Coelenterata (Cnidaria) Notes for BSc Zoology | Characteristics, Digestion, Nervous System & Polymorphism

 Introduction

Coelenterata (Cnidaria) are simple aquatic, mainly marine animals with radial symmetry and a diploblastic body. They have stinging cells called cnidoblasts for defense and feeding. Examples include Hydra, jellyfish, and sea anemones.

Habitat of Coelentrates

• Coelenterates are predominantly marine organisms, distributed across oceans worldwide.
• A few species — most notably Hydra — inhabit freshwater environments.
• They are found from shallow coastal waters to deep oceanic zones.
• Body form varies: some species are sessile (fixed to substrate), while others are free-swimming (pelagic).

Body Symmetry

• Coelenterates show radial symmetry the body can be divided into equal halves along multiple planes passing through the central axis.
• Some members, such as sea anemones, exhibit biradial symmetry.
• Radial symmetry is considered an evolutionary adaptation to a sessile or slow-moving aquatic lifestyle

 Grade of Organisation

• Coelenterates are at the tissue grade of organisation.
• Cells performing similar functions are grouped into tissues, but there are no true organs or organ systems.

Diploblastic Body Wall 

The body wall of Coelenterata is composed of two primary germ layers:

Layer	Position	Function
Epidermis / Ectoderm	Outer layer	Protection, sensation
Gastrodermis / Endoderm	Inner layer	Digestion, absorption

• Between these two layers lies a non-cellular, jelly-like mesoglea.
• No true mesoderm is present — this is a critical distinguishing feature.

Cnidoblasts and Nematocysts

• The most distinctive feature of phylum Coelenterata is the presence of cnidoblasts (also called cnidocytes).
• Each cnidoblast contains a nematocyst a capsule enclosing a coiled, hollow, thread-like tube.
• Nematocysts are concentrated mainly on the tentacles.

Functions of Nematocysts:

• Prey capture — paralysing or entangling prey
• Defence — against predators
• Locomotion — in certain species

Digestion in Coelenterata 

Digestion in Coelenterata is both extracellular and intracellular.It is a unique and frequently examined feature.

Part 1 Extracellular Digestion (Cavity Digestion)

• Captured prey is brought into the coelenteron (gastrovascular cavity) through the hypostome (mouth).
• Gland cells of the gastrodermis secrete digestive enzymes (proteases and lipases) directly into the coelenteron.
• Digestion begins outside the cells, within the cavity — hence "extracellular."
• Muscular contractions of the body wall churn food for better enzyme mixing.
• Large food particles are broken into smaller fragments — but digestion is not yet complete.

Part 2 Intracellular Digestion (Cellular Digestion)

• Partially digested fragments are engulfed by nutritive-muscular cells of the gastrodermis via phagocytosis or pinocytosis.
• Digestion is completed inside food vacuoles within these cells.
• Lysosomes fuse with food vacuoles and release enzymes to complete breakdown.
• Final products (amino acids, fatty acids, sugars) are absorbed into the cytoplasm.
• Nutrients diffuse to the epidermis — there is no circulatory system.

Nervous System of Coelenterata 

• Coelenterates possess the most primitive type of nervous system — a diffuse nerve net.
• There is no brain, ganglia, or nerve cord.
• Nerve impulses travel in all directions from the point of stimulation (non-polar conduction).
• Specialised sensory cells (sensory neurons) are present in the epidermis.
• Some medusae possess:
• Statocysts — balance/equilibrium organs
• Ocelli — simple light receptors

Reproduction in Coelenterata

Coelenterates reproduce by both asexual and sexual methods.

Asexual Reproduction:

• Budding — most common method (e.g., Hydra)
• Binary fission — e.g., Hydra
• Fragmentation — e.g., sea anemones
  

Sexual Reproduction:

• Gonads are epidermal or gastrodermal in origin — no true reproductive organs.
• May be monoecious (hermaphrodite) or dioecious (separate sexes).
• Fertilisation may be internal or external.
• Development involves a free-swimming planula larva.

Respiration and Excretion in Coelenterata

• No specialised respiratory or excretory organs are present.
• Respiration occurs by simple diffusion of O₂ and CO₂ across the body surface.
• Excretion occurs by diffusion of metabolic wastes — primarily ammonia — through the body wall.
• Coelenterates are therefore ammonotelic (ammonia-excreting).

What is Polymorphism in Coelenterata

• Polymorphism is defined as the occurrence of two or more morphologically and functionally distinct types of individuals (called zooids) within the same species, all arising from the same zygote (i.e., they are genetically identical).
• Each distinct individual is called a zooid (or person).
• Each zooid is specialised to perform a specific function for the benefit of the entire colony.
• Polymorphism is best developed in the class Hydrozoa, especially in the order Siphonophora (e.g., Physalia, Velella).
• It represents a division of labour at the individual level — often called zoidal division of labour

Types of Polymorphism Based on Number of Zooid Types

Dimorphism (Two Types of Zooids)

Definition: When a colony contains only two morphologically and functionally distinct types of zooids, it is called dimorphism.

Example: Obelia (a common hydrozoan)

Obelia is the classic textbook example of dimorphism. Its colony contains exactly two types of zooids:

 Gastrozooid (Nutritive Polyp / Hydranth)

• It also called hydranth.
• Cylindrical polyp with a hypostome (mouth) at the top.
• Surrounded by hollow tentacles armed with nematocysts.
• Function: Captures prey and performs digestion for the entire colony.
• It cannot reproduce sexually.

Gonozooid (Reproductive Polyp / Gonangium)

• It also called gonangium or blastostyle.
• A vase-shaped, mouthless and tentacle-less polyp enclosed in a transparent cup called the gonotheca.
• Contains a central stalk (blastostyle) on which medusa buds (gonophores) are produced by asexual budding.
• The medusae are released and reproduce sexually.
• Function: Exclusively reproduction.

 Trimorphism (Three Types of Zooids)

Definition: When a colony contains three morphologically and functionally distinct types of zooids, it is called trimorphism.

Example: Millepora (Fire coral) and Velella

In Millepora, three zooids are present:

1.Gastrozooid (Nutritive Zooid)

• Possesses a mouth and short tentacles.
• Functions in feeding and digestion.

2.Dactylozooid (Defensive/Sensory Zooid)

• It is elongated, slender, mouthless individuals.
• It armed with nematocysts.
• Functions in defence, prey capture, and sensation.

3.Gonozooid (Reproductive Zooid)

• Mouthless, tentacle-less.
• Produces medusoid buds for sexual reproduction.
• Functions exclusively in reproduction.

Polymorphism (Many Types of Zooids — True Polymorphism)

Definition: When a colony contains four or more morphologically and functionally distinct zooids, it is called true polymorphism or pleomorphism.

Best Example: Physalia (Portuguese Man-o'-War)

• Physalia is the most spectacular example of polymorphism in Coelenterata. It is a siphonophore colony that floats on the ocean surface. The colony contains the following zooids:

1. Pneumatophore (Float / Gas Bladder)

• A modified medusa — the most prominent structure of the colony.
• A large, gas-filled, bladder-like float filled with nitrogen, oxygen, CO₂, and carbon monoxide.
• Keeps the entire colony afloat on the water surface.
• Has a gas gland that secretes gas to regulate buoyancy.

• Function: Flotation and buoyancy regulation.

2.Nectophores (Swimming Bells)

• Modified medusae — bell-shaped, muscular, without mouth or tentacles.
• Contract rhythmically to propel the colony through water.

Function: Locomotion / swimming.

3.Gastrozooids (Feeding Zooids)

• Modified polyps — with a large mouth and long tentacles armed with nematocysts.
• Capture and digest prey — nutrients shared with the whole colony via the coenosarc (common living tissue).
• Function: Feeding and digestion.

4.Dactylozooids (Protective / Sensory Zooids)

• Modified polyps — elongated, mouthless, with very long tentacles (can reach up to 50 metres).
• Heavily armed with powerful nematocysts — responsible for the deadly sting of Physalia.
• Function: Defence, prey capture, sensation.

5.Gonodendra / Gonozooids (Reproductive Zooids)

• Modified medusae or polyps — produce gonophores (reproductive buds).
• May bear gonophores (either medusoid or sporosacs) that produce eggs and sperm.
• Function: Sexual reproduction.

6.Palpon (Palpacle / Digestive Zooid)

• Mouthless but with tentacle-like palpacle.
• Assists in digestion and nutrient distribution.
• Considered a modified dactylozooid in some classifications.

Conclusion

Coelenterates show a simple body organization with specialized stinging cells and unique polymorphism, making them an important group in animal evolution.

Reference 

•                   Kotpal invertebrates