Obelia: Structure, Life Cycle & Metagenesis | Class 11 Biology

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Obelia Structure, Life Cycle & Metagenesis Explained (Class 11 Notes + Diagram + MCQs)

labelled diagram of Obelia
labelled diagram of Obelia

Introduction

Obelia is a marine, colonial cnidarian that provides a classic example of metagenesis (alternation of generations). It belongs to class Hydrozoa and exhibits two distinct body forms in its life cycle: a sessile polyp and a free-swimming medusa.

This topic is important for Class 11 Biology, practical exams, and competitive exams like NEET, as it helps in understanding life cycles and division of labour in simple multicellular animals.

Classification

  • Phylum: Cnidaria
  • Class: Hydrozoa
  • Order: Leptothecata (Hydroida in older classifications)
  • Genus: Obelia

Habit and Habitat

Obelia is found in marine environments, especially in shallow coastal waters.

  • Habit: Colonial and sessile (polyp stage)
  • Habitat: Attached to rocks, shells, seaweeds, and submerged objects

  • The colony remains fixed, while reproductive stages become free-swimming

Structure of Obelia Colony

Obelia exists as a branched, tree-like colony composed of interconnected individuals called zooids. The colony shows division of labour, where different zooids perform specialized functions.

Main Parts of the Colony

1. Hydrorhiza


  • Root-like structure

  • Anchors the colony to the substratum

2. Hydrocaulus


  • Upright stem-like axis

  • Supports and connects the colony

3. Zooids (Polyps)

There are two main types:

(a) Gastrozooids (Hydranths)


  • Feeding polyps

  • Possess mouth and tentacles

  • Capture and digest food using nematocysts

(b) Gonozooids (Gonangia)


  • Reproductive polyps

  • Lack mouth and tentacles

  • Produce buds that develop into medusae

Protective Covering


  • Each feeding polyp is enclosed in a hydrotheca

  • Each reproductive polyp is enclosed in a gonotheca

  • The entire colony is covered by a non-living protective layer called perisarc

Medusa Stage (Sexual Form)

The medusa is the free-swimming sexual stage of Obelia.

Key Features:


  • Umbrella-shaped (bell-shaped) body

  • Mouth located on a projection called manubrium

  • Tentacles present along the margin

  • Gonads located on radial canals

  • Usually dioecious (separate male and female individuals)

Life Cycle of Obelia (Step-by-Step)

Obelia shows a complex life cycle involving two alternating phases:

1. Polyp Stage (Asexual Phase)


  • Colonial and sessile

  • Gonozooids produce medusae by budding

2. Medusa Stage (Sexual Phase)


  • Free-swimming

  • Produces gametes (sperm and ova)

  • Fertilization occurs in water

3. Development


  • Fertilized egg forms a zygote

  • Zygote develops into a ciliated planula larva

  • Planula settles on a substrate

  • Develops into a new polyp colony

Metagenesis in Obelia

Metagenesis is the alternation between asexual and sexual generations.

  • Polyp (asexual) → produces medusae
  • Medusa (sexual) → produces gametes

This cycle ensures both rapid reproduction and genetic variation.

Diagram of Obelia

Labelled diagram showing hydrorhiza, hydrocaulus, gastrozooids, gonozooids, and medusa stage.

Labelled diagram of Obelia
Labelled diagram of Obelia



Identification Features


  • Marine, colonial organism

  • Presence of two types of zooids

  • Chitinous protective covering (perisarc)

  • Alternation of polyp and medusa stages

  • Presence of planula larva

Importance


  • Demonstrates division of labour in simple animals

  • Classic example of metagenesis

  • Important for understanding evolution of multicellularity

  • Useful in zoology practicals and exams

Exam-Oriented Key Points


  • Shows dimorphism (polyp and medusa forms)

  • Exhibits tissue-level organization

  • Life cycle includes planula larva

  • Reproduction involves both asexual and sexual phases

Frequently Asked Questions (FAQs)

What is Obelia?

Obelia is a colonial marine cnidarian that shows alternation of polyp and medusa forms.

What is metagenesis in Obelia?

It is the alternation between asexual polyp and sexual medusa stages.

Which stage is sessile in Obelia?

The polyp stage is sessile.

Which stage produces gametes?

The medusa stage produces gametes.

What is the planula larva?

It is a free-swimming, ciliated larval stage that develops into a polyp colony.

MCQs

1. Obelia belongs to which class?

(a) Scyphozoa

(b) Hydrozoa

(c) Anthozoa

(d) Calcarea

Answer: (b)

2. Feeding zooids in Obelia are called:

(a) Gonozooids

(b) Gastrozooids

(c) Medusae

(d) Planula

Answer: (b)

3. Obelia shows:

(a) Only sexual reproduction

(b) Only asexual reproduction

(c) Metagenesis

(d) Binary fission

Answer: (c)

4. The larval stage of Obelia is:

(a) Trochophore

(b) Planula

(c) Nauplius

(d) Veliger

Answer: (b)

Keywords (SEO Focus)

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Final Note

This topic is frequently asked in Class 11 exams, NEET, and practical viva, especially focusing on life cycle and metagenesis. Understanding the alternation between polyp and medusa stages is essential for scoring well.