PHYLUM  PLATYHELMINTHES

Definition and Introduction

Bilateral and protostominal  organ  grade eumetazoans  without  a body cavity ( acoelomates)  platyhelminthes means flatworms  (Gr. Platys= flat; helmins  =  worms. )  their  body is  dorsoventrally  flattened. About  10,000 species  known.

Brief History

Aristotle mentioned tapeworms. But  scientific studies of flatworms began only  in the 18 th  century. It was  Gegenbaur  (1859)  who  placed these in a separate  group  and suggested  the  name of  the phylum.

Important Characters

1.       Mostly  parasitc , cheif  invertebrates  , including man. Some  pathogenic,  i.e.  disease producing .

2.        Differentiation  of head   ( cephalisation )  with  a brain and , in some  primitive  eye spots, at anterior  begins  in eumetazoans with  platyhelminths .

3.       In  many forms body  is divided into  segments,  but this  segmentation  is not  true  metamerism.

4.       Sukers, hooks  spines, etc. Often  developed  for  attachment to host tissue .

5.       Soft and  delicate,  dorso ventrally flattened  bilateral  body  is triploblastic,  both triploblastic  organization  and bilateral symmetry  also  begin  in eumetazoans  with these  animals .

6.       Similary , development  of distinct organs  and organ system  also  begins in eumetazoans  with platyhelminths .     Distinct digestive,  excretory , nervous and  anus. Reproductive  systems occur, but  no skeletal,  vascular and respiratory  system ,No  anus  some  have secondarily  lost  the digestive  system .

7.       No  body  cavity  ,Space  between  body  wall  internal organs  occupied by s loose  mesodermal  tissue  called  parenchyma.

8.        Units  of excretory  system  are peculiarly specialized  protonephridial    tubules  with terminal  flame  cells  or bulbs.

9.       As  an  adaptation to parasitic  mode of life , the  reproductive  system