PHYLUM  PORIFERA

Definition and Introduction

Lowest  multicellular animals or metazoans without  true  tissues, i.e. , at  Cellular level or body organization.

Familiar as sponges, these animals are  well known for their  ability to  absorb  and without  fluids . Now  a days  artificial sponges are made from  rubber nylon., etc, but a natural sponge is  the dried skeleton of some  members  of this  phylum,  The  word porifera means pore  bearers (Gr. Porus= pore, ferre= to bear ), Their   body wall  has numerous minute  pores, called ostia, through which  a continuous   current  of outside  water is drawn into  the body, About 5,000 species  are known .

Brief History

Aristotle (384-322 B.C.)  knew sponges Until 1765, these  were  through  to be plants  when ellis  first  observed  that sponges constantly  draw  and pump Outside water. He  therefore,  recognized   these  animals .Due  to their  plant like  form  Linnaeus. 1758, Cuvier 1796 and Lamarck1801 included sponges, together with coelenterates,  under the group  Zoohyta plant like animals .Robert  grant  1825  finally proved that sponges  are animals , and  coined the name porifera  for  sponges from  other  metazoans  on basis of embryological  studies, and  suggested  a separate group Parazoa for these.

Important Characters

1. All aquatic, mostly marine. Solitary or  colonial, all sessile-attached  to various objects  in water ,mostly  look like nonliving  outgrowths.

2.   Body  shape  and size  variable , shape cylindrical vase shaped  cup shaped,  spherical or dise like. Some radially symmetrical ,but  most  are irregular  and asym metrical  in shape , white  or variously  coloured.

3. Body  organization  of cellular  level  i.e.  true  tissues, movable  parts  or  appendages   not formed . Although  there  is some physiological division of labour  accompanied with  structural  differentiation  amongst  body  cells. But similar  cells  are  arranged neither in permanent layers  nor  masses  to form  tissues,  Moreover ,  cell differentiation   not  permanent  cells  one kind  can change  intro  other  kinds ALso   cells   mostly  function independently without  co ordination , because  there is   no co ordinating  nervous  system. Thus sponge body  is liked  a mere mass  of cells  held  together by a nonliving  cementing  substance .

4.     Sponge architecture  is constructed around  a system of water canals , esta blushing  a canal system .Outside  water  is continuously  drawn  into the canal  system through  numerous  dermal  pores  or ostia of body  wall ,collects  into  the main  body  cavity called   spongocoel and then escapes out  through  a single  large  exhalent  opening  the osculum, situated  at the  free end  of body .Cells obtain  food  and oxygen   from this   water current , and  release CO2  and excretory  substance  into it. Thus, there is neither  a distinct mouth, nor  a digestive  cavity.

5.Sponge body  is diploblastic  with an outer dermal ( ectodermal)  layer of  flattened  pinacocyte cells  ( pinacoderm )  and an inner  gastral  ( endodermal )  layer  of  peculiar  collared  cells  choanocytes  (choanoderm)  .In  between is a jelly like  mesenchyme  or mesohyl  layer  of dispersed collagen  fibers. It  contains  some  amoeboid, migratory  cells of both  layers.

6. A skeleton  calcareous or  siliceous s spicules ,  or of sponging   fibres , or  of  both  siliceous spicules and  sponging fibres present in most  sponges for  supporting  the  body  and maintaining its  gross anatomy.

7. Digestion  intracellular  in food  vacuoles  as in protozoa.

8. Gaseous  exchanges  and excretion  by diffusion  through  plasma  membrane  as in protozoa. Ammonia  is called chief  excretory waste.

9.No  nervous  system  reactions  largely  local.

10. Reproduction  both  asexual and sexual.  Asexual  mostly  by buds and  gemmules .Most forms  are hermaphrodite. Zygote  develops  into  a free swimming  ciliated  larva.