🪸 Introduction

Phylum Coelenterata, also called Cnidaria, includes aquatic, mostly marine animals with a simple body organization and distinct tissues. They are diploblastic, radially symmetrical, and possess specialized stinging cells (cnidoblasts) used for defense and prey capture.

  • Name Origin: The name Cnidaria is derived from the cnidoblasts or cnidocytes (stinging cells) present on the tentacles and the body. These cells contain stinging capsules called nematocysts.

  • Functions of Cnidoblasts: Anchorage, defense, and capture of prey.

  • Examples: Physalia (Portuguese man-of-war), Adamsia (Sea anemone), Pennatula (Sea-pen), Gorgonia (Sea-fan), Meandrina (Brain coral), Aurelia (Jellyfish), Obelia, Hydra.

🧬 General Characteristics

  • Habitat: Mostly marine, few freshwater (Hydra).

  • Symmetry: They exhibit radial symmetry.

  • Germ Layers: They are diploblastic (ectoderm and endoderm).

  • Body Cavity: They have a central gastro-vascular cavity (coelenteron) with a single opening, the mouth on the hypostome, which serves for both ingestion and egestion (incomplete digestive system).

  • Body organization: They exhibit the tissue level of organisation.

  • Body wall: Two layers — outer ectoderm, inner endoderm (diploblastic).

  • Body forms: Two morphological types

    1. Polyp: Sessile and cylindrical form (e.g., Hydra, Adamsia - Sea anemone).
    2. Medusa: Umbrella-shaped and free-swimming form (e.g., Aurelia - Jellyfish).

  • Digestion: Extracellular and intracellular within gastrovascular cavity.

  • Skeletal types: Hydrostatic, calcareous, or chitinous (in corals). Some cnidarians, like corals, have a skeleton composed of calcium carbonate.

  • Special cells: Cnidoblasts (nematocysts) for defense and capturing prey.

  • Digestion: Digestion is both extracellular (in the gastro-vascular cavity) and intracellular (within the cells lining the cavity).

  • Alternation of Generations (Metagenesis): Some cnidarians exist in both forms and exhibit alternation of generation (metagenesis). Polyps produce medusae asexually, and medusae form the polyps sexually (e.g., Obelia).

🧫 Polymorphism

  • Occurrence of multiple body forms within the same species.
  • Example: Obelia exhibits both polyp and medusa stages — alternation of generations (metagenesis).

🌊 Reproduction

  • Asexual: By budding (polyp stage).
  • Sexual: Medusa produces gametes → fertilization → planula larva → polyp.

🪸 Coral Formation

  • Polyps of Anthozoa secrete calcium carbonate, forming coral reefs.
  • Coral reefs provide rich marine biodiversity (e.g., Great Barrier Reef, Australia).

🧠 Classification

Class Features Examples
Hydrozoa Both polyp and medusa; colonial Hydra, Obelia, Physalia
Scyphozoa Predominantly medusa Aurelia (Jellyfish)
Anthozoa Only polyp; coral formation Adamsia, Gorgonia, Pennatula

🔍 Summary Table

Feature Cnidaria
Level of organization Tissue
Germ layers Diploblastic
Symmetry Radial
Digestion Extracellular & intracellular
Unique cells Cnidoblasts
Body forms Polyp & Medusa
Reproduction Asexual (budding), Sexual (gametes)
Example Hydra, Obelia, Aurelia, Adamsia

💡 Questions and Answers

Q1. Name the specialized cells found in coelenterates and their function.
A1. Cnidoblasts or nematocysts — used for defense and capturing prey.

Q2. What are the two body forms in cnidarians?
A2. Polyp (sessile) and Medusa (free-swimming).

Q3. What is polymorphism in Cnidaria?
A3. Occurrence of different body forms (e.g., polyp and medusa) in one life cycle.

Q4. What type of symmetry is shown by cnidarians?
A4. Radial symmetry.

Q5. Name one coral-forming animal.
A5. Gorgonia or Adamsia (class Anthozoa).

Q6. What are cnidoblasts, and what are their functions?

A6: Cnidoblasts (or cnidocytes) are specialized stinging cells unique to Phylum Cnidaria. They are present on the tentacles and body. Each cnidoblast contains a stinging capsule called a nematocyst. Their functions include anchorage (sticking to surfaces), defense against predators, and capture of prey by injecting toxins.

Q7. Explain the two basic body forms found in Cnidarians.

A7: Cnidarians exhibit two main body forms:

  1. Polyp: A sessile (attached), cylindrical form with the mouth and tentacles facing upwards. Examples include Hydra and sea anemones (Adamsia).

  2. Medusa: A free-swimming, umbrella-shaped form with the mouth and tentacles facing downwards. Example includes jellyfish (Aurelia).

Q8: What is metagenesis? Give an example.

A8: Metagenesis is the phenomenon of alternation of generations observed in some cnidarians where the life cycle includes both polyp and medusa forms. Typically, the polyp reproduces asexually (by budding) to form medusae, and the medusae reproduce sexually to form gametes, which fuse to form a zygote that develops into a polyp. Example: Obelia.

Q9. What type of symmetry and level of organization is found in Coelenterates?

A9: Coelenterates exhibit radial symmetry and the tissue level of organisation. They are also diploblastic, having only ectoderm and endoderm as embryonic germ layers.