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Cnidaria – Definition, Classification, Reproduction
Phylum Cnidaria or Coelenterata (Gr., koilos – hollow; enteron – gut)
1. Aquatic (living in water), mostly marine, a few such as Hydra are fresh water. Some of these species live in colonies (corals, Physalia), while others live solitary (Hydra).
2. Cnidarians or coelenterates are multicellular, diploblastic animals with tissue grade of organization. A gelatinous layer called mesoglea or mesohyl persists between the ectoderm (epidermis) and endoderm (gastrodermis).
3. Body shows radial symmetry.
4. A central gastrovascular cavity coelenteron is present which lacks an anus but has a mouth that is surrounded by short and slender tentacles.
5. Possess specialized sting cells (cnidoblasts) bearing stinging cell organelles called nematocysts. Nematocysts serve the function of paralyzing the prey by injecting poison or holding the prey.
6. Respiratory, circulatory, and excretory organs are absent.
7. Nervous system is primitive and has only a network of nerve cells.
8. Exhibit the phenomenon of polymorphism (Gr., poly – many; morphe – shape) which is the specialization of individuals of colonial species for the division of labour. For example, hydrozoans have feeding individuals or zooids, the gastro zooids, protective zooids, dactylozooids, and reproductive zooids, and the gonozooids producing sexual medusae.
9. Body shows two main forms, the polyps, and the medusae. The polyp is diploid, cylindrical, and sedentary. It is either solitary or colonial and reproduces asexually. The medusa is a diploid, umbrella-like, and free-swimming sexual phase of the animal.
10. Asexual reproduction by budding and sexual reproduction by gametes formed by medusae.
11. Many forms possess a hard exoskeleton of lime to form corals.
Phylum Coelenterata is divided into the following three classes:
Class 1. Hydrozoa – Their life history includes both polypoid and medusoid forms.
Examples: Hydra, Obelia, Millepora (coral), Physalia (Portuguese man-of-war), and Porpita.
Class 2. Scyphozoa – Their life history lacks polypoid phase and has only medusoid form.
Examples: Aurelia (jellyfish), Rhizostoma.
Class 3. Anthozoa – Their life history has only polypoid form, the medusoid forms are absent.
Examples: Pennatula (sea pen), Tubipora (organ pipe coral), Gorgonia (sea fan), Corallium 01 (red coral), Metridium (sea anemone), Fungia (mushroom coral).
Differences between Porifera and Coelenterata
| Porifera | Coelenterata (Cnidaria) |
| 1. They have a cellular level of organization. | 1. They have a tissue level of organization. |
| 2. Their bodies have a number of inhalent (incurrent) pores or ostia and a single exhalant (excurrent) pore or osculum. | 2. Their bodies have only a single opening. |
| 3. Digestion is intracellular. | 3. Digestion is both extracellular and intracellular. |
| 4. Muscle and nerve cells are absent. | 4. Muscle and nerve cells appear for the first time in coelenterates. |
| 5. Appendages are absent in poriferans. | 5. Coelenterates have appendages in the form of tentacles. |
| 6. Poriferans contain special cells called collar cells or choanocytes. | 6. Coelenterates contain special cells called cnidoblasts which contain nematocysts. |