Phylum Coelenterata/ Coelenterates (2nd Phylum)

• Radially symmetrical, diploblastic multicellular animals with a tissue grade of organisation.

• Aquatic, freshwater or marine solitary or colonial forms which may be free swimming or sedentary.

• Body has a mouth at the oral end which leads into a spacious cavity called gastrovascular cavity or coelenteron.

• Presence of long, hollow structures called tentacles used for locomotion and food capturing.

• Presence of peculiar type of cells called cnidoblasts, nematocysts or stinging cells in the ectoderm, especially in the tentacles, used for offence and defence.

• Digestion is both intracellular and extracellular.

• Respiration and excretion by simple diffusion.

• Presence of a network of nerves spread all over the body.

• Many forms exhibit polymorphism, wherein different types of individuals are present in a colony for different functions. These individuals are called Zooids.

• Reproduction asexually (external budding) or sexually (formation of gametes).

    fig. 10.7 - Examples of Coelenterata

The phylum is divided into three classes

Characteristics	Class Hydrozoa	Class Schyphozoa	Class Anthozoa
Habitat	Fresh water or marine	Marine	Marine
Body Form	Polypoid or Medusoid	Medusoid	Polypoid
Example	Hydra, Obelia Physalia (Portuguese man of war)	Aurelia Cassoprea (Jelly fishes)	Sea anemone Astrea Fungia and other Corals

                    fig. 10.8 - Anatomy of Hydra

TYPES  POLYMORPHIC FORMS IN CNIDARIANS: (COELENTERATA_)

  There are three types of polymorphism  (polymorphic forms) found in the Cnidarians-(Obelia sp.) The class Anthozoa consists only of  polypoid forms.

POLYPOID  FORMS  OF CNIDARIAN   POLYMORHISM 

• The polypoid form or the polyp is the hydroid form, generally sessile .
•  Body cylindrical hollow with a common stem of colony by proximal end .
• The distal; end with a conical elevation called manubrium or oval cone with tentacles at its base in a circle.
• Both body and manubrium contain coelenteron with a mouth at the top of the manubrium
• .Body composed of  epidermis and gastrodermis with mesoglea inbetween.
• Tentacles, manibrium and polyp wall well provided with muscular cells for contraction
• .Cnidoblast cells with nematocysts  in tentacles                                                                                                                                                                                                                                                                                      Obelia  colony                                                        SIngle polyp,magified   
  
                                                    
                                                                                                           
                         CNIDOBLAST CELL                                                   DISCHARGED CNIDOBALST