CNIDARIA (GENERAL)

• The body wall is composed of two layers, the epidermis and gastrodermis.
• Epidermal cells have microvilli on their surface to increase surface area.
• Gas exchange occurs through diffusion across the epidermis.
• Cnidarians have two body plans: Medusa, which is an upside-down cup and motile, and Polyp, which is tubular is shape and sessile
• Some cnidarians are polymorphic, that is, they

  have two body places during their life cycle.

  Example: colonial hydroid, Obelia.
• Anemones and corals (Anthozoans) only exist in the polyp form
• Radial Symmetry- basic body plan in which the organism can be divided into similar halves by passing a plane at any angle along a the central axis, characteristic of sessile and bottom-dwelling animals, such as sea anemone and sea stars.
• Epidermis: covers the body
• Gastrodermis: inner digestive layer; lines gastrovascular cavity and contains gland cells
• Mesoglea: Layer between epidermis and mesoglea with amoebocytes for digestion, nutrient transport, and storage, wound repair, and antibacterial defense. Is the hydrostatic skeleton of cnidarians
• Epitheliomuscular Cells- epithelial cell of

  cnidarians, modified to function in

  contraction and has single elongated

  fibrillar base that functions in the same

  manner as a muscle cell.
• Nutritive Muscular Cells- Tall, columnar

  cells that have laterally extended bases

  containing myofibrils.
• In hydrozoans, water is brought into the cavity

  through the mouth by the beating

  of the cilia on nutritive muscular cells
• In anthozoans, water is brought into the cavity by ciliated cells in the pharynx (as hydrostatic skeleton)
• Interstitial Cells- scattered among the

  bases of the nutritive cells and can

  transform into other cell types.
• Gland Cells- tall cells that secrete

  digestive enzymes
• Cnidocyte- or cnidoblast during its

  development; “stinging cells” ; produce organelles called cnidae,

  present around the mouth, and tentacles, and serve to immobilize prey with toxins contained within the cells.
• Nematocyst- one type of cnidae; tiny capsule composed of material similar to chitin; contains a coiled thread that may

  bear barbs. These are characteristic of and exclusive to cnidaria
• Operculum- a little lid that covers the end

  of the capsule.
• Cnidocil- hairlike projections (modified

  cilium) in the outer wall of the cell and are

  sensitive to touch; has a trigger-like structure found in

  all cnidocytes
• Anthozoans have ciliary mechanoreceptors instead of cnidocils
• Cnidae- venom-filled harpoon that fire when triggered(coiled within cnidocyte and covered by hinged operculum)
• A type of cnidae, penetrant (stenolele) is the largest and most

  complex nematocyst. It pierces the skin and is the chitinous exoskeleton of the prey. It injects a poisonous fluid called hypnotoxin which paralyzes the victim or kills it.
• A type of cnidae, Glutinant (ptychocysts), has a sticky surface used to stick to prey. It is found on burrowing (tube) anemones.
• A type of cnidae, Volvent (desmoneme or spirocysts), are small and pear-shaped nematocysts with a lasso-like string. They contain a short, thick, spineless, smooth, and elastic thread tube forming a single loop and closed at the far end.
• Penetrant in Hydrozoa, Scyphozoa, and Anthozoa
• Volvent in Hydroida and Siphonophora
• Glutinant in all classes of cnidaria
• Cnidarian swim by a form of jet propulsion, by wiggling their base, and by pedal disc gliding.
• Cnidarians have no brains or central nervous system and instead have decentralized nerve nets
• Decentralized nerve nets consist of

  • Sensory Neurons
  • Motor Neurons
  • cobwebs of Intermediate Neurons
• Sensory neurons generate signals in response to various types of stimuli.
• Motor neurons- tell muscles to contract.
• Intermediate Neurons- to connect sensory and motor neurons; Also form “signal cables” and ganglia– that act as local coordination centers.
• Most communication between nerve cells are via chemical

  synapses– small gaps across which chemicals flow.
• • Hydra- has multipolar neurons.
  • Aurelia sp.- bipolar neurons run through conduction tracts so sending of impulses is faster
• Rhopalia- rhopalium, receptors of light, balance,

  touch, and chemical detection.

  Aurelia sp. Has 8 rhopalia or tentaculocysts (small modified tentacles).
• Each rhopalium is lying between paired marginal lappets-touch receptors.
• Sensory epithelium of cnidarians is olfactory
• Ocellus- an ectodermal pigment spot at the outer side of the tentaculocyst. Contain pigments and photoreceptor cells. Sensitive to light.