Phylum – Platyhelminthes

The members of this phylum have a body that is flattened from the upper to the lower surface (dorso-ventrally flattened), which is why they are commonly called flatworms. Most of them live as endoparasites inside other animals, including human beings.

Symmetry, layers and body cavity

• They are bilaterally symmetrical, so a single plane divides the body into matching left and right halves.
• They are triploblastic, being built from three germ layers.
• They are acoelomate, meaning there is no true body cavity between the gut and the body wall.
• They show an organ level of organisation, with tissues arranged into organs.

Parasitic adaptations and nutrition

The parasitic forms are equipped with hooks and suckers that let them hold on to the host. Some of these worms take in nutrients straight through their body surface, absorbing food directly from the host rather than through a mouth.

Excretion and osmoregulation

Excretion and the control of water balance (osmoregulation) are carried out by specialised cells known as flame cells.

Reproduction and regeneration

• The sexes are not separate, so each animal carries both sets of reproductive organs. …