Phylum Coelenterata is also called Cnidaria due to the presence of stinging cells on their body. The word Cnidaria is derived from Greek word, Knide meaning stinging cells. It’s synonyms Coelenterate is also a Greek word meaning Koilos=cavity and enteron = intestine. The term Coelenterate and Cnidaria was coined by Leuckart and Hyman respectively. The study of Coelenterate is called Cnidology. It consists of about 9,000 living species.
Characteristic Features of Coelenterata
Habitat: They all are aquatic, mostly marine and few freshwater forms. They may be Solitary (living independently) or colonial (living in groups).
Body Organization: They are multicellular with tissue grade of organization. Organs are absent. Tissues are specialized for performing different activities in the body.
Germ layers: They are diploblastic. Outer layer is called ectoderm while inner layer is called endoderm (or gastrodermis). Between two layers, a gelatinous layer called mesogloea is present.
Symmetry: They are usually radial symmetry.
Coelom: The organisms are without coelom i.e. acoelomate
Nematocytes are the unique cells present in Cnidarias. Nematocytes are also called as Cnidoblasts or stinging cells or host cells or battery cells. It is present on tentacles and secretes a hypnotizing chemical called hypnotoxin. Hypnotoxin are chemically made up of protein and phenol. It is used to paralyze pray as well as for defensing enemies.
Locomotion: Among the two forms of Coelenterate, Polyp forms like Hydra move slowly by creeping, crawling or by somersaulting. Whereas medusa form are free swimming forms. Medusa locomote by jet propulsion created by muscles in the rim of bell.
Nutrition: It is holozoic type.
Digestion: No specialized digestive organs are present. The opening of gastrovascular cavity acts as both mouth and an anus. The mode of digestion is first extracellular and then intracellular. Extracellular digestion includes action of enzymes on food particles and semi absorption of nutrition by cells lining the gastrovascular cavity.
Intracellular digestion occurs in endodermal cells. The nature of food in intracellular digestion is first acidic and then alkaline.
Respiration and excretion take place trough general body surfaces.
Excretion: It also occurs by simple diffusion. Excretory wastes are mainly composed of ammonia are ammoniotelic.
Nervous system: It is of primitive type. First appearance of nerve cells is found in coelenterate. Neurons are non-polar and are net like scattered irregularly in the body.
Polymorphism: It is an ability of an organism to appear in two or more forms. Two forms of organism or zooids are found in coelenterates. Polyp and medusa forms.
- Body is cylindrical with long stalk.
- Mesoglea is reduced type.
- Mouth located in hypostome.
- It is asexual reproducing form.
- Gonads absent except Hydra.
- Example of polyp form of zooids: Hydra
Image source: nicerweb
- Umbrella or cup shaped mouth with reduced stalk.
- Mesoglea is thick layered.
- Mouth is located in manubrium.
- It is sexual reproducing form.
- Gonads present.
- Example of medusa form of zooids: Aurelia (Jelly fish)
Reproductive system: They reproduce both by asexual and sexual methods. Asexual reproduction takes place by budding, fission and regeneration. Sexual reproduction occurs by formation of male and female gametes.
Fertilization: It is external. Cleavage formation in zygote is holoblastic.
Development: It is direct or indirect. Indirect development includes formation of larva such as Hydrula (in Hydra), Planula (in Obelia) and Ephyra (in Aurelia).
Alternation of generation: Coelenterates shows alternation of generation or metagenesis. Asexual generation (polyp form) alternates with sexual generation (medusa form).
Classification of Coelenterata
Coelenterata are classified into 3-classes on the basis of dominance of polyp and medusa forms.
(Gr. hydros=water, zois=animal)
Hydrozoa characteristics features.
- Mostly marine with few fresh water habitat.
- Presence of both polyp and medusa forms.
- Asexual from (polyp) is dominant.
- Mesoglea is simple and non-cellular.
- Gonads are ectodermal in origin.
- Examples: Hydra, Obelia, Physalia, Millipora etc.
(Gr. skyphos=cup, zois=animal)
Scyphozoa characteristics features.
- Exclusively marine.
- Polyp form is primitive or absent while medusa form is dominant.
- Mesoglea is cellular.
- Gonads are endodermal in origin.
- Examples: Aurelia (jelly fish), Rhizostoma etc.
(Gr. anthoas=flower, zois=animal)
Anthozoa characteristics features.
- Exclusively marine.
- Only asexual form (polyp) is present. Medusa is totally absent.
- Mesoglea consists of fibrous connective and amoeboid cells.
- Gonads are endodermal in origin.
- Examples: Metridium (sea anemone), Pennatula (sea pen), Heliopora (blue corals), Corallium (red corals) etc.
Examples of Coelenterata
|S.N||Examples (Scientific name)||Common name|
|4.||Physalia||Portuguese man of war|
|8.||Corallium||Precious red coral/Coral stone|
|10.||Alcyonium||Dead man’s finger|
|12.||Pennatula||Sea pen/sea feather|
Importance of Coelenterata
- They maintain balance in ecosystem either eating small organism or being eaten by predator.
- Many species of coelenterates are used as food which contribute to economic growth of the nation.
- Jellyfish is commonly used as food in Japan, Korea and western countries.
- Many marine species lives with symbiotic association with different organisms in which both are benefited.
- Some of the coelenterates are used for decorative purposes.
- Coral reef provides habitat and spawning for fishes.
- They are also used in lab and for research purposes.
- According to a research paper published in ncbi, they are the source of bioactive compounds.
- It has been used in traditional medicines since long time.
- Corals are used in cosmetic and substrate for new bone growth.
- The stings of some Cnidaria are fatal to human and other animals.
Some important points of coelenterates can be concluded as:
- They have unique stinging cells called nematocyst or Cnidoblast.
- Nematocysts secretes poison called hypnotoxin which is protein plus phenolic nature.
- Members of this phylum is called as zooids.
- Polymorphism is the characteristic features of coelenterates.
- Corals are skeleton remaining from dead coelenterates.
- Mesoglea present between ecto and endoderm is secreted by both layers.
- Phylum with first appearance of nerve cells.
- They show higher power of regeneration due presence of totipotence cells called interstitial cells.
- Physalia exhibits three types of polymorphism. Dactylozooid, Gastrozooid and Gonozooid.
1. Jordan EL and Verma PS. 2018. Invertebrate Zoology. 14th Edition. S Chand Publishing.