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


Related topics

Bivalves, cephalopods, gastropods, nematodes

What are flatworms?

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

Characteristics of flatworms

The main characteristics of flatworms are the following:

Types of flatworms

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

Digestive system of flatworms

The digestive system of flatworms consists of the mouth, which is located in the midline of the belly and can sometimes also change position. They have a pharynx in which can be found a series of glandular cells that have the function of making mucus to help 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 one, the bulbous one and the folded one.

Circulatory system

Flatworms are beings that have no heart, veins, arteries, capillaries, hemolymph or any type of circulatory fluid. The circulation they possess occurs between cells or intercellularly and is responsible for transporting substances by diffusion from cell to cell.

Gases enter the body surface and then travel from one cell to another by diffusion. Because they are parasitic animals that live at the expense of the host, they do not really need a digestive or circulatory system.

Excretory system

The excretory system is a protonefridial system, consisting of a pair of branched longitudinal ducts or lateral channels. These side channels are equipped with a special cell known as flamingos.

Life cycle

The 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 the one of feeding in which the nutrients and the substances that help to satisfy the energy needs are incorporated depending on the relation that they have with the environment and also with other individuals, already be of its same or another species and finally it includes also the reproduction that consists in engendering new beings continuing with the species.


Depending on their characteristics, flatworms can be found in either salty or freshwater bodies of water. They can also be found in terrestrial habitats, as long as these are quite humid, some of them may 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 may remain within some types of hosts, such as freshwater specimens. Another habitat of flatworms is the human body, for example, the tapeworm Saginata, are housed in the first areas of the small intestine of humans and can reach lengths of 2 to 15 meters.


Some curiosities of flatworms are the following:

Examples of flatworms

Written by Gabriela Briceño V.

How to cite this article?

Briceño V., Gabriela. (2019). Flatworms. Recovered on 23 February, 2024, de Euston96:

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