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.

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.

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