyfish), Pennatula (sea-pen), Gorgonia (sea-fan), Meandrina (brain coral), and Obelia.

Do corals belong to Coelenterata?

Yes. Corals are cnidarians. Some corals build a skeleton made of calcium carbonate (CaCO3). Meandrina (brain coral) is a common NEET example.

Where are cnidoblasts located on the animal?

Cnidoblasts are present on the tentacles and the body surface. On the tentacles they help catch prey and sting; over the body they help in defense and anchorage.

Related concepts:
Polyp, Medusa and Metagenesis in CnidariansDiploblastic vs Triploblastic: The Germ Layers ExplainedPhylum Porifera: Sponges and the Canal SystemPhylum Ctenophora: Comb Plates and BioluminescenceSymmetry in Animals: Asymmetrical, Radial and Bilateral
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