Cnidaria

• CNIDARIA

  • Radially symmetrical
  • Diploblastic
  • Single gastrovascular cavity
  • No head, brain, or CNS
  • Possession of cnidae
  • Exhibit polymorphism
• CNIDARIA are radially symmetrical which allows food capturing from all sides
• CNIDARIA are diploblastic; the epidermis and gastrodermis are separated by jelly-like mesoglea and lack organs
• CNIDARIA have a single gastrovascular cavity that opens only through the mouth
• CNIDARIA have no anus
• CNIDARIA have no head, brain, and CNS
• CNIDARIA have a simple interconnecting nerve cell which forms a nerve net
• Statocysts and oceli are usually the only sensory structures in CNIDARIA
• Skeleton of CNIDARIA

  • Horny or calcareous
  • Internal or external
  • Continuous
• CNIDARIA have no special structures for respiration, excretion, and transport system
• Some large jellyfish have sea water channels

  
• Despite lacking reproductive, circulatory, digestive, or excretory systems, CNIDARIA can reproduce, exchange gas, capture and digest prey, and distribute organic molecules to all their cells
• The gastrovascular space may serve as a hydrostatic skeleton
• CNIDARIA possess a unique stinging or adhesive structure called cnidae

  
• Cnidocyte


  Cell that contains cnidae
• Nematocyst


  The most common type of cnidae
• CNIDARIA usually exhibit polymorphism
• Polyp


  Cylindrical with a mouth surrounded by tentacles at the end
• Medusa

  Discoidal or umbrella-shaped with a mouth surrounded by tentacles on the sub-umbrella side
• Most polypoid hydrozoans are colonial and polymorphic, e.g., Obelia
• Four classes of CNIDARIA

  • Hydrozoa
  • Scyphozoa
  • Cubozoa
  • Anthozoa
• HYDROZOA


  • Enteron not divided by mesenteries
  • Tentacles usually solid
  • Acellular mesoglea
  • Cnidocytes only found in epidermis
  • External chitinous skeleton

  
• Hydromedusa are small and are the gamete producing individuals
• Hydromedusa possess velum; craspedote
• The gametes in HYDROZOA are ectodermal
• SCYPHOZOA

  • Medusa is usually the dominant form
  • Lack velum; acraspedote
  • Margin of medusa fringed by short tentacles
  • Mesoglea contains amoeboid mesenchyme cells
  • Cnidocytes occur in gastrodermis and epidermis

  
• Gametes in SCYPHOZOA are gastrodermal in origin
• CUBOZOA (BOX JELLY)

  • Medusa is square-shaped
  • Marginal self or velum
  • Tentacles hang from corners
  • Strong swimmers and voracious predators

  
• CUBOZOA have elaborate image-forming eyes
• The stings of some CUBOZOA species can be lethal to humans
• ANTHOZOA (SEA ANEMONES AND CORAL)


  • Only the polyp predominates
  • Oral end expanded into an oral disc
  • Gastrovascular cavity divided by mesenteries
  • Mesenteries bear cnidocytes and gonads

  
• The mouth of ANTHOZOA leads into a pharynx
• At one or both ends of the mouth is a siphonoglyph which moves water into the gastrovascular cavity
• The polyp of a coral secretes an exoskeleton of calcium carbonate
• The dead skeleton accumulates below coral polyps, forming coral reefs
• Coral reefs are economically important as they serve as refuge for young crustaceans and fishes
• Coral reefs protect coasts of many tropical islands