Phylum-Cnidaria /Coelenterata General Characters

Phylum – Coelenterata (Cnidaria)
Phylum Cnidaria: Cnidarians have radial symmetry, a gastrovascular cavity, and cnidocytes There are more than
10,000 species in the phylum Cnidaria, most of which are marine. The phylum contains hydras, jellyfish, sea
anemones and coral animals.The name Coelentrata was coined bt Lauckart.Hatschek called them cnidaria.Study of
cnidaria is cnidology
• They are aquatic, mostly marine, sessile or free-swimming,
• radially symmetrical animals.
• The name cnidaria is derived from the nidoblasts or cnidocytes (which contain the stinging capsules
or nematocytes) present on the tentacles and the body. Cnidoblasts are used for anchorage, defense and
for the capture of prey.
• Cnidarians exhibit tissue level of organisation and are diploblastic.
• They have a central gastro-vascular cavity with a single opening, hypostome.
• Digestion is extracellular and intracellular.
• Some of the cnidarians, e.g., corals have a skeleton composed of calcium carbonate.
• Cnidarians exhibit two basic body forms called polyp and medusa. Polyp is a sessile and cylindrical
form like Hydra, Adamsia, etc. whereas, medussa is umbrella-shaped and free-swimming like Aurelia or jelly
fish. Those cnidarians which exist in both forms exhibit alternation of generation (Metagenesis), i.e., polyps
produce medusae asexually and medusae form the polyps sexually (e.g., Obelia).
• Gastrovascular cavity, a central digestive cavity with only one opening(functions as mouth and
anus)
• There are two possible cnidarian body plans: sessile polyp and motile, floating medu
• Some species of cnidarians exist only as polyps, some only as medusae, and others are dimorphic
(both polyp and medusa stages in their life cycles).
• Polyp (diagram-b)= Cylindrical form which adheres to the substratum by the aboral end of the body
• stalk and extends tentacles around the oral end to contact prey
• Medusa(diagram-a) = Flattened, oral opening down, bell-shaped form; moves freely in water by
passive drifting and weak bell contractions; tentacles dangle from the oral surface which points downward.
• Cnidarians are carnivorous.
• Tentacles around the mouth/anus capture prey animals and push them through the mouth/anus
into the gastrovascular cavity.
• Digestion begins in the gastrovascular cavity with the undigested remains beinexpelled through the
mouth/anus.
• Tentacles are armed with stinging cells, called cnidocytes—after which the Cnidaria are named.
• Cnidocytes = Specialized cells of cnidarian epidermis that contain eversible capsule-like organelles,
or cnidae, used in defense and capture of prey .Nematocysts are stinging capsules.
• The simplest forms of muscles and nerves occur in the phylum Cnidaria.
• Epidermal and gastrodermal cells have bundles of microfilaments arranged into contractile fibers.
• The gastrovascular cavity, when filled with water, acts as a hydrostatic skeleton against which the
contractile fibers can work to change the animal's shape.
• A simple nerve net coordinates movement; no brain is present.
• The nerve net is associated with simple sensory receptors radially distributed on the body. This
permits stimuli to be detected and responded to from all directions.
Examples:
• Hydra(Water devil)
• Physalia (Portuguese man-of-war)
• Adamsia (Sea anemone)
• Pennatula (Sea-pen)
• Gorgonia (Sea-fan)
• Meandrina (Brain coral)