Phylum Coelenterata (Cnidaria) NEET Notes: Features, Examples & 25 MCQs

Phylum Coelenterata (also called Cnidaria) is one of the most scoring topics of the NEET chapter Animal Kingdom. In these notes, based on NCERT Class 11 Biology, we cover every line on coelenterates: cnidoblasts, tissue level of organisation, the gastrovascular cavity, polyp and medusa, metagenesis, corals, coral reefs and the key examples. A set of 25 NEET-style MCQs is given at the end.

Phylum Coelenterata at a glance

According to NCERT, coelenterates are aquatic, mostly marine, sessile or free-swimming, radially symmetrical animals. In Phylum Coelenterata the body has no true organs; cells are grouped into tissues. They are the simplest animals to show tissue-level organisation along with ctenophores, and they come right after Porifera (sponges) in the classification of the animal kingdom.

Why the name Coelenterata or Cnidaria?

The term Coelenterata derives from coelenteron (gastrovascular cavity). This phylum is also called Cnidaria because it contains cnidoblasts – a specific type of cell called a sting cell or cnidocyte.

In NCERT words, the name cnidaria is derived from the cnidoblasts or cnidocytes (which contain the stinging capsules or nematocysts) present on the tentacles and the body. Note that the coelenteron is not a true coelom; members of Phylum Coelenterata are acoelomate.

General characteristics of Phylum Coelenterata

General Characteristics:

• Aquatic, mostly marine, solitary or colonial – tissue level of organisation.
• Radial symmetry, blind sac body plan
• Diploblastic (epidermis and gastrodermis)
• Coelenteron / gastrovascular cavity
• Digestion: extracellular as well as intracellular
• Respiration, excretion: diffusion
• Reproduction: asexual and sexual
• Fertilization: gametes are produced by the sexual stage (medusa in forms showing metagenesis)
• Development: Indirect (Planula larva)
• Examples: Sea anemone, Corallium (red coral), jellyfish, physalia (Portuguese man of war), aurelia, sea fan

Additional features to remember

• They can be sessile (attached, e.g. Hydra, Adamsia) or free-swimming (e.g. Aurelia).
• Body is radially symmetrical: any plane through the central axis divides it into identical halves.
• Blind sac body plan: the digestive system is incomplete, with one opening that works as both mouth and anus.
• There is no body cavity (coelom), no segmentation, and no circulatory or respiratory system.
• A simple nerve net is present.

Cnidoblasts and nematocysts

Cnidoblasts are the special cells that give the phylum its name. Each cnidoblast contains a stinging capsule (nematocyst) with a coiled thread. They are present on the tentacles and the body (NCERT Figure 4.7 shows a diagrammatic view of a cnidoblast).

NCERT lists three uses of cnidoblasts in Phylum Coelenterata:

• Anchorage – attaching to the substratum
• Defence – stinging enemies
• Capture of prey – paralysing small animals

Body wall and organisation

Cnidarians exhibit tissue-level organization and are diploblastic. The outermost layer is called the epidermis, and the innermost layer is called the gastrodermis. Between the two layers is the mesoglea. They have a central gastrovascular cavity with a single opening (mouth on the hypostome). They exhibit two basic body forms called polyp and medusa. The former is sessile and cylindrical, such as hydra and Adamsia, while the latter is umbrella-shaped and free-swimming, such as Aurelia (jellyfish). Cnidarians that exist in both polyp and medusa forms show alternation of generations, or metagenesis; the classic example is Obelia. Physalia is a colonial form. Coelenterates have a simple nerve net.

Diploblastic body

In diploblastic animals the cells are arranged in two embryonic layers: an external ectoderm and an internal endoderm. An undifferentiated layer, the mesoglea, is present in between. (Triploblastic animals, from Platyhelminthes onwards, have a third layer, the mesoderm.)

Gastro-vascular cavity and digestion

The central gastro-vascular cavity (coelenteron) opens to the outside by a single opening, the mouth, located on a raised cone called the hypostome. Digestion is extracellular (enzymes act on food inside the cavity) and intracellular (food particles are taken into gastrodermal cells and digested in food vacuoles). The same cavity distributes food, which is why it is called gastro-vascular.

Polyp and medusa: the two body forms

Members of Phylum Coelenterata exhibit two basic body forms called polyp and medusa (NCERT Figure 4.6).

Metagenesis

You might have read about alternation of generations in plants, where diploid and haploid stages interchange in the life cycle. However, in coelenterates, asexual and sexual stages alternate, which is called metagenesis. It is a characteristic feature of this phylum. The asexual stage is called a polyp, a cylindrical, sessile structure. The sexual stage is called a medusa, which is umbrella-shaped and shows movement. Example: Obelia (sea fur). Physalia (Portuguese man-of-war) is a colonial cnidarian, not the NCERT example of metagenesis.

NCERT states it precisely: those cnidarians which exist in both forms exhibit alternation of generations (Metagenesis), i.e., polyps produce medusae asexually and medusae form the polyps sexually (e.g., Obelia). In Phylum Coelenterata, the medusae release gametes; the zygote develops into a ciliated planula larva, which settles and grows into a new polyp colony.

Draft reference: https://www.aakash.ac.in/important-concepts/biology/phylum-coelenterata-cnidaria

Corals and coral reefs

Coral is a generalized term for all kinds of coelenterates whose bodies (exoskeleton) are surrounded by a calcium carbonate structure. These corals form large colonies, which we call coral reefs. Coral reefs generally form around islands. For instance, the Great Barrier Reef surrounds Australia.

Some members of Phylum Coelenterata, e.g., corals, have a skeleton composed of calcium carbonate. Meandrina (brain coral) is the NCERT example of a coral.

Types of Coral Reefs

• Fringing reef: If the coral reef is present at the base of a mountainous island
• Barrier reef: If the coral reef is present around an island, but the distance between the island and the reef is about 2 to 4 km, then such a coral reef is called a barrier reef. The area between the island and the coral reef is filled with water, commonly called a lagoon.
• Atoll: If the island is submerged in water and the coral reef only encircles the water, such a coral reef is called an atoll.

Examples of Coelenterata

Coelenterata in the salient features table (NCERT Table 4.2)

Next: Phylum Ctenophora →

Coelenterata vs Ctenophora

Phylum Coelenterata: Common doubts

Is the coelenteron a true coelom?

No. The coelenteron is the gastro-vascular (digestive) cavity. Coelenterates have no coelom and are acoelomate.

Why are coelenterates called diploblastic?

Their body develops from two layers, an outer ectoderm (epidermis) and an inner endoderm (gastrodermis), with an undifferentiated mesoglea between them.

Does Hydra show metagenesis?

No. Hydra exists only as a polyp. Obelia, which has both polyp and medusa stages, is the NCERT example of metagenesis.

What is the function of cnidoblasts?

Cnidoblasts contain nematocysts and are used for anchorage, defence and the capture of prey.

Is a jellyfish a fish?

No. Aurelia (jelly fish) is a cnidarian in the medusa form. True fishes are chordates.

Is digestion in coelenterates intracellular or extracellular?

Both. Food is partly digested in the gastro-vascular cavity (extracellular) and completed inside gastrodermal cells (intracellular).

Coelenterata NEET MCQs (25 questions)

Test yourself with these 25 NCERT-based NEET practice questions on Phylum Coelenterata. They cover every NCERT line on Phylum Coelenterata, from cnidoblasts to coral examples.