Phylum: Coelenterate(Cnidaria)

Made up of two Greek words Koilos – hollow, Enteron – intestine

1. Habit and Habitat:
   They are all aquatic, mostly marine but few are fresh water forms such as hydra. They are solitary or colonial. Many are sedentary and sessile and a few are active swimming or free floating. Mostly colonial.

2. Body form:
   These are the simplest of metazoan with hollow tubular body.

3. Symmetry:
   The body is bilaterally or radially symmetrical with central gastro-vascular cavity.

4. Level of Body organization:
   They are multicellular with low tissue grade of body organization cells are mostly scattered and specialized for different functions. Some cells (like of nerve fibers) from definite tissue.

5. Germ Layers:
   They are diploblastic animals with an outer ectoderm and inner ectoderm separated by a gelatinous (non-cellular) jelly like cementing layer of mesogloea.

6. Coelem:
   They are a coelomate animal with true coelom.

7. Body Plan:
   The body encloses a single or branched central cavity known as coelenteron or gastro-vascular cavity surrounded by endoderm which performs the functions of intestine in higher animals. It opens to the exterior by a single opening, the mouth. There is no anus.

8. Appendages:
   The mouth is situated at a raised portion of the body at the anterior end, the hypostome, and is encircled by short, slender and extensible projections called tentacles. These serve food capacture, its ingestion and also for defense. Nematocysts found in tentacles are the organs of defense and offense, adhesion and food capturing.

9. Skeleton:
   They do not possess skeletal structures but coral polyps secrete calcareous skeleton.

10. Morphism:
    Two types of individuals occurs, they are dimorphic animals. These are polyps and medusa. The polyp is sessile, cylindrical and fixed to substratum and reproduces by asexual means. The medusa is free swimming umbrella shaped and reproduces by sexual means. Some species even exhibit polymorphism.

11. Locomotion:
    Locomotion is brought about by smooth muscle fibers and tentacles. Some like corals and sea anemone are sedentary.

12. Digestion:
    The digestion is intracellular and take place in the coelenteron by the Actium of enzymes secreted by glands of endoderm. A limited amount of digestion, however, takes place within endoderm cells.

13. Respiration:
    Gaseous exchange takes place through general body surface.

14. Circulatory System:
    It is absent.

15. Excretion:
    Excretion takes place through the general body surface.

16. Nervous system:
    Nervous system is primitive consisting of a diffuse network of nerve cells in the mesogloea.

17. Sensory Organs:
    Ocelli and statocysts are the sense organs. They are the first invertebrate to develop sense organs.

18.Reproduction:
Asexual reproduction occurs by budding and fission. The budding leads to extensive colony formation. Sexual reproduction occurs by gametic fusion. Sexual forms are ethermonoeciocy or diocious.

19.Development:
The development is indirect with larval, free-swimming stage, plannula larva (ciliated larva).

1. Regeneration:
   Regeneration power is well-developed.

2. Alternation of Generation:
   They exhibit alternation of generation or metagenesis. The asexual polypod form alternates with the sexual medusoid form. E.g. Hydra, Corals, Jelly fish etc.

Summarized:

1. Body possesses internal hollow cavity. The gastrovascular cavity is also known as Coelenteron.
2. Tissue grade is found only in the nerve fibres where diffused nerve network is formed.

On the basis of presence or absence of zooids(morhism) and nature of mesogloeze, the phylum coelenterate is divided into following classes:

1. Hydrozoa

1. They are mostly colonial and fresh water and few solitary.
2. They exist as polyp and medusa with polyp being the dominant.
3. They exhibit alternation of generation between the asexual polyp and sexual medusa form.
4. The medusa is generally minute in size with velum (circular inwardly projecting shelf)
5. Mesoglea is simple, non-cellular and jelly-like. E.g. Hydra, Obelia etc.

2. Scyphozoa:

1. Body contains cellular mesoglea.
2. Medusoid form is dominant in them whereas polypoid stag may be present or absent.
3. The medus are large bell or umbrella shaped without velum.
4. They are exclusively marine in which adults are solitary and free swimming.E.g. Jellyfish, Rhixostoma, etc.

3. Actinozoa or Anthozoa:

1. Body consists of thick and cellular mesogloea containing fibres and connective tissues.
2. They exist only in polyps. Medusod stage is entirely absent. So, there is no alternation of generation.
3. They are exclusively marine, fixed to substratum, solitary or colonial. Majority possess skeletonE.g. Tubipora etc.

NOTE:Mesogloea cell is ameboid/irregular.
Alternation of generation in coelenterates is known as metagenesis.