Flatworms

Scientific name: Platyhelminthes

This planarian (Dugesia) belongs to one of the four groups of flatworms alive today.
This planarian (Dugesia) belongs to one of the four groups of flatworms alive today. Photo © Ed Reschke / Getty Images.

Flatworms (Platyhelminthes) are a group of invertebrate animals that includes planarians, tapeworms, and flukes. There are about 20,000 species of flatworms alive today. Members of this group are soft-bodied animals that have no body cavity, no circulatory system, and no respiratory system. Oxygen and nutrients must pass through their body wall by means of diffusion. This limits their body structure and is why these organisms are flat.

Flatworms range in length from less than one millimeter to several meters long. The group includes species that are free-living such as planarians but also internal and external parasites such as flukes and tapeworms.

The digestive system of most flatworms is relatively simple. It consists of a mouth, pharynx, and either a simple or branched intestine. Any food that is not digested and absorbed in the intestine is expelled back out through the pharynx and mouth. The complexity of the nervous system varies from one species of flatworm to the next. Flatworms have a distinct head region and their nervous system is cephalized (which means the nervous tissue is concentrated towards the anterior end of the body). In some flatworms, the nervous system is little more than a net-like nerve plexus but in other flatworms there is, in addition to a nerve plexus, one to five pairs of nerve cords that run the length of the body that are connected by transverse nerves to form a ladder-like nerve structure.

Many flatworms reproduce asexually as well as sexually. Among those that reproduce asexually, some do so using a process called fission in which a constriction forms behind the pharynx and the animal separates into two individuals that then regenerated the missing parts of their body.

There are a variety of flatworms that are known parasites to humans.

Among these are blood flukes Chinese liver flukes, lung flukes, and sheep liver flukes. Modes of infection with these flatworms varies but include eating raw fish, shellfish, and aquatic vegetation.

Key Characteristics

The main characteristics of flatworms include:

  • inhabit marine, freshwater, and moist land habitats
  • includes free-living as well as internal and external parasitic species
  • many members of the group exhibit complex parasitic lifestyles
  • bilateral symmetry
  • polarity (distinct head and tail)
  • body flattened dorsoventrally
  • lacks respiratory, circulatory, and skeletal systems

Classification

Flatworms are classified within the following taxonomic hierarchy:

Animals > Invertebrates > Flatworms

Flatworms are divided into the following taxonomic groups:

  • Planarians and relatives (Turbellaria) - There are about 4,500 species of planarians and their relatives alive today. Members of this group are mainly free-living, nonparasitic flatworms. Planarians and their relatives have a soft, flat body that is covered with ciliated skin.
  • Digenic flukes and relatives (Trematoda) - There are about 18,000 species of digenic flukes and their relatives alive today. Members of this group have complex parasitic live cycles. Their hosts are mollusks and vertebrates.
  • Monogenic flukes (Monogenea) - There are several thousand species of monogenic flukes and their relatives alive today. Members of this group have a leaf-like or cylindrical body with suckers and hooks that enable them to attach themselves to their hosts. All members of this group are parasitic to fish (either attaching themselves to the skin or gills of fish).
  • Tapeworms and relatives (Cestoda) - Tapeworms have a tape-like body with suckers and hooks that enable them to attach themselves to their hosts. Tapeworms lack a gut or digestive organs. They are parasites that reside in the digestive tracts of their host. Members of this group have especially long bodies compared to members of other flatworm groups.
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Klappenbach, Laura. "Flatworms." ThoughtCo, Jun. 5, 2015, thoughtco.com/flatworms-profile-130274. Klappenbach, Laura. (2015, June 5). Flatworms. Retrieved from https://www.thoughtco.com/flatworms-profile-130274 Klappenbach, Laura. "Flatworms." ThoughtCo. https://www.thoughtco.com/flatworms-profile-130274 (accessed November 23, 2017).