Wednesday, October 11, 2017

Phylum Cnideria ( Coenlentrata )

 Cniderians were proved as animals first time by Peysonnel and Trembley.
Leukart used the term coelentrata and Hatscheck used the term cnideria.
The phylum Cnideria includes about 9,000 species.

General characters
1. Habitat- All are aquatic and mostly marine , except some like hydra, are fresh water.
2. Body Form- Body form varies considerably. Many colonial - like Obelia are trimorphic , having three kinds of zooids-polyps, blastocysts and medusae. Occurrance of more than one type of individuals in their colonies performing different functions is called polymorhism.
3. Symmetry- shows radial symmetry.
4. Germ layers- diploblastic animals., i.e.,derived only from two germ layers endoderm and ectoderm.
5. Level of organization- first multicellular animals from evolution point of view showing tissue level of organization.
6. Body wall- consists of two layers of cells ; outer- epidermis and inner gastrodermis. There is a non- cellular gelatinous layer called mesoglea, between the epidermis and gastrodermis.
  1. Epidermis- epidermis consists of following cells .(i) Epitheliomuscular cells - provides protection and acts as muscles. (ii) Cnidoblasts ( stinging cells ) - Cnidoblast has nematocyst ( the stinging organ ) .The nematocyst consists of capsule, shaft and thread tube. These are used for defence and offense.  (iii) Interstitial cells - Reserve cells and called totipotent cells which can be converted into any type of cells ( iv) Nerve cells - form primitive nervous system. (v) Sensory cells- sensory in function.
  2. Gastrodermis- The gastrodermis comprises of (i)Nutritive muscular or digestive cells- Inracellular digestion takes place inside these cells and these also acts as muscles; (ii) Gland cells- secretes digestive enzymes for extracellular digestion; (iii) Interstitial cells (iv) Nerve cells  and (v) Sensory cells. Functions of interstitial, nerve and sensory cells are similar to that of epidermis cells.
7.Digestive tract- have a central gastrovascular cavity ( coelenteron ) with a mouth, which also acts as anus . 
8. Digestion - both intracellular and extracellular type of digestion present.
9. Respiration and Excretion are carried out through body surface by diffusion. Main excretory product is ammonia.
10. Nervous system- primitive type of nervous system present which consists of a network of nerve cells and their processes. Statocyst is a sense organ for balance which is first time developed in Cnideria.
11. Skeleton - In some Coelentrates body is supported by horny or calcareous exoskeleton or endoskeleton.
12.Reproduction- both by asexual ( budding) and sexual methods. Both gonads and buds arise from the interstitial cells The power of regeneration is also developed.
13. Development- cleavage is holoblastic . Direct or indirect development is found . In obelia planula larvae is present. However in aurelia planula, scyphistoma and ephyra larvae are found.
14. Metagenesis - In obelia , polyps reproduce medusae asexually and medusae form the polyps sexually. Such alternation of asexual and sexual phases in the life cycle of obelia is called metagenesis. It shouldn`t be confused with alternation of generation as found in plants where one phase is haploid and other is diploid. Here  both phases are diploid.

Distinctive features-
presence of cnidoblast for defense and offense , network of nerve cells acting as primitive nervous system.

 Advancement over porifera-  tissue level of organization, digestive tract, nerve and sensory cells.


Chiefly on the basis of the dominance of medusoid or polyploid phase in the life cycle, the phylum Cnideria is divided in three major classes- 
Class 1 - Hydrozoa-

  • both marine and fresh water.
  • Solitary or colonial.
  • Either only polyps are found or polyps and medusae are present exhibiting polymorphism. 
  • Medusae are always sexual forms and bear true velum which is marked by a narrow shelf- like ridge on the inner border of the circular margin of the medusae.Such a medusae is called as Craspedote.
  • Many hydrozoa exhibit alternation of generations (-metagenesis) 
  • gonads are usually ectodermal in origin and sex cells are discharged externally ,
  • ex.-Hydra ( fresh water polyp ), Obelia(sea-fur) , Physalia( portuguese man of water ).
Class 2 - Schyphozoa-
  • Exclusively marine.
  • solitary and represented by a bell shaped medusae which are without velum. They are commonly called jelly fishes.
  • Polyp form is reduced or absent.
  • mesoglea is much enlarged and gelatinous.
  • gonads are endodermal in origin. The sex cells are discharged into the gastrovascular cavity.
  • Examples,- Aurelia(jelly fish) , Rhizostoma. 
Class 3- Anthozoa- 
  • exclusively marine .
  • attach themselves with the rocks and other submerged animals.
  • They are solitary or colonial.
  • Only polyp form is present. Medusae form is absent.
  • The gastrovascular cavity is divided into compartments by complete or incomplete mesenteries. 
  • Gonads are endodermal and develop into mesenteries . Sex cells are discharged into the gastrovascular cavity.
  • Examples- Gorgonia ( sea fan ), Metridium ( sea anemone ) , Alcyonium ( dead man`s finger ), Fungia ( mushroom coral ) , Penntula ( sea pan or sea feather ), Corallium ( red coral ), Astraea ( star coral ), Madrepora (stag-horn coral), Tubipora(organ pipecoral), Heliopora( blue-coral) ,Meandrina ( brain coral) .
Polymorphism- Occurrence of the same species in more than one type of individuals , which differ in the form and function from one another, is known as polymorphism. This ensures an efficient division of labour between the several individuals which often get united to form colonies, e.g., polyp and medusae are examples of such organisms which differ in structure and function and the cnidarian possessing both of them are said to be exhibiting polymorhism., e.g., Obelia, Physalia etc.
  1. Physalia (Portuguese man of war) : Physalia exhibit a remarkable example of polymorphism and division of labour. A gas gland is present inside pneumatophore secretes a gas which helps the animal to float over the water surface.
  2. Pennatula (Sea pan) :It looks like a quill( a type of feather or pan ). It is carnivorous.
  3. Gorgonia(sea fan) : All the branches form a hand held fan like structure .So, Gorgonia is commonly called sea fan.
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