Cnidaria (Types of Classes)

Phylum Cnidaria

  • Aquatic invertebrates including jellyfish, sea anemones, corals, and hydras.
  • Characterized by:
    • Radial symmetry.
    • Gastrovascular cavity for digestion.
    • Specialized stinging cells called cnidocytes.
  • Cnidocytes contain nematocysts used for defense and capturing prey.
  • Two main body forms:
    • Polyp: Sessile.
    • Medusa: Free-swimming.
  • Most species are marine, playing a vital role in marine ecosystems.
  • Coral reefs are formed by colonial anthozoans.

Classes of Cnidaria

  1. Hydrozoa
  2. Scaphozoa
  3. Cuboza
  4. Antozoa
  5. Trazoa
  • Phylogenetic Tree: Antozoa is the most basal group, while medusa-bearing classes (capezoa, cubosa, stratozoa) evolved later.

Characteristics (Form and Function)

  • Habitat: Aquatic, mostly marine.
  • Symmetry: Radial or biradial.
  • Body Types: Free-swimming medusae and sessile polyps.
  • Germ Layers: Diploblastic (epidermis and gastrodermis), with mesoglea (extracellular matrix between ectodermis and gastrodermis).
  • Gut: Incomplete.
    • Extracellular digestion occurs in the gastrovascular cavity.
    • Intracellular digestion occurs in gastrodermal cells.
  • Features: Tentacles, muscular contractions (via epitheliomuscular cells), and sense organs for balance (statocysts) and photosensitivity (ocelli).
  • Reproduction: Asexual (budding) and sexual (monoecious or dioecious, mostly dioecious).
    • Monoecious: Both female and male reproductive cells in one individual.
  • Excretory/Respiratory Organs: Absent (lower form of animal).
  • Coelom: Acoelomate.

Body Plans: Polyp and Medusa

  • Different classes exhibit either a dominant polyp or medusa stage.

Polyp Stage

  • Hydroid Form: Adaptation to a sedentary life.
  • Tubular body with mouth directed upward.
  • Mouth leads to a blind gastrovascular cavity.
  • Aboral end attaches to the substratum via a pedal disc.
  • Reproduces asexually by budding, fission, or pedal laceration.
  • In colonial forms, polyps are specialized for feeding, reproduction, or defense.
  • In class Hydrozoa (e.g., hydrades), feeding polyps (gastrozoids) are distinguished from reproductive polyps (gonagia) by the absence of tentacles.

Medusa Stage

  • Bell or umbrella-shaped.
  • Usually free-swimming, with mouth directed downward.
  • Tentacles extend down from the rim of the umbrella.
  • Equipped with statocysts (for balance) and ocelli (for light sensitivity).
  • Vellum: structure that differentiates hydromedusae from scyphomedusae.

Nematocysts

  • Located in cnidocytes.
  • Tiny capsule made of chitin-like material containing a coiled filament.
  • Filament may have tiny barbs or spines.
  • Used for defense.
  • Except in Anthozoa, cnidocytes are equipped with a trigger-like cnidocil (modified cilium).
  • Tactile stimulation causes nematocyst discharge.
  • After discharge, the cnidocyte is absorbed, and another develops.

Classes of Cnidaria (Details)

1. Hydrozoa

  • Mostly marine and colonial.
  • Both polyp and medusa forms are present.
  • Example: Colonial Obelia
    • Typical hydroid has a base, stalk, and one or more terminal zooids (individual polyp animals).
    • Individual zooids are attached to the coenosarc.
    • Hydranths (gastrozoids): Feeding polyps with tentacles surrounding the mouth.
    • Colonial hydroids reproduce through budding off new individuals.
    • Theca: A protective case or sheath for hydranths.
  • Obelia Colony: Polyp stage and medusa stage (male and female).
    • Medusae release sex cells into the water for fertilization.
    • Fertilization leads to the formation of a blastula, then larvae.
    • Larvae settle to start a new colony.
    • Gonangium: Contains the gonopore for reproduction.
    • Hydrant: Smaller than scyphozoan hydroids.
    • Hydrotheca: individuals protective sheet covering or protection
  • Hydra: Reduced or absent hydroid stage with gonophores responsible for reproduction.
    • Planula larva develops inside and emerges from gonopore.
    • Free-swimming actinula larva grows and attaches to form an adult polyp.
    • Freshwater hydra found on aquatic leaves and lily pads.
    • Cylindrical tube body with aboral end (basal or pedal disc) for attachment.
    • Ring of 6-10 hollow tentacles encircles the mouth.
    • Reproduction: sexual and asexual.
      • Asexual reproduction via budding.
      • Sexual reproduction in dioecious species.
    • Temporary gonads appear stimulated by lower temperatures or organic stagnation.
    • Sperm and egg shed externally to form a zygote.
    • The zygote undergoes holoblastic cleavage to form a hollow blastula.
    • A cyst forms around the embryo for overwintering.
    • Young hydras hatch in the spring.

2. Schipozoa

  • Larger jellyfishes belong to this class.

  • True jellyfish.

  • Life form is dominated by medusa.

  • Radial body symmetry.

  • Mostly solitary.

  • Distinguishing Features:

    • Bell-shaped, composed mostly of mesoglea.
    • Vellum: Lax shelf-like velum found in hydrozoa medusae.
    • Margin of the umbrella has indentations (each bearing a pair of lappets).
    • Equilibrium sense organ called rhopalium located between lappets (containing statocysts).
    • Mouth located beneath the umbrella, manubrium forms four oral arms.
    • Tentacles and manubrium contain nematocysts.
    • The nervous system consists of a nerve net.
    • The sexes are separate.

  • Reproduction and Life Cycle:

    • Fertilization is internal (gastric pouch of the female).
    • Zygote develops into a ciliated larva.
    • Larva attaches and develops into a scyphistoma.
    • Scyphistoma undergoes strobilation forming buds called ephyrae that break loose to form jellyfish.

  • Process

    • Sperm fertilizes egg.
    • Zygote develops into ciliated larvae.
    • Larvae grows and develops into scyphistoma
    • Scyphistoma continually develops into early strobila stage
    • Mature layer pinches off.
    • Grows into jellyfish

3. Staurozoa

  • Stalk jellyfish.
  • Life form is polyp-like medusa (no medusa stage).
  • Commonly called as medusae.
  • Solitary polyps with stalks using adhesive disc to attach to seaweeds and objects on the sea bottom.
  • Polyp top resembles a medusa with extensions ending in tentacle clusters surrounding the mouth.
  • Reproduce asexually.

4. Cubozoa

  • Box jellyfish.
  • Medusa form is dominant.
  • True polyp is inconspicuous or unknown.
  • Umbrella is square-shaped.
  • Base of each tentacle has a flat blade called pedalium.
  • Umbrella edge turns inward to form a velarium (increasing swimming efficiency).
  • Strong swimmers, feeding mostly on fish in nearshore areas.
  • Polyp stage is tiny.
  • New polyps unilaterally don't produce FRA but directly change.
  • Can be deadly (e.g., sea wasp).
  • Examples:
    • Chironex fleckeri (potentially fatal cubomedusa from Australia).
    • Caribdea marsupialis.
  • Structures: Gastric filaments, manubrium, gonads, rhopallium, pedallium, and tentacles.

5. Anthozoa

  • Lack medusa stage.

  • All are marine (deep and shallow water) and vary in size.

  • Three subclasses:

    1. Zoantaria
    2. Cerianthipatharia
    3. Alcyonaria

  • Gastrovascular cavity: Large, partitioned by septa (mesenteries - inward extensions of body wall).

  • Characteristics: Polyp only, radial body symmetry.

  • Usual appearance is a polyp stage with tentacles and radial symmetry.

  • They have pharynx, retractor muscles, and pedal disc for attachment to the substrate.

  • Cnidocytes also present, and gastrovascular cavity.

  • Tentacles withdraw into the oral disc when in danger.

  • Most harbor symbiotic algae.

  • Some have mutualistic relationships with hermit crabs.

Hexacorallia
  • Members of the order Scleractinia (true or stony corals).
  • Described as miniature sea anemones that live in calcareous cups they secreted.
  • Gastrovascular cavity is hexamerous (six parts).
  • Lack pedal disc.
  • Sheet of living tissue forms over coral surface connecting all gastrovascular cavities.
  • Parts: Calcium carbonate skeleton and septal filament.
  • Gastrovascular cavity connected to the pharynx and mouth.
Tube Anemones and Thorny Corals
  • Members of subclass Cerianthipatharia.
  • They do have coupled but unpaired septa.
  • Tube anemones: solitary and live in soft sediments.
  • Thorny or black corals: colonial and attached to a hard substrata.
  • Both groups have few species and live in warmer seas.
Alcyonaria (Octocorallian Corals)
  • Eight tentacles and eight unpaired septa.
  • All are colonial.
  • Gastrovascular cavities communicate through tubes called solenia.
  • Solenia pass through an extensive mesoglea.
  • Show great variation in colony form.

Summary of Classes

  • Hydrozoa: Mostly polyp, radial or biradial symmetry, mostly marine (some freshwater), many are colonial (e.g., hydra and Obelia).
  • Schipozoa: True jellyfish, dominated by medusa, marine only, mostly solitary.
  • Staurozoa: Stalk jellyfish, polyp-like medusa, no true medusa, solitary, marine.
  • Cubozoa: Box jellyfish, dominated by medusa, true polyp, mostly marine species, solitary.
  • Antozoa: Anemones and corals, polyp life stage, marine, mostly solitary or colonial (e.g., black corals or stony corals).
    • Three subclasses: Hexacorallia, Cerianthipatharia, and Alcyonaria.