Phylum Ctenophora- characteristics, classification, examples

Ctenophora Definition

Ctenophores are free-swimming, transparent, jelly-like, soft-bodied, marine animals having biradial symmetry, comb-like ciliary plates for locomotion, the lasso cells but nematocytes are wanting. They are also known as sea walnuts or comb jellies.

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Phylum Ctenophora Characteristics

  • They are free-swimming, marine, solitary, pelagic animals. No polymorphism and no attached stages were found.
  • The body is transparent, gelatinous, pear-shaped, cylindrical, or flat or ribbon-shaped.
  • They have a biradially symmetrical body along an oral-aboral axis.
  • They have an external surface with comb-like 8 ciliary plates for locomotion. Hence name as comb jellies.
  • They have a pair of long, solid, retractile tentacles.
  • Their body organization is cell-tissue grade.
  • Their body is acoelomate and “diploblastic” having ectoderm and endoderm. The body wall has outer epidermis, inner gastrodermis, middle jelly-like mesoglea with scattered cells, and muscle fibers. So, Ctenophora may also be considered as “triploblastic”.
  • Their digestive system contains the mouth, stomodaeum, complex gastrovascular canals, and 2 aboral anal pores.
  • They lack nematocysts.
  • They have special adhesive and sensory cells i.e. colloblasts or lasso cells present in tentacles which helps in food captures.
  • They lack skeletal, circulatory, respiratory, and excretory organs.
  • Their nervous system is diffused types and the aboral end bears a sensory organ, called statocyst.
  • They are monoecious (hermaphrodite); gonads are endodermal situated on walls of digestive canals.
  • Their development direct with characteristic cydippid larva.
  • They lack asexual reproduction and alternation of generation.
  • Regeneration and paedogenesis are common in them.

Phylum Ctenophora

Figure: Pelagic ctenophores: (a) Beroe ovata, (b) Euplokamis sp., (c) Nepheloctena sp., (d) Bathocyroe fosteri, (e) Mnemiopsis leidyi, and (f) Ocyropsis sp. Image Source: Wikipedia.

Phylum Ctenophora Classification

Phylum Ctenophora contains about 100 know species and grouped in 2 classes

Class 1. Tentaculata

  • Adults with 2 long aboral tentacles.
  • In some larva has tentacles, while adults have oral lobes.
  • Mouth narrow and pharynx small.

Order 1. Cydippida

  • Body simple, round, and oval.
  • Digestive canals terminate blindly; no anal pores.
  • Tentacles are two long and branched.
  • Tentacles are retractile into pouches or sheath.
  • Examples: Mertensia, Pleurobrachia, Hormiphora

Order 2. Lobata

  • Body oval, laterally compressed.
  • Adults with 2 large oral lobes and 4 slender flap-like auricles around the mouth.
  • Pouched or sheath tentacles in the larva.
  • Tentacles reduced and without sheath in adults.
  • Gastrovascular canals are connected by a ring at oral ends.
  • Examples: Mnemiopsis, Bolinopsis

Order 3. Cestida

  • Body elongated compressed/flat, ribbon-like.
  • Two main tentacles in the sheath but reduced.
  • Many small lateral tentacles along the oral edge.
  • Combs plates in 4 rows but rudimentary.
  • Examples: Cestum, Velamen

Order 4. Platyctenea

  • Body greatly compressed/flat in the oral-aboral axis.
  • 2 well- developed tentacles with sheath.
  • Comb plates reduced in adults.
  • Adapted for creeping.
  • Examples: Ctenoplana, Coeloplana

Order 5. Thalassocalycida

  • They are found surface waters down up to 2,765 Ms in Atlantic oceans and the Mediterranean Sea.
  • The body is a bell of Medusa shaped and may be up to 15 cm in diameter.
  • Mouth slit holds by a central cone-shaped peduncle.
  • A pair of small tentacles hang from the side of the peduncle.
  • Com jelly is with its transparent and colorless body. Usually different to see.
  • They hold the bell wide opens to captures prey i.e. Zooplankton.
  • Presumably hermaphroditic.
  • This species has limited swimming ability compared to other comb jellies.
  • Examples: Thalassocalyce inconstans.

Class 2. Nudu

  • Body large, conical, and compressed laterally.
  • Without tentacles and oral lobes.
  • Wide mouth and large pharynx.
  • Voracious feeder.

Order 1. Beroida

  • No tentacles and oral lobes.
  • Body large, conical, and laterally compressed.
  • Mouth large.
  • Voluminous Stomach.
  • Examples: Beroe

References

  1. Kotpal RL. 2017. Modern Text Book of Zoology- Invertebrates. 11th Edition. Rastogi Publications.
  2. Jordan EL and Verma PS. 2018. Invertebrate Zoology. 14th Edition. S Chand Publishing.

Phylum Ctenophora

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Laxmi Neupane

Laxmi Neupane is doing her Ph.D. in Pharmaceutical Biotechnology at Helmholtz Institute for Pharmaceutical Research Saarland (HIPS), Saarbrucken, Germany. She did her M.Sc. in Medical Microbiology from the Central Department of Microbiology, Tribhuvan University, Kathmandu, Nepal, and her B.Sc. in General Microbiology from Pinnacle Academy, Kathmandu, Nepal. Her research interest is in isolating antimicrobial myxobacteria from the soil sample.

7 thoughts on “Phylum Ctenophora- characteristics, classification, examples”

    • Thank you for the correction. Some say they are also triploblastic, so we have now written both diploblastic and triploblastic.

      Reply
    • Ctenophores can also be considered as triploblastic for the fact that the mesoglea in them contain many ameobocytes, elastic fibres and muscle cells. Thus it introduces a third layer inspite of ectoderm and the endoderm.

      Reply
    • Thank you for the correction. Some say they are also triploblastic, so we have now written both diploblastic and triploblastic.

      Reply
    • Ctenophores can also be considered as triploblastic for the fact that the mesoglea in them contain many ameobocytes, elastic fibres and muscle cells. Thus it introduces a third layer inspite of ectoderm and the endoderm.

      Reply

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