The flatworms , also called flatworms , are a group of invertebrates that have their body soft , and are generally very crushed . Several species of flatworms can live outdoors , but about 80 percent of all flatworms are parasites , that is, they live on or in another organism and feed on it. Are symmetrical bilaterally so the right and left sides are similar and have systems respiratory, skeletal and circulatory specialist. The body is not segmentedand the spongy connective tissue constitutes the so-called parenchyma and fills the space between the organs.

What are flatworms?

They are organisms that do not have vertebrae . Most of them are parasites and do not have a respiratory or circulatory system and many of them do not have a digestive system or anus either.

  • Characteristics of flatworms
  • Types of flatworms
  • Digestive system of flatworms
  • Circulatory system
  • Excretory system
  • Lifecycle
  • Habitat
  • Curiosities
  • Examples of flatworms

Characteristics of flatworms

The main characteristics of flatworms are as follows:

  • The soft body of a flatworm is ribbon- shaped , flattened back, and bilaterally symmetrical .
  • They are the simplest triploblastic group animals that have organs.
  • Organ systems are formed from three germ layers: an external ectoderm and an internal endoderm , with a mesoderm between them.
  • Flatworms do not have a true circulatory or respiratory system , but like all other animals, flatworms consume oxygen.
  • They breathe in their integument ; and the gases diffuse directly through its wet outer surface, which is known as integumentary exchange.
  • They are carnivores and dig for food.
  • They can be found mainly in rainy , marine and terrestrial environments that are humid.

Types of flatworms

We can mention four different types of flatworms, which are:

  • Turbellarians : they are also known as planarians and have an autonomous life. They move by means of cilia and live in the sea, although some have adapted to fresh water and land.
  • Monogeneous : they are mainly parasitic species of amphibians and fish . They cannot act as parasites in man. They are hermaphrodites with cross reproduction.
  • Trematodes : they are found in animals and some species can affect man . They can measure several centimeters and have suction cups to fixate on the host. They are hermaphroditic and reproduce sexually by expelling all the eggs with the feces. Then the eggs hatch and first they transform into crabs or mollusks , then they adhere to aquatic plants and there they are eaten by some animals infecting them.
  • Cestodes : they are the most recognized and are parasites found in man . They have a head full of hooks that they use to adhere to the host’s intestine . They reproduce by self- fertilization, that is, they have sexual and male organs.

Digestive system of flatworms

The digestive system of flatworms is formed by the mouth , which is located in the mid-ventral line and on some occasions it can also change position. They have a pharynx in which a series of glandular cells can be found whose function is to make mucus to aid swallowing. They are also responsible for producing enzymes that begin extracellular digestion by evagination of the pharynx.

We find several types of pharynx in them, the simple, the bulbous and the folded.

Circulatory system

Flatworms are beings that do not have a heart, veins, arteries, capillaries, hemolymph, or some type of circulatory fluid. Their circulation occurs between cells or intercellularly and they are responsible for transporting substances by diffusion from cell to cell.

Gases enter the body’s surface and then travel from one cell to another through diffusion. Because they are parasitic animals that live off the host, they do not really need the digestive or circulatory system.

Excretory system

The excretory system of flatworms is a protonephridial system , consisting of a pair of branched longitudinal ducts or lateral canals . These lateral channels are provided with a special cell known as flamboyant.


Flatworms are in charge of carrying out a set of activities known as the Life Cycle In this cycle, one of the most important actions is feeding in which nutrients and substances that help to satisfy energy needs are incorporated depending on the relationship they have with the environment and also with other individuals, either of the same or another species and lastly it also includes reproduction, which consists in generating new beings continuing with the species .


Depending on their characteristics, flatworms can be found in either salty or fresh water bodies . They can also be located in terrestrial habitats , as long as they are quite humid , some of them can even inhabit the sand of the watery bottoms . A good part of them are benthic, coexisting at the bottom of the waters on rocks or mixed with algae.

In their parasitic form they can remain within some types of hosts , such as freshwater specimens. Another of the habitats of flatworms is the human body , for example, the Tapeworm Saginata , they live in the first areas of the small intestine of humans and can reach 2 to 15 meters in length.


Some curiosities of flatworms are the following:

  • Flatworms or flatworms do not have feet so they must move through vibrations of the ciliated epithelium.
  • It is estimated that approximately 60 million people have been invaded by flatworms, especially tapeworms .
  • Although medical treatment is given, the best way to eliminate them is through surgery .
  • Some of them can regenerate parts of their body when one of them is missing.
  • They are very sensitive to water pollution .

Examples of flatworms

  • Tapeworm: intestinal parasite of vertebrates
  • Notoplana acticola
  • Planaria
  • Staves
  • Cercaria
  • Taenia solium

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