Cnidaria

• Phylum Cnidaria from Greek knide ‘nettle’ or stinging cells
• GENERAL CHARACTERISTICS:

  • Most marine, some in freshwater.

  • Radial symmetry or biradial symmetry.

  Diploblastic tissue-level organization.

  • Two types of individuals: polyps and medusae.

  • Gelatinous mesoglea between the epidermal and gastrodermal tissue layers. It functions as hydrostatic skeleton (for support and flexibility).

  • Gastrovascular cavity (an incomplete gut, which has only one opening that serves as both mouth and anus)
• Specialized cells, called cnidocytes, used in defense, feeding, and attachment.
• Cnidocytes house stinging organelles called nematocysts. It helps to paralyze their preys by injecting venom.
• Cnidocil triggers the nematocyst
• Muscular contractions via epitheliomuscular cells.
• statocysts (a balance sensory organs) and ocelli (photosensitive organs)
• Nerve cells organized into nerve net which coordinates body movements.

  • No excretory or respiratory system. No coelomic cavity
• Asexual reproduction by budding in polyps forms clones and colonies;
• Sexual reproduction by gametes in all medusae and some polyps; monoecious or dioecious
• Polyp is usually asexual and sessile. It attaches to a substrate at the aboral end, and has a cylindrical body, called the column, and a mouth surrounded by food-gathering tentacles
• Medusa is dioecious (sexes occur in different individuals) and free swimming.
• Phylum Cnidaria

  Radial or biradial symmetry, diploblastic organization, a gastrovascular cavity, and cnidocytes. More than 10,000 species
• Class Hydrozoa

  Cnidocytes present in the epidermis; gametes produced epidermally and always released to the outside of the body; mesoglea is largely acellular; medusae usually with a velum; many polyps colonial; mostly marine with some freshwater species.

  Hydra, Obelia, Gonionemus, Physalia.
• Class Scyphozoa

  Medusa prominent in the life history; polyp small; gametes gastrodermal in origin and released into the gastrovascular cavity; cnidocytes present in the gastrodermis as well as epidermis; medusa lacks a velum; mesoglea with wandering mesenchyme cells of epidermal origin, marine.

  Aurelia
• Class Staurozoa

  Medusae absent; develop from benthic planula larvae; eight tentacles surrounding the mouth; attachment to substrate by adhesive disk; sexual reproduction only; marine. Haliclystis
• Class Cubozoa

  Medusa prominent in life history; polyp small; gametes gastrodermal in origin; medusa cuboidal in shape with tentacles that hang from each corner of the bell; marine. Chironex.
• Class Anthozoa

  Colonial or solitary polyps; medusae absent; cnidocytes present in the gastrodermis; cnidocils absent; gametes gastrodermal in origin; gastrovascular cavity divided by mesenteries that bear nematocysts; internal biradial or bilateral symmetry present; mesoglea with wandering mesenchyme cells; tentacles solid; marine. Anemones and corals. Metridium
• Rhopalia can sense light, an eye spot, sensory struc of certain Scyphozoan (true jellyfish) and Cubozoan (box jellyfish)
• Class Hyrdozoa portuguese man o' war