Phylum Ctenophora

Phylum Ctenophora- characteristics, classification, examples

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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.

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 triploblastic, with the outer epidermis, inner gastrodermis, middle jelly-like mesoglea with scattered cells, and muscle fibers.
  • Their digestive system contains the mouth, stomodaeum, complex gastrovascular canals, and 2 aboral anal pores.
  • They lack nematocysts.
  • They have special adhesive and sensory cell 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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