ANIMAL KINGDOM

 PHYLUM- PORIFERA

Members of this phylum are commonly known as sponges. They are generally marine and mostly asymmetrical animals.These are primitive multicellular animals and have cellular level of organisation.

GENERAL CHARACTERS

The phylum porifera includes strange, apparently lifeless and plant-like creatures commonly called the sponge

• Body form:- The body form varies considerably.  Some sponges are vase-like or cylindrical,  but the majority form flat, globular or irregular,  branching masses. 

• Symmetry:- They are Asymmetrical organisms. 

• Germ Layers:- The sponges develop from two germ layers only.  Hence,  they are diploblastic animals. 

• Level of organisation :- The sponges are simple in structure. There are no tissue, organ,  and organ -system. Thus they are a cellular  level of organisation. 

• Head:- There is no head in sponges. 

• Body wall:- The body wall consists of two imperfect layers: outer dermal layer,  or pinacoderm  and inner chanocytic layer or choanoderm. Ther dermal layers consist of flat cells called pinacocytes. 

• The choanocyte layer consists of characteristic flagellated collar cells, or choanocytes. 

• Between these layers is a gelatinous material named mesohyl,  or mesenchymal. It contains numerous free amoeboid cells, the amoebocytes,  of several kinds,  such as scleroblast,  spongioblast, thesocytes, phagocytes, trophocytes etc. 

• Body cavity:- The body wall encloses a large cavity,  the spongocoel . Either  the spongocoel  or certain canals are lined by choanocytes with flagella. 

• Skeleton :- Almost  all sponges possess an internal skeleton . It may consist  of tiny cancerous  or dolorous spicule,  or fine sponging fibers, or both located in the mesenchymal. The spicules are secreted by a cell called scleroblast. The sponging fibers secreted  by cells are termed spongioblast. 

• Digestive tract :-  Mouth  and digestive cavity are lacking. Nutrition  is holotropic. Digestion is intracellular and occurs in food vacuole as in protozoan. Sponges are filter feeders,  also called suspension feeders. 

• Circulation :- Distribution of food from the ingesting cells to others is brought about by the wandering amorous cells, the trophocytes, of mesenchyme. 

• Respiration :- Respiration occurs  by diffusion of gases through the body surface, which are direct contact with water. This is called body surface respiration. 

• Excretion:- excretion also occurs by diffusion  through  the body surface. Excretion matters mainly with ammonia. The sponges are ammonotelic animals. 

• Sensation :- Sponges do not give visible response to stimuli and are the only animals without nervous system and sense organs. 

• Reproduction :-  reproduction  is asexual as well as sexual. The former occur by budding or by special cell masses,  termed granules. Sexual reproduction involve  the formation of Obama and spermatozoa. These develop from the archaeocytes or choanocytes,  or both. Sperms leave one sponge and enter another with water current to fertilize the eggs called internal fertilization. 

Classification 

The professor is divided  into three classes on the basis of spicules( skeleton) 

1. Calcarea or Calcispongiae

2. Hexactinellida or hyalospongiae

3. Demospongiae or sclerospongiae

Class I Calcarea:- The vacates have the following characters-

• The skeleton consists of calcareous domicile. 

• The characters are relatively large. 

• They are all Marin and inhabit shallow water. 

Examples : Leucosolenia, Scypha(sycon)

class II Hexactinellida:- The hexactinellida have the following  characters -

• The skeleton consists of 6 rated,  siliceous spicules. 

• The choanocytes are small. 

• They are all Marin and inhabit  deep water. 

Examples : Euplectella-  the Venus's flower basket,

  Hyalonema - the Glass Rope sponge. 

Class lll Demospongiae :- The Desmpspongia have the following  characters -

• The skeleton  consists of 1 or 4 rayed siliceous domiciles or sponging fibers or of both. 

• The choanocytes are small. 

• They occur in shallow and deep sea as well as in freshwater. 

Examples: spongia(Euspongia)- the bath sponge,  

spongilla - the freshwater sponge,  cliona- boring sponge. 

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