Phylum Platyhelminthes/Flatworms



Body Plan

They have bilateral symmetry.

Triploblastic animal,

          1.Ectoderm
          2.Mesoderm
          3.Endoderm

They are true mesoderm.
segmentation is absent.
Cephalization is present.
Sensory organs, nervous system concentrate in the head region.
They possess some sort of attachment organs on the body surface-
adhesive areas, suckers, hooks.
Epiderm is made in a single layer.
Each epidermal cell has cilia & tegument(non-ciliated syncytium).


The body is made of 3 germinal layers,
        1.Ectoderm
        2.Mesoderm
        3.Endoderm
They have mesodermal derivatives(parenchyma).
They arranged loosely.
Some parenchyma fixed & united into a network. It provides a low resistant path for internal transport.

Platyhelminthes are acoelomates. They do not have a true body cavity & between their internal organs
filled by parenchyma.

Their circular & longitudinal muscles well developed for their elongation and retraction movements.
They have an organ level of organization.
But circulatory, respiratory & Skelton systems are absent.

Digestive system


Digestive system

Incomplete digestive system
The mouth is present but the anus is absent. They use mouth to ingest & egestion.
Their food is digested intracellularly & extracellularly.
large members gut has lateral branches to increase their absorption & digestion.
(class Cestoda completely absent their digestive system)

Reproductive system

Reproductive system.
 Male reproductive system            Female reproductive system


                     
They are hermaphroditic.
Fertilisation is internal.
During copulation, transfer of sperm from both partners to other partners.
The sperm that transfer will be stored in the female body system.
Fertilize eggs over a long period in their body.

They have different asexual reproduction abilities.
Asexual reproduction occurs by fission & regenerate a missing part of the body.

Nerves system

Nerves system


Consists pair of ganglia or nerve ring.
Give rise 2 or 3 pairs of longitudinal nerves that connected by transverse nerves.
They have several types of sensory cells.

They are,
        1.Photoreceptors   
        2.Chemoreceptors  
        3.Mechanoreceptors
        4.Gravityreceptors 

Excretory system

Excretory system


Formed by Protonephridia.
that connect to ducts that open the exterior by excretory pores.
Flame cells are unique to flatworms.
These flame cells embedded in the parenchyma of the body.
A flame cell is a single cell with a tuft of flagella that extends to the excretory tube.
Excretory tubule connects to collecting duct.
Their nitrogenous waste Ammonia. This Ammonia is diffuse through the body wall.