ANIMAL KINGDOM

 PHYLUM- CNIDARIA

The phylum cnidarians, also called coelenterates, include the hydroids,  jellyfish, sea anemones and corals. They may be solitary or colonial.  

GENERAL CHARACTERISTICS

• Body form -: The colonial forms two main types of individuals or zooids: polyp and Medusa.

• The polyp is cylindrical and usually  fixed, but may be solitary or colonial. 

• The Medusa is umbrella-like and generally free -swimming but always solitary. 

When both polyp and Medusa forms are found in the same animal,  the two forms alternate in the life cycle. This phenomenon is termed alternation of generation ( metagenesis). 

• Symmetry :- Symmetry is usually radial. 

• Germ Layers:- The cnidarians develop two germ layers 

• Ectoderm

• And endoderm

Hence,  they are diploblastic like the sponges. 

• Level of organisation -:The cnidarians have tissue but lack organ and organ system. Thus, they have a tissue level of organisation.

• Head-: There is no head in either  form of cnidarians. 

• Appendages-: There are no locomotory appendages in cnidarians.

• Body wall-: The body wall consists  of two distinct layers of cells.

• Outer epidermis and inner gastrodermis.

• The cells characteristic  of the cnidarians include stinging cells ( cnidocytes or cnidoblasts or nematoblasts) for offence and defence. 

• Between the two layers is a sheet of gelatinous material, the mesogloea, which may contain free cells. 

CNIDOBLAST

• The stinging cells,  when discharged, give out form a sack,  the cnide or cnidocyst or nematocysts,  a long  thread tube that may coil round the prey, or attached to it, or inject a toxin, called hypotoxin. 

• Hypnotoxin is a toxin that paralyzes prey. 

• Body cavity:- There is no separate body cavity or coelom.

• Skeleton :- The soft body may be supported by horny or calcareous exoskeleton  or endoskeleton. 

• Digestive Tract:- The body enclose a single cavity, the coelenteron. It provides the phylum it's alternative name. It has a single apparatus,  the mouth. 

• There is no annus. The mouth serves both for taking food and for throwing out faces. Such a digestive cavity is said to be incomplete. 

• Digestion is partly intracellular, which takes place in food vacuole within the gastrodermal cells.

• Respiration :- Cnidarians have body surface respiration. It occurs by diffusion of gases through the body surface, which are direct contact with water. 

• Excretion :- Excretion also occurs by diffusion through the body surface. Excretory matter is chiefly ammonia. Cnidarians are also ammonotelic like sponge. 

• Circulations :- Colenteron distributes food beside partly digesting. On account of its dual role,  the celebration is also called gastrovascular cavity. 

• Nervous system :- A Primitive nervous system occurs in both the forms. It consists of a network of nerve cells and their processes. Sensory cells are also present. 

• Reproduction :- saxes may be separate or united.  Reproduction is usually asexual ( budding)  in the polyp form and sexual in the Medusa form. Gonads have no ducts. Fertilization may be external or internal. 

classification of cnidarians :-

The phylum Cnidaria is divided into three classes :

1. Hydrozoa

2. Scyphozoa

3. Anthozoa or Actinozoa

Class l. Hydrozoa

The hydrozoa are commonly called hydroids. 

• They usually have both polyp and Medusa forms and often show polymorphism and metagenesis. 

• The polyps have a manubrium

• The Medusae have a velum. 

• Mesogloea is non cellular

• Gonads are ectodermal and shed gamete directly into the surrounding water. 

Examples : Hydra- The freshwater polyp. 

Obelia-The seafur

Physalia-the portugueses man-of-war. 

class ll Scyphozoa

The scyphozoa are commonly called jellyfishes. 

• The Medusa form is predominantly.  It lacks a true velum

• The polyp form is reduced,  but it bears a short manubrium. 

• Mesogloea  is thick, gelatinous and contains cells. 

• Gastrovascular cavities lack mesenteries. 

• Gonads are endodermal and shed the gamete into the digestive tract when they escape through the mouth. 

Examples :  Aurelia-The jellyfish

Class lll Anthozoa. 

The Anthozoa are commonly called sea-anemones and corals. 

• There is only polyp form

• It is without a manubrium

• Mesogloea contains cells and fibers. 

Examples : Admasia-The sea 

anemone,

Astarea- The star coral.

Pennatula- The sea pen.

Gorgonia- The seafan.

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