Phylum Cnidaria

20.6.2

PHYLUM COELENTRATA /

CNIDARIA

Characteristics of Cnidaria:-

1)Radially Symmetrical, diploblastic.
2)Body multicellular, few tissues, some

          organelles.
3)Body contains an internal cavity and a mouth.
4)Two different forms exist (dimorphic), medusa

      and polyp.
5)Reproduction is asexual or sexual.

•       Asexual reproduction by budding.

•       Sexual reproduction produces characteristic planula larva.

6)Has a simple net like nervous system.

7)Has a distinct larval stage which is planktonic.

8)Lives in aquatic environments, mostly marine.

9)Mostly carnivorous otherwise filter feeders.

10)May have a minimal skeleton of chiton or calcium carbonate.

There are three major groups of Cnidarians

Class	Life Style	Form	Habitat	Genera
Hydrozoa	Solitary or colonial, sessile as adult	Sexual polyps and asexual medusa either of which may be absent.	Freshwater and Marine	Hydra, Obelia, Physalia, Tubularia
Scyphozoa	Solitary, nearly all free swimming	Sexual medusa with a reduced or absent polyp	Marine only	Aurelia (jelly fish), Cassiopeia, Chironex, Rhizostoma
Anthozoa	Solitary or colonial sessile as adult	Polyp only	Marine only	Actinia (anemone), Madrepora (coral)

Hydra

•        Freshwater Cnidarian.

•        Polyps only.

•        Usually reproduce asexually (budding).

•        It has tentacles that surround the mouth-

          anus.

•        The tentacles and body have stinging cells, cnidocytes which contain small harpoons called nematocysts.

Obelia

•        The plantlike colony consists of numerous branches, which terminate in two kinds of polyps. 

•        The feeding, or nutritive polyps possess tentacles and resemble Hydra. 

•        The reproductive polyps are club-shaped and lack tentacles. 

•        The branches of the colony are covered by a transparent sheath - the perisarc, which extends around the nutritive and reproductive polyps as the hydrotheca and the gonotheca, respectively.

•        All cnidarians are carnivores, feeding on live prey which they usually capture using tentacles armed with cnidocytes.

Life Cycle of Obelia

•        This animal alternates between a polyp stage (which is the asexual generation) and a medusa stage (which is the sexual generation). 

•         sexual reproduction occurs, when some buds develop into structures called blastostyles which are reproductive polyps.

•        They produce tiny, free-swimming sexual medusae with tantacles and gonads.

Medusae are unisexual.

•        Gonads release egg cells and sperms into the water. Fertilization produces a zygote which develops into a planula larva consisting of a ball of cells with a ciliated outer layer.

•        The planula larva is dispersed by currents before settling onto a rock or seaweed where it starts a new colony.