Cnidaria

• Phylum Cnidaria comprises the Hydras, jellyfish, corals, and sea anemones.
• They are tentacle-bearing organisms with an internal cavity called the gastrocoel.
• In terms of symmetry, they are either radial or radio-bilateral. 
• Cnidarians belong to the simplest form of metazoans, capable of forming primitive organs while the rest of its body remains in the tissue level of organization.
• Their defining characteristics are the following: secretion of complex intracellular organelles called cnidae and a planula larvae in the life cycle.
• Their integument is a two-layered epithelium (epidermis and gastrodermis) with an in-between connective tissue, the mesoglea, which may possess motile amorphous cells called amoebocytes. 
• Longitudinal and circular muscle layers are present among member of this phylum.
•  They feed on small crustacean, which is brought to the mouth via tentacles. 
• Although they do not possess a digestive system, food is broken down via first extracellular digestion followed the phagocytosis and filtration. 
• Respiration and excretion is accomplished via diffusion thru the integument. There is no circulatory system.
• They are present marine waters but can also be found in shallow freshwater, sessile on rocky coast or on coral formations. Most of the time they are dismissed as an algae layer.
• The unique characteristic of this group is its alternation of the polyp and medusa forms during its life cycle or metagenesis.
• Cnidarians are known to have radial symmetry, diploblastic, , polyp, and medusoid, gastrovascular cavity, tentacles, and cnidocyte with nematocyst.
• Cnidaria are typically grouped into two major subphyla: Medusozoa and Anthozoa.
• Three major classes are: Hydrozoa, Scyphozoa, and Cubozoa.
• Hydrozoa

  
• Scyphozoa

  
• Cubozoa

  
• Velum is the extension of bell inwards.
• Rhopalia are sensing organs that contain light-sensing structures and gravity-sensing structures called statoliths.
• Hydra

  
• C: Hydrozoa

  Obelia sp.

  
• C: Hydrozoa

  Aglaophenia sp.

  
• C: Hydrozoa

  Sea fir/ Air fern

  Sertularia sp.

  
• C: Scyphozoa

  Aurelia sp.

  Moon Jelly

  
• C: Scyphozoa

  Upside-down Jelly

  Cassiopeia sp.

  
• C: Scyphozoa

  Spotted Jelly

  Mastigias sp.

  
• Cassiopeia sp. has oral arms composed of zooxanthelae
• C: Cubozoa

  Box Jelly

  Chironex fleckeri

  
• C: Cubozoa

  Box Jelly

  Chiropsalmus

  
• SP: Anthozoa

  C: Octocorallia

  • polyp
  • tentacles in 8x, hairy/branched

  
• SP: Anthozoa

  C: Hexacorallia

  • polyp
  • tentacles in 6 multiplier, unbranched

  
• SP: Anthozoa

  C: Ceriantharia

  • Ptychocysts
  • Oral disk with two whorls of tentacles

  
• C: Octocorallia

  Soft Corals

  Alcyonacea

  
• C: Octocorallia

  Organ Pipe Coral

  Stolonifera

  
• C: Octocorallia

  Blue Coral

  Helioporacea

  
• C: Hexacorallia

  Anemones

  Actiniaria

  
• C: Hexacorallia

  Stony Star Corals

  Scleractinia

  
• C: Hexacorallia

  Black Corals

  Antipatharia