Phylum Ctenophora: General Characters, Classification & Examples | BSc Zoology Notes

 Introduction 

• Phylum Ctenophora is a small but scientifically significant phylum of the Kingdom Animalia, comprising approximately 150-200 known species all of which are exclusively marine. These elegant invertebrates are commonly called "Sea Walnuts" or "Comb Jellies" names that reflect both their appearance and their most remarkable feature.

The name Ctenophora is derived from two Greek words:

                           Ktenos means "Combs"
                           Phoros means "Bearing"

General Characters of Phylum Ctenophora

 1. Habitat & Habit

•  All members of Phylum Ctenophora are exclusively marine. There is no freshwater species are known.
• They are solitary, free-swimming (pelagic) organisms found throughout the world's oceans.
• No polymorphism is seen in it all individuals of a species are structurally identical.
• No attached (polyp) stage — the polyp stage is completely absent, which is a major difference from Phylum Cnidaria

2. Body Shape, Symmetry & Transparency

• The body of ctenophores is transparent (allowing them to be nearly invisible in water).
• They exhibit "Biradial Symmetry" — a combination of radial and bilateral symmetry — along the oral–aboral axis.
• This biradial symmetry is unique to ctenophores and is a key distinguishing character from the radial symmetry of Cnidaria.

3. Level of Organization

•  Body organization in ctenophores is at the Tissue Level of Organization.

The body is:

•              Acoelomate → No true body cavity (coelom) is present
•              Triploblastic → Three germ layers are present

4. Body Layers – Epidermis, Gastrodermis & Mesoglea

The body wall of ctenophores consists of three distinct layers:

• The Mesoglea is thick and gives the body its characteristic gelatinous, transparent appearance.
• Unlike Cnidaria, the mesoglea of ctenophores contains true cells and collagen fibres, making ctenophores triploblastic.

5. Comb Plates – The Hallmark Feature of Ctenophora

•  The most defining and unique feature of Phylum Ctenophora is the presence of 8 vertical rows of "Comb Plates" on the external body surface.
• Each comb plate is made up of fused cilia thousands of cilia joined together to form a paddle-like structure.
• These 8 comb rows run from the aboral pole toward the oral pole of the body.
• The beating of these comb plates propels the animal through water making them the primary organs of locomotion.
• The iridescent rainbow shimmer seen in comb jellies is caused by light diffraction through these moving cilia.

 6. Digestive System of Ctenophora

The digestive system of ctenophores is complex and well-organized:

> Mouth → Stomodeum (Pharynx) → Complex Gastrovascular Cavity → 2 Anal Pores

• Food enters through the mouth at the oral pole.
• It passes into the Stomodeum (also called the pharynx or gullet).
• From here, a complex gastrovascular cavity (canal system) distributes digested nutrients throughout the body.
• Undigested waste is expelled through 2 Anal Pores located at the aboral pole.

 7. Colloblasts – Unique Food Capturing Cells 

•  Nematocysts (stinging cells) are completely absent unlike Cnidaria.
•  Instead, special "Adhesive and Sensory Cells" called "Colloblasts" (also called Lasso Cells) are present on the tentacles.
• Colloblasts secrete a sticky substance that traps prey — they function like biological glue.
• Colloblasts help in the capture of small marine organisms such as zooplankton and copepods.

 8. Tentacles

• A pair of solid, retractile tentacles are present in most ctenophores.
• These tentacles are lined with colloblasts for food capture.
• The tentacles can be retracted into tentacle sheaths (pouches) in some orders (e.g; Cydippida)

 9. Absent Organ Systems

The following organ systems are completely absent in Phylum Ctenophora:

Absent System	Significance
Skeletal System	No hard or cartilaginous support
Circulatory System	Nutrients distributed via gastrovascular canals
Excretory System	Waste removed via anal pores and diffusion
Respiratory System	Gas exchange occurs by simple diffusion

 

 10. Nervous System & Statocyst

• The nervous system of ctenophores is of the diffused (nerve net) type.
• The aboral end of the body bears a highly specialized sensory structure called the "Statocyst".
• The Statocyst is a gravity receptor that helps the animal detect its orientation in water.
• It functions in balance, equilibrium, and coordination of the comb plate beating rhythm.
• The statocyst contains calcareous statolith granules supported by four balancer cilia.

 11. Reproduction in Ctenophora 

• Ctenophores are Hermaphrodite (Monoecious). A single individual possesses both male (testes) and female (ovaries) gonads.
• Gonads are located side by side along the walls of the digestive canals.
•  The characteristic larva of ctenophores is called "Cydippid Larva" — it resembles an adult Cydippida.
•  Asexual reproduction is absent in ctenophores.
•  Alternation of generations is absent.
•  Regeneration is present :- ctenophores can regrow lost body parts.
•  Paedogenesis (sexual reproduction in larval stage) is also observed.

🔬 Classification of Phylum Ctenophora

Phylum Ctenophora  

├── Class 1: TENTACULATA (With tentacles)  

│   ├── Order 1: Cydippida  

│   ├── Order 2: Lobata  

│   ├── Order 3: Cestida  

│   ├── Order 4: Platyctenea  

│   └── Order 5: Thalassocalycida  

│  

└── Class 2: NUDA (Without tentacles)  

    └── Order 1: Beroida

 Class 1 – TENTACULATA

Defining Feature: Ctenophores with 2 long aboral tentacles present. Divided into 5 orders.

 Order 1 – CYDIPPIDA

•  Body is rounded or oval in shape.
• Tentacles are branched and retractile into well-developed pouches (sheaths).
• This is considered the most primitive and ancestral order of Tentaculata.
• The Cydippid larva which is the larval form of all ctenophores resembles an adult Cydippida.

 Examples: Pleurobrachia (Sea gooseberry), Hormiphora, Mertensia

 Order 2 – LOBATA

•  Body is oval and laterally compressed.
• Two large oral lobes are present on either side of the mouth — used in feeding.
• 4 slender flap-like auricles are present around the mouth — these help funnel prey inward.
• Tentacles are pouched in larva but become reduced and lose their pouches in the adult.

 Examples: Mnemiopsis leidyi (Venus's girdle lookalike), Bolinopsis

 Order 3 – CESTIDA

•  Body is dramatically elongated, flat, and ribbon-like.
• Two main tentacles are present in sheaths but are greatly reduced.
• Many small lateral tentacles are present along the oral edge of the ribbon.
• Four comb rows are prominent and functional; the remaining four are rudimentary.

 Examples: Velamen parallelum, Cestum veneris 

 Order 4 – PLATYCTENEA

• Body is extremely flattened and oral-aborally compressed.
• Tentacles are well-developed and present with sheath (pouch).
• Comb plates are reduced or completely absent in adult adaptation.
• The body is specially adapted for creeping on the substrate (benthic lifestyle).
• These are the only ctenophores adapted for a bottom-dwelling (benthic) life.

 Examples: Ctenoplana, Coeloplana

Order 5 – THALASSOCALYCIDA

•  Body is bell-shaped resembling a medusa (jellyfish).
• A central cone-shaped peduncle projects inward and holds the mouth slit open.
• A pair of small tentacles hangs from the sides of the peduncle.
• Body is transparent and colorless — nearly invisible in open water.
• They are Hermaphrodite.
• Feed by capturing zooplanktons and small marine organisms as prey.
• They have very limited swimming ability largely drift with ocean currents.

 Example: Thalassocalyce inconstans

 Class 2 – NUDA 

Defining Feature: Ctenophores completely without tentacles at any stage of life.

 Order 1 – BEROIDA

•  No tentacles at any life stage.
•  No oral lobes.
•  Body is conical and laterally compressed  thimble-shaped.
• Mouth is extremely large and wide which occupies most of the oral surface.
• Stomach is voluminous and it is capable of swallowing prey almost as large as itself.
• Beroe are active predators. They prey on other ctenophores.

 Example: Beroe cucumis, Beroe ovata

Reference 

•                   Kotpal invertebrates
•                   E.L. Jordan & P.S. Verma – Invertebrate Zoology
•                   R.C. Brusca et al. – Invertebrates