Cnidaria and Ctenophora

Cnidaria: 

  • Also known as coelenterata.
  • They are aquatic, mostly marine, sessile or free-swimming.
  • The name is derived from the cnidoblasts or cnidocytes (which contain the stinging capsules or nematocysts) present on the tentacles and the body.
  • Corals have a skeleton composed of calcium carbonate.
  • Digestion is extracellular and intracellular.

 

Characteristics-

  • Body Organization:
    • Cnidarians exhibit radial symmetry, meaning their bodies are arranged around a central axis, allowing multiple planes of symmetry and are diploblastic.
    • They have two basic body forms: polyps and medusae.
      • Polyps are cylindrical and typically sessile.
      • Medusae are umbrella-shaped and free-swimming.
    • Cnidarians have specialised cells called cnidocytes, which contain stinging structures called nematocysts used for prey capture and defence.

 

 

  • Locomotion:
    • Many cnidarians, such as jellyfish, exhibit a pulsating movement by contracting and relaxing their bell-shaped medusa body form, allowing them to swim in the water.
    • Polyps are generally immobile and use their tentacles to capture prey.
  • Reproduction:
    • Cnidarians can reproduce both sexually and asexually.
    • Asexual reproduction occurs through budding, where new individuals develop as outgrowths from the parent organism.
    • Sexual reproduction involves the production of eggs and sperm. Cnidarians can have separate sexes (male and female) or be hermaphroditic.
    • Fertilisation may occur internally or externally, depending on the species. The resulting zygote develops into a larval form called a planula, which eventually settles and grows into a polyp.
    • Those cnidarians which exist in both forms exhibit alternation of generation called Metagenesis, i.e., polyps produce medusae asexually and medusae form the polyps sexually (e.g. Obelia).
  • Feeding Strategies:
    • Cnidarians are carnivorous and use their specialised tentacles armed with stinging cells (cnidocytes) to capture prey.
    • They typically feed on small organisms, such as plankton and small fish.
    • Once prey is captured, cnidarians use their tentacles to immobilise and bring the prey into their mouth, which leads to a central digestive cavity called the gastrovascular cavity. The prey is then digested, and nutrients are distributed throughout the body.
  • Examples:

Aurelia (Medusa), Adamsia is a Polyp (Sea anemone) , Physalia (Portuguese man-of-war), Pennatula (Sea-pen), Gorgonia (Sea-fan) and Meandrina (Brain coral).

 

 

Ctenophora

  • Commonly known as sea walnuts or comb jellies.
  • They are marine.
  • Shows bioluminescence (the property of a living organism to emit light).
  • Digestion is both extracellular and intracellular.

 

Characteristics-

  •  Body Organization:
    • They have a gelatinous, transparent body with a radial symmetry, diploblastic with tissue level or organisation.
    • They have eight comb-like rows of cilia, called ctenes, which they use for locomotion.
  • Locomotion:
    • Ctenophores use their cilia to move through the water in a coordinated, swimming motion.
    • By beating their cilia in a synchronised pattern, they create water currents that propel them forward.
  • Reproduction:
    • Ctenophores have separate sexes (male and female).
    • They reproduce sexually, with males releasing sperm into the water, which is then captured by females. Fertilisation occurs externally.
    • After fertilisation, ctenophores develop into a larval form known as a cydippid, which eventually transforms into the adult body form.
  • Feeding Strategies:
    • Ctenophores are also carnivorous, feeding primarily on small planktonic organisms.
    • They possess specialised sticky cells called colloblasts, located on their tentacles, which they use to capture prey.
    • Once prey is captured, ctenophores bring it into their mouth, where it enters a gastrovascular cavity for digestion. Waste is expelled through the same opening.
  • Examples:

Pleurobrachia and Ctenoplana.