Phylum Cnidaria (Coelentrata)
- Habitat -
- They are aquatic mostly marine animals, they are sessile or free-swimming.
- Level of organization -
- They exhibit tissue level of organization.
- Body wall -
- They are diploblastic animals, body wall is made of 2 layers, outer ectoderm and inner endoderm.
- Body symmetry -
- They are radially symmetrical.
- Digestive system -
- They have a central gastro-vascular cavity with a single opening hypostome, which serves as both the mouth and the anus.
- Digestion-
- is extracellular and intracellular.
- Specialized cells known as the cnidoblasts or cnidocytes which contain the stinging capsules or nematocytes are present on the tentacles and the body. Cnidoblasts are used for anchorage, defense and for the capture of prey.
- Some cindarians like corals have skeleton composed of calcium carbonate.
- Cnidarians exhibit two basic body forms called polyp and medusa. The polyp is a sessile form ann cylindrical in shape like the Hydra, Adamsia etc. The medusae form is umbrella shaped and free-swimming forms like Aurelia or jelly-fish.
- Metagenesis -
- Cnidarians which exhibit both polyp and medusae form are exhibit alteration of generation, this is known as metagenesis. Poplyps produce medusae asexually and medusae form the polyps sexually.
- Locomotion -
- The body contains nerve network that allows movement of tentacles and body.
- Examples:
- Aurelia (medusa), Adamsia (polyp)
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