PHYLUM  CNIDARIA (= COELENTERATA )       

Definition and Introduction

Tissue  grade eumetazoans with  a radial  symmetry .The  term  coelenterate  signifies the  presence of  a single internal cavity  called  coelenterons,  or gastrovascular,  cavity combining  function  of both  digestive  and body   cavities, The  term cnidaria  indicates  the presence of  cells (Gr. Knide= nettle  or stinging  cells) About 9000 species known.

Brief History

Aristotle  knew  cnidarians due to their  stinging  nature  and called  these  cnide  He  regarded  these  intermediate  between  animals  and plants .Peyssonel (1723)and   Trembley( 1744)  provd  these to be animals .Hence  Linnaeus (1758) cuvier  (1796)  and Lamard (1801) included  sponges and cnidarians under his  phylum  Coelenterata. Finally ,Hatschek (1888)  divided Coelenterata  into  three  phyla spongiaria (= porifera )  cnidaria  and ctenophore.

Important Characters

1.All  aquatic, mostly  marine. Solitary  or colonial.

2. Body typically  dimorphic (of 2 types ) sessile , cylindrical  and hydra like  polyp form  and free  swimming  saucer, cup bell  or umbrella shaped  medusa form  many  species  unclude both  forms  in life  cycle .In  some  ,the two basic  types  are further modified  into  polymorphic individuals .

3. Anterior end either  swollen into  a hypostome, bearing   terminal  mouth  or flattened into  an oral  disc with  central mouth.

4. Mouth  typically  surrounded by  highly  contractile , thread like  sensory  tentacles,

5.Basic  radial symmetry   is   a diagnostic  character.

6. Next   to  sponges advancement of body  organization .Physiological   division of labour  and consequent  cell differentiation  more elaborate and  permanent. Also  similar cells  form  permanent epithelial tissues, definite  organs not formed  .Thus  body   organization  is of tissue  grade.

7. The  single body  cavity, called  coelenterons  gastro vascular  cavity  is  simple  or branched, or  divided into  compartments  by longitudinal  plate  like  partitions called  mesenteries. It opens out by  a single   mouth .No anus.

8. Body  wall diploblastic, i.e.  formed  from two embryonic  germinal layers ecto derm  and endoderm , respectively forming its outer epidermis  and inner gastrodermis. In  between  is a noncellular jelly like layer called mesogloea. In some  advanced members , the mesogloea  contains some  cell derived from  epidermis and  gastrodermis.

9. Both  epidermis and gastroderimis  or only epidermis   contain certain highly  specialized and  unique stinging cells or  nematoblasts  found  nowhere else in the animals  kingdom ,These cells  are used  for food  capture, defense  and attachment .

10. Most  cnidarians are carnivorous and mainly  feed upon  small  crustaceans  of zooplankton. Digestion     partly extracellular  in gastrovascular  cavity  and partly  intracellular  in food vacuoles  of some gastrodermal cells.

11. Gaseous exchanges  and excretion  (ammonia)  through  general body  surface .

12. Reproduction  asexual (building ) or  sexual .Life  cycle  usually   exhibits  an alternation  of generations ( metagenesis)    between  asexual  polypoid  and sexual  medusoid  phase .A free  swimming  ciliated  stereoastrula   called  planula larva,  occurs in life cycle  of most cnidarians.

  13. A primitive  type of nervous system ,without brain  is found  in the form of an in traepidermal nerve net.  Eye  spots  (= ocelli)   and statocysts for  equilibrium  occur  in some.