Flatworms

What is a Flatworm? Flatworms are in the phylum Platyhelminthes. These are soft, flattened worms that have tissues and internal organ systems. They have bilateral symmetry and cephalization. Flatworms are aceolomates – this means that they do not have a fluid filled body cavity. A coelom is a fluid filled body cavity. This means that the organs of a flatworm are imbedded in their body.

Form and Function in Flatworms Flatworms use diffusion for respiration, excretion, and circulation. There are 2 types of flatworms – free-living – do not depend on other organisms to live and parasitic – have to live in or on another organism.

Feeding Free-living flatworms are carnivores that eat tiny aquatic animals or recently dead animals. They have an incomplete digestive system – they have one opening which food and wastes pass through. They have a pharynx – this is a tube near the mouth, that moves food into the digestive cavity. Parasitic worms do not have a very complex digestive system. They feed on blood, tissue fluids, or pieces of cells in a hosts body. They absorb already digested food so they do not need to digest it.

Response Free-living flatworms have several ganglia – a group of nerve cells, that control the nervous system. This is not a brain. They also have 2 nerve cords that run along the body. Free-living flatworms have an eyespot – this is an area that can detect changes in the amount of light.

Movement Free-living flatworms move one of 2 ways: by cilia that help them move in water. Muscle cells allow them to twist and turn.

Reproduction Free-living flatworms are hermaphrodites – they have both male and female reproductive organs. They cannot self fertilize, they need another worm to reproduce. Asexual reproduction can take place by fission – an organism can split in two and each part becomes a new organism. Parasitic flatworms have very complex life cycles.

Tubellarians This is the free-living flatworms. Most live in marine or fresh water. A common member is the planarian.

Flukes These are parasitic flatworms that live in the internal organs of their host. Schistosoma is a blood fluke that can infect humans. Snails are intermediate hosts to the blood fluke. This causes schistosomiasis – this is where the eggs of the fluke clog blood vessels, causes swelling, tissue decay in lungs, liver, spleen, or intestines.

Tapeworms Tapeworms are long, flat parasitic worms that live in the intestines of their hosts. They have no digestive tract. The head of an adult tapeworm is called a scolex – it contains hooks or suckers to attach to the intestine wall. Behind that it has many proglottids – segments that makes up the body and carry reproductive structures. The proglottids will break off and burst to release fertilized eggs.