NCERT grounding
Phylum Coelenterata (Cnidaria) is treated in section 4.2.2 of the NCERT Class 11 Biology chapter Animal Kingdom. The chapter places it immediately after Porifera in the broad classification of Kingdom Animalia, because it marks the first advance from the cellular level of organisation of sponges to the tissue level of organisation. Every fact on this page is grounded in that NCERT section and in the salient-features comparison of Table 4.2.
"They are aquatic, mostly marine, sessile or free-swimming, radially symmetrical animals. The name cnidaria is derived from the cnidoblasts."
NCERT Class 11 Biology · Animal Kingdom · §4.2.2
NCERT lists the diagnostic feature of the phylum in Table 4.2 simply as "Cnidoblasts present", with the level of organisation recorded as tissue, symmetry as radial, coelom absent, segmentation absent, and the digestive system incomplete. Those table entries are the exact pegs on which NEET match-the-column questions are built, so they are reproduced and explained throughout this page.
Defining features of Coelenterata
Coelenterates are aquatic and mostly marine animals that live either as sessile, attached forms or as free-swimming forms. They are radially symmetrical: any plane passing through the central axis of the body divides the animal into two identical halves. This is the same body plan that NCERT also assigns to ctenophores and to adult echinoderms, which is why the three are often grouped together in symmetry questions.
The phylum carries two names that students must keep separate. Coelenterata refers to the single body cavity, the coelenteron or gastrovascular cavity. The newer name Cnidaria is derived from the cnidoblasts — the stinging cells unique to the phylum. NCERT uses "Coelenterata (Cnidaria)" as the heading, so both names are examinable and they describe the same phylum.
Internally, the body is built from only two embryonic cell layers, an outer ectoderm and an inner endoderm, making the phylum diploblastic. Between these two layers lies an undifferentiated, non-cellular jelly called the mesoglea. Because there is no third layer (mesoderm), no true body cavity lined by mesoderm can form — coelenterates are therefore recorded as acoelomate in NCERT Table 4.2.
Read this grid as the NCERT Table 4.2 row for Coelenterata. Each card is one column entry that NEET converts into a match or statement question.
Symmetry
Radial. Any plane through the central axis gives two identical halves.
Germ layers
Diploblastic. Ectoderm and endoderm only, with mesoglea between them.
Organisation
Tissue level. Cells of like function grouped into tissues; no organs.
Body cavity
Acoelomate. No mesoderm, hence no true coelom; one gastrovascular cavity.
Tissue level of organisation
In Porifera, cells are merely loose aggregates with some division of labour — the cellular level of organisation. In coelenterates, NCERT states that the arrangement of cells is "more complex", with cells performing the same function arranged into definite tissues. This is the tissue level of organisation, and Coelenterata is the lowest phylum to reach it. The phylum does not, however, build organs or organ systems; that step appears only from Platyhelminthes onward.
The gastrovascular cavity and digestion
The body of a coelenterate encloses a central gastrovascular cavity with a single opening, the mouth, situated on a raised cone called the hypostome. Because this one opening serves as both mouth and anus, the digestive system is described as incomplete in NCERT Table 4.2. Digestion in coelenterates is both extracellular and intracellular: food captured and pushed into the cavity is first broken down extracellularly within the cavity, and the partly digested fragments are then engulfed and digested intracellularly by the lining cells.
Porifera (sponges)
- Cellular level of organisation
- Mostly asymmetrical body
- Intracellular digestion only
- Canal system with ostia and osculum
- Choanocytes line the spongocoel
Coelenterata (cnidarians)
- Tissue level of organisation
- Radially symmetrical body
- Extracellular and intracellular digestion
- Gastrovascular cavity with one opening
- Cnidoblasts on tentacles and body
The contrast above is the clean way to remember the phylum's place in the syllabus. Coelenterata is the immediate successor of Porifera in NCERT's classification figure, and almost every defining feature is a one-step advance: cellular becomes tissue, asymmetry becomes radial symmetry, and digestion that was purely intracellular now adds an extracellular phase inside a true gastrovascular cavity.
Figure 1. The diploblastic body wall: an outer ectoderm and an inner endoderm enclose a non-cellular mesoglea. The two layers wrap a single gastrovascular cavity that opens to the outside only through the mouth borne on the hypostome.
The cnidoblast and nematocyst
The structure that names the phylum Cnidaria is the cnidoblast, also called the cnidocyte. NCERT states that cnidoblasts are present on the tentacles and the body, and that each one contains the stinging capsule, the nematocyst. Cnidoblasts are the diagnostic cell of the phylum — no other phylum possesses them, which makes "cnidoblasts present" a frequent match-the-column answer.
NCERT assigns the cnidoblast three explicit functions. They are used for anchorage, for defence, and for the capture of prey. The nematocyst inside discharges a coiled thread when triggered; in some cnidarians this thread injects a toxin that paralyses small prey, allowing the slow, sessile or weakly swimming animal to feed without active pursuit.
Figure 2. A cnidoblast (cnidocyte) carries the stinging capsule, the nematocyst, with its coiled thread. NCERT lists three roles for the cnidoblast: anchorage, defence, and the capture of prey.
Polyp, medusa and metagenesis
NCERT states that cnidarians exhibit two basic body forms, called polyp and medusa. The polyp is a sessile and cylindrical form, like Hydra and Adamsia. The medusa is umbrella-shaped and free-swimming, like Aurelia, the jelly fish. Both forms are radially symmetrical and diploblastic; they differ in shape, lifestyle and the part they play in reproduction.
Polyp
Hydra
Standard NCERT polyp example
- Sessile — attached, does not swim
- Cylindrical body shape
- Produces medusae asexually
- Other example: Adamsia
Medusa
Aurelia
Standard NCERT medusa example
- Free-swimming — moves through water
- Umbrella-shaped body
- Produces polyps sexually
- Commonly called the jelly fish
Many cnidarians pass through both forms in their life cycle. When a species exists in both polyp and medusa forms, it shows alternation of generations, which NCERT names metagenesis. In metagenesis the sessile polyp produces medusae asexually, and the free-swimming medusa produces polyps sexually. NCERT's stated example of metagenesis is Obelia.
Metagenesis — alternation of generations in Cnidaria
-
Step 1
Polyp stage
Sessile, cylindrical form attached to the substratum.
Asexual -
Step 2
Medusae budded
The polyp produces medusae by asexual budding.
Polyp → Medusa -
Step 3
Medusa stage
Umbrella-shaped, free-swimming form drifts in water.
Sexual -
Step 4
Polyp formed
The medusa reproduces sexually to give rise to polyps again.
Medusa → Polyp
Which form is sexual, which is asexual?
Students routinely reverse the two halves of metagenesis. Remember the direction by the lifestyle: the fixed polyp can only bud, so it reproduces asexually and gives medusae; the mobile medusa carries gametes through water, so it reproduces sexually and gives polyps.
Rule: Polyp → medusa is asexual; medusa → polyp is sexual. NCERT's example of metagenesis is Obelia.
Examples and corals
NCERT names a fixed set of coelenterate examples, and NEET draws its match-the-column options directly from this list. Each example must be tied to its common name and, where relevant, to its body form. The list below is exactly as given in NCERT section 4.2.2.
| Scientific name | Common name | Note |
|---|---|---|
| Hydra | — | Polyp form; sessile, cylindrical |
| Adamsia | Sea anemone | Polyp form |
| Physalia | Portuguese man-of-war | Colonial cnidarian |
| Aurelia | Jelly fish | Medusa form; free-swimming |
| Pennatula | Sea-pen | — |
| Gorgonia | Sea-fan | — |
| Meandrina | Brain coral | Coral with calcium carbonate skeleton |
| Obelia | — | Exists as polyp and medusa; shows metagenesis |
Among these, the corals deserve special attention. NCERT states that some cnidarians, such as corals, possess a skeleton composed of calcium carbonate. Meandrina, the brain coral, is the named coral example. This calcareous skeleton must not be confused with the calcareous shell of molluscs or the calcareous ossicles of echinoderms — those belong to entirely different phyla, and NEET uses that confusion to set traps.
Body forms in Cnidaria
The phylum has exactly two basic body forms: the sessile cylindrical polyp and the free-swimming umbrella-shaped medusa. A species showing both performs metagenesis.
Physalia, the Portuguese man-of-war, appears repeatedly in NEET match questions and must be linked to its common name without hesitation. Pennatula (sea-pen) and Gorgonia (sea-fan) are the two examples most often dropped from memory, so they are worth a deliberate revision pass. Together with Hydra, Adamsia, Aurelia, Meandrina and Obelia, they form the complete examinable set for this phylum.
Worked examples
A diploblastic animal with a single gastrovascular cavity, radial symmetry and stinging cells on its tentacles belongs to which phylum, and what is its level of organisation?
The stinging cells are cnidoblasts, the diagnostic feature of Phylum Coelenterata (Cnidaria). The diploblastic body, radial symmetry and single gastrovascular cavity all confirm this. Its level of organisation is the tissue level — cells of like function are grouped into tissues, one step above the cellular level of sponges and one step below the organ level of Platyhelminthes.
In the metagenesis of Obelia, the medusa stage produces the next generation of polyps by which mode of reproduction?
By the sexual mode. In metagenesis the polyp produces medusae asexually, while the free-swimming medusa produces polyps sexually. The mobile medusa is the gamete-bearing, sexual generation; the fixed polyp is the asexual, budding generation.
Match each cnidarian with its common name: Physalia, Adamsia, Pennatula, Meandrina.
Physalia is the Portuguese man-of-war; Adamsia is the sea anemone; Pennatula is the sea-pen; and Meandrina is the brain coral, the coelenterate with a calcium carbonate skeleton. All four are NCERT-listed examples of Phylum Coelenterata.
Why is the digestive system of a coelenterate described as incomplete, and what types of digestion occur in it?
The gastrovascular cavity has only a single opening, the mouth on the hypostome, which serves as both mouth and anus. A digestive system with one opening is, by NCERT's definition, incomplete. Digestion in coelenterates is both extracellular and intracellular: food is first broken down extracellularly within the cavity, then the fragments are engulfed and digested intracellularly by the lining cells.
Common confusion & NEET traps
Coelenterata sits in a cluster of three radially symmetrical phyla, and NEET exploits the overlap. The versus card below isolates the differences that examiners test most often.
Coelenterata (Cnidaria)
- Cnidoblasts present on tentacles and body
- Polyp and medusa body forms; metagenesis possible
- Digestion extracellular and intracellular
- Examples: Hydra, Aurelia, Physalia
Ctenophora
- Eight rows of ciliated comb plates for locomotion
- No polyp–medusa alternation
- Bioluminescence well-marked
- Examples: Pleurobrachia, Ctenoplana