Phylum Ctenophora Definition

Phylum Ctenophores are translucent, jelly-like, soft-bodied, free-swimming marine organisms that lack nematocytes but have biradial symmetry, comb-like ciliary plates for motility, and lasso cells. They are sometimes called comb jellies or sea walnuts.

Phylum Ctenophora Characteristics

• They are pelagic, solitary, free-swimming, sea creatures. There were no connected phases or polymorphisms discovered.
• The body may be translucent, gelatinous, spherical, flat, or ribbon-like in form.
• They have an oral-aboral axis that runs along their biradially symmetrical body.
• They feature eight comb-like ciliary plates on their outside surface, which they use for motility.
• Hence, comb jellies as a name.
• They have two long, sturdy tentacles that retract.
• Their bodily structure is at a cell-tissue level.
• Their bodies are “diploblastic” and acoelomate, with ectoderm and endoderm.
• The body wall is made up of muscle fibres, an exterior epidermis, an inner gastrodermis, a middle mesoglea that resembles jelly, and dispersed cells. Ctenophora might thus be categorised as “triploblastic”.
• Their mouth, stomodaeum, intricate gastrovascular canals, and two aboral anal apertures make up their digestive system.
• The nematocysts are absent.
• Colloblasts, also known as lasso cells, are particularly sensitive and sticky cells found in their tentacles that aid in the capture of prey.
• Skeletal, circulatory, respiratory, and excretory organs are absent in them.
• Their neural systems are composed of scattered kinds, and the aboral end contains the statocyst, a sensory organ.
• They have endodermal gonads that are located on the walls of their digestive canals and are monoecious (hermaphrodite).
• Their growth is directly related to that of the typical cystipid larva.
• They lack generational alternation and asexual reproduction.
• They often undergo paedogenesis and regeneration.

Phylum Ctenophora Classification

There are roughly 100 species in the phylum Ctenophora that are categorised into two groups.

Class 1. Tentaculata

• Two lengthy aboral tentacles on adults.
• While some adults have oral lobes, some larvae have tentacles.
• narrow mouth and a tiny pharynx.

Order 1: Cydippida

• basic, oval, and circular in shape.
• There are no anal apertures, and the digestive tubes end blankly.
• Tentacles have two branches and are long.
• Tentacles may retract into sheaths or pouches.
• Examples include Hormiphora, Pleurobrachia, and Mertensia

Order 2: Lobata

oval body with compressed sides.

Adults have four thin flap-like auricles surrounding the mouth and two huge oral lobes.

The larva’s tentacles are sheathed or puffed.

Adults have shorter and bare tentacles.

At their oral ends, gastrovascular canals are joined by a ring.

Mnemiopsis and Bolinopsis, as examples

Order 3 : Cestida

• Body compressed, flat, ribbon-like, extended.
• Reduced to two major tentacles in the sheath.
• Along the oral border, several tiny lateral tentacles
• They’re comb plates in four rows, although they’re basic.
• Cestum and Velamen, as examples

Order 4: Platyctenea,  

• The body in the oral-aboral axis is severely crushed or flat.
• Sheaths around two strong tentacles
• Adult comb plates have decreased.
• Developed for creeping.
• Ctenoplana and Coeloplana, as examples

Order 5: Thalassocalycida, 

• In the Atlantic Ocean and the Mediterranean Sea, they may be found in surface waters that are as deep as 2,765 Ms.
• The body has a bell-like Medusa form and may have a diameter of up to 15 cm.
• Held by a central peduncle with a cone-like form.
• From the side of the peduncle, a pair of tiny tentacles protrude.
• Com jelly is a clear and colourless jelly. Typically different to observe
• They keep the bell open wide to catch zooplankton as their meal.
• Most likely hermaphrodite.
• Compared to other comb jellies, this species has a reduced capacity for swimming.
• Thalassocalyce inconstans, as an example.

Class 2. Nudu

• Large, conical, and laterally compressed body.
• Without oral lobes and tentacles.
• Big throat and a wide mouth.
• Ferocious eater.

Order 1: Beroida

• Oral lobes and no tentacles.
• Large, conical, and lateralized body.
• mouth wide.
• A large stomach.
• Instances: Beroe

References

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