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Phylum Coelenterata

They are first true metazoans to reach the tissue grade of organisation.

Definition: They are sedentary or free swimming radially symmetrical, low and definite tissue grade of organisation, diploblastic with non-cellular mesoglea. They possess cnidocil (spine of nematocyst) and enteron or the gastro¬vascular cavity with a single large opening osculum.

Phylum Coelenterata General Characters-

1. Tissue grade system: Low grade.

2. Habit and habitat: Exclusively aquatic, mostly marine few fresh water. Solitary or colonial and sedentary or free living.

3. Symmetry: Radial symmetry.

4. Tentacles encircle the mouth.

5. Body wall: Diploblastic and non¬-cellular intermediate supporting layer known as mesoglea.

6. Characteristic interstitial cells and nematocyst: Nematocyst serves for stinging, adhesive, offence and defence and food capture.

7. Gastro-vascular cavity: Lined with endoderm called coelenteron.

8. Digestion: Extracellular.

9. Skeleton: External or internal lining or in the form of sclerites or as continuous mass.

10. Locomotion: Due to smooth muscle fibrils in the epithelial cells.

11. Nervous system: Primitive made up of diffused network of nerve cells and neurites in the body wall.

12. Body structure: Two types-polyp and medusa. Many species are provided with the phenomenon of alternation of generation.

13. Reproduction: Asexual by budding and sexual by ova and sperms. Cleavage holoblastic.

14. Development, Indirect with stereogastrula know as planula.

Classification-Includes 10,000 known species. Classified mainly into three classes-

Class 1-Hydrozoa e.g. hydra, obelia, physalia.

Class 2-Scyphozoa e.g. aurelia, cassiopeia.

Class 3-Anthozoa e.g. pennetula, alcyonium, metridium.

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