Commonly known as flatworms, are relatively simple invertebrate animals. The name is derived from the Greek platy for flat, and helminthes for worms. There are approximately twenty thousand species identified and described so far.

Characteristics of Platyhelminthes

Platyhelminthes are bilaterally symmetrical organisms, having identical parts on either side of an axis, and they are triploblastic, meaning they are composed of three layers of tissue. The body is dorsoventrally flattened, in the manner of a ribbon, giving rise to the common name. These organisms have no body cavity, and possess a blind gut, having a mouth but no anus. Waste is expelled through the same opening the organism uses to ingest food. The gut is branched to transport nutrients to the extremities of the body. Respiration is via diffusion in the absence of more sophisticated organs.

The flat nature of the body allows no cell to be very far from the outside, and consequently, the process of diffusion is adequate to oxygenate the cells. The reproduction of these organisms is hermaphroditic, whereby each individual produces both eggs and sperm. Cross fertilisation between individuals is typical.

Classes of Platyhelminthes

Platyhelminthes are commonly known as flatworms.

Platyhelminthes are commonly known as flatworms.

Traditionally, Platyhelminthes are divided into four discrete basic groups or Classes: Turbellaria, Trematoda, Cestoda and Monogenea.

Turbellaria are mostly non-parasitic animals found in oceans and in fresh water. Primarily carnivorous, Turbellaria are characterised by their soft epidermis. Most are less than a centimetre long.

Trematoda, commonly referred to as flukes, are parasitic and have oral suckers, additionally with hooks in some cases. Larval worms live inside small animals, while the adults live inside larger vertebrate hosts including humans.

Cestoda, known familiarly as tapeworms, like the Trematoda, are also parasitic. Adults live in the digestive tract of vertebrates, including humans, while larval forms are found in both vertebrate and invertebrate hosts. They have no digestive system of their own, instead absorbing nutrients directly from the host’s gut through the surface of their body.

Monogeneans are primarily external parasites. Consequently, the Monogenea has a complex arrangement of attachment organs, including suckers, hooks, spines and clamps which enable them to remain attached to their host.

Phil Platyhelminthes examples

Arthurdendyus triangulates, a New Zealand native parasite which preys on earthworms and which has become a concern in North West Europe.

Taenia solium, the pork tapeworm infests human intestines.

Clonorchis sinensis, the liver fluke causes serious diseases in humans.