Phylum- Platyhelmintes

Characters

•      Free living, commensal/parasitic forms

•      Tissue-organ grade of organization i.e., body cells aggregate into definite tissues & tissues make up the organs

•      Triploblastic i.e., body has 3-embryonic germ layers-ectoderm, mesoderm & endoderm

•        Bilaterally symmetrical with definite polarity: Anterior (head) & posterior (tail) ends

•       Dorsoventrally flattened

•       Well defined ventral surface bearing mouth & gonopore

•       Body unsegmented except in class Cestoda

•      Acoelomate i.e., without any body cavity

•       Spaces between various body organs filled with special mesodermal tissue, the mesenchyma /parenchyma

•       Adhesive structures like hooks, spines & suckers & adhesive secretions common in parasitic forms

•      Epidermis cellular/syncytial, frequently ciliated, absent in some

•       Muscular system of mesodermal origin

•       Longitudinal, circular & oblique muscle layers beneath epidermis

•      Digestive system branched & incomplete without anus,  altogether absent in Acoela & Cestoda

•       Skeletal, respiratory & circulatory systems wanting

•       Excretory system includes lateral canals & protonephridia (flame cells), absent in some primitive forms

•      Nervous system primitive, ladder-like, comprises a pair of anterior ganglia with longitudinal nerve cords connected by transverse nerves

•       Sense organs simple, eye spots/photoreceptors in free living forms

•      Mostly monoecious (Hermaphrodite) with complex reproductive system

•       Well developed gonads, gonoducts & accessory organs

•       Eggs mostly devoid of yolk

•       Yolk produced separately in yolk/vitelline glands

•      Fertilization internal/cross/self

•       Development direct/indirect

•       Indirect in endoparasites with a complicated life cycle involving many larvae & hosts

Phylum- Platyhelminthes

1.Turbellaria                     2.Cestoda              3. Trematoda