Chp 4

• Cnidaria


  • Body has a radial symmetry
  • Distinguishing feature is cnidocytes, specialized cells used mainly for capturing prey
  • Body consists of mesoglea, a non-living jelly-like substance, sandwiched between two layers of epithelium - an epidermis and a gastrodermis
  • Diploblastic organisms
• Types of cnidocysts


  • Penetrant - armed with spines, and an open filament
  • Glutinant - lacks spines, with closed filament
  • Volvent - with closed filament, secretes toxic substances
• Cnidaria


  Have two basic body forms: swimming medusae and sessile polyps, both radially symmetrical with mouths surrounded by tentacles that bear cnidocytes
• Cnidarian super-classes


  • Anthozoa (sea anemones, corals, sea pens)
  • Scyphozoa (jellyfish)
  • Cubozoa (box jellies)
  • Hydrozoa (freshwater cnidarians and many marine forms)
• Hydrozoa


  • Mostly marine, some live in fresh water
  • Life cycle includes polyp, medusa, or an alternation of medusa/polyp generations or vice versa
  • Mesoglea is acellular
  • Gametes develop only in the epidermis
  • Cnidocysts are found only in the epidermis
  • Medusa has a velum for active swimming
  • Some colonial forms secrete a skeleton of chitin and protein, or of calcium carbonate
  • Medusa may possess statocysts and ocelli
• Obelia


  • Worldwide distribution except the high-arctic and Antarctic seas
  • Medusa stage is common in coastal and offshore plankton around the world
  • Usually found no deeper than 200m from the water's surface, growing in intertidal rock pools and at the extreme low water of spring tides
• Obelia


  • Diploblastic, with two layers - an epidermis (ectodermis) and a gastrodermis (endodermis), with a jelly-like mesoglea filling the area between the two layers
  • Gastrovascular cavity is present where digestion starts and later becomes intracellular
  • Incomplete digestive tract where food enters, is digested, and expelled through the same opening
  • Polyp stage has mouth situated at the top of the body structure, or manubrium, surrounded by tentacles
  • Medusa stage has mouth situated at the distal end of the main body structure
  • Medusa has a ridge-like structure on the inner margin, called velum
  • Eight statocysts act as hollow balance organs attached to the bases of the eight tentacles
  • Four gonads lie in the main body
• Obelia life cycle


  1. Polyp colony reproduces asexually

  2. Medusae are released from gonozooids by budding

  3. Medusae reproduce sexually, releasing sperm and eggs that fertilize to form a zygote, which later morphs into a blastula, and then a ciliated swimming larva called a planula

  4. Planulae attach to a solid surface and develop into one feeding polyp

  5. As polyp grows, it begins developing branches of other feeding individuals, forming a new generation of polyps by asexual budding
• Scyphozoa (True jellyfish)


  • Exclusively marine
  • Medusa is much larger than in hydrozoans
  • Medusa is dominant, polyp is absent or restricted to an immature stage
  • Mesoglea is cellular
  • Gametes and cnidocysts are produced in the gastrodermis
  • Medusa lacks velum
  • Stomach extends into four gastric pouches bearing gastric filaments
  • Possess special sensory organs called rhopalia
• Aurelia aurita


  • Also called the moon jelly, moon jellyfish, common jellyfish, or saucer jelly
  • Translucent, usually about 25–40 cm in diameter, with four horseshoe-shaped gonads visible through the top of the bell
  • Feeds by collecting medusae, plankton, and mollusks with its tentacles, and bringing them into its body for digestion
  • Capable of only limited motion, drifts with the current, even when swimming
• Aurelia


  • Basic body plan is convex above and concave below, surrounded by a row of closely delicate marginal tentacles
  • Exterior surface is covered by epidermis, lining of digestive system and canals from the mouth inward and the gonads are of gastrodermis
  • Lacks respiratory, excretory, and circulatory systems
  • Respiration and excretion occur by diffusion on all body surfaces
  • Has four bright gonads under the stomach
  • Has a nerve net responsible for contractions in swimming muscles and feeding responses
• Aurelia life cycle


  1. Sexes are alike but separate

  2. Sperm from male gonads pass out its mouth and enter the stomach of the female to fertilize the eggs in her gonads

  3. Zygote emerges to the oral arms and develops into a planula larva

  4. Planula larva settles on a substrate and changes into a "scyphistoma" polyp

  5. Scyphistoma polyp feeds and grows, may produce lateral buds

  6. In autumn and winter, a transverse fission called strobilation occurs resulting into small ephyrae that swim off to grow up as medusae
• Anthozoa (sea anemones)


  • Marine
  • Polyp is dominant, medusa is absent
  • Solitary or colonial
  • Mesoglea is thick and contains cells and fibers
  • Gametes are produced in the gastrodermis, fertilization occurs in the gastric cavity or externally in sea water
  • From the mouth extends a pharynx, usually with one or two siphonoglyphs as ciliated grooves for respiration
  • Between the body wall and pharynx, or gastro-vascular cavity extend mesenteries
  • Many species contribute in the construction of coral reefs
• Anthozoa


  • Actinia
  • Metridium
• Stony corals (Madreporaria or Scleractinia)


  • Solitary or colonial with a hard skeleton
  • Founding polyp settles on the seabed and starts to secrete calcium carbonate to protect its soft body
  • Occur in all the world's oceans
  • Hermatypic corals are mostly colonial and tend to live in clear, shallow tropical waters, they are the world's primary reef-builders
  • Ahermatypic corals are either colonial or solitary and are found in all regions of the ocean and do not build reefs
• Cubozoa (box jellyfish)


  • Exclusively marine
  • Some species produce extremely potent venom from nematocysts
  • Medusa is dominant with a box-like bell shape, polyp is absent
  • Stomach extends into four gastric pouches
  • Possess special sensory organs called rhopalia and ocelli used for vision and statolith that help the animal orient itself in water
  • The Australian stinger Chironex fleckeri is among the deadliest creatures in the world, having caused human fatalities
• Phylum Ctenophora


  • Comprises about 90 species of free-swimming marine animals with transparent gelatinous bodies, often called comb jellies
  • Found in warm sea, mostly in surface water, but a few live at various depths, even 3000m
  • Rest vertically in the water and can swim only weakly
  • Food includes larva of mollusks and crustaceans, fish eggs, and small fish
• Ctenophora


  • Biradial symmetry (radial + bilateral) on an oral-aboral axis, with large mesoglea, body is not segmented
  • Usually have eight external rows of comb plates, no nematocysts, tentacles with adhesive colloblast cells
  • Cellular epidermis and gastrodermis, mesoglea with amoebocytes and muscle cells
  • Digestive tract with mouth, pharynx, stomach and anal pores
  • Diffuse nervous system with statocyst
  • Monomorphic, hermaphrodites
  • Development with or without larval stage: cydippid, no asexual development
  • Some are bioluminescent
• Ctenophora


  • Pleurobranchia