Flatworms: characteristics and classification

The edge of flatworms is formed by flatworms such as tapeworms. Its main characteristics are:

  • metazoans (i.e. multicellular animals),
  • triblastics (ectoderm, endoderm and mesoderm are distinguished in their development)
  • acellomed (they do not have a coelom, that is, they do not have an internal cavity, they are solid because they have a mesenchyme where the different organs are embedded).

The c nowledge of flatworms is important for man, both health and economics because, as we mentioned, are parasites of both humans and other animals.

Classification of flatworms

Flatworms have traditionally been classified into three classes: Turbellaria, Trematoda, Monogenea, and Cestoda.


All flatworms with the exception of peatlands (where planarians are included ) are parasites. All flukes and cestodes are endoparasites and monogene also have ectoparasitic species. Monogeneans mainly parasitize fish and amphibians, flukes and tapeworms of all kinds of animals, including humans.

The Turbellaria class is the only harmless one, they are free, marine or freshwater worms, although they can live on land: in tropical forests with humidity up to saturation. As we will see, they breathe by diffusion through the integument, and therefore, require habitats where there is water or high levels of humidity. The class Turbellaria refers to a paraphyletic group (that is, they are not all organisms from the same common ancestor).

As can be seen, the classification and phylogeny of flatworms is not easy at all. Ehlers (1986) defines the following characteristics for the flatworm taxon:

  • a) absence of mitosis in somatic cells, for example: somatic cells differentiate from blastomeres or totipotent cells in the post-embryonic state;
  • and b) multiciliated cells, where the cilia lack an accessory centriole , the monophyly of the group has not been able to be established with certainty up to now.

But to date no apomorphy has been found to define the Turbellaria; that is, no unique and common characteristics have been found to a common ancestor that allows them to be grouped under a common class.

It has been shown that characters such as free-living and a body covered by multiciliated epidermis do not represent characteristics for the taxon. The term Turbellaria is currently used in a descriptive way.

In systematic biology and, in particular, in cladistics, an  apomorphy  or  apomorphy is an evolutionarily novel biological trait or character, an evolutionary novelty derived from another trait belonging to a phylogenetically close ancestral taxon. Source: Wikipedia


At present, it is more correct to speak of two subphiles: Catenulida and Rhabditophora. The subphylum Rhabidtophora groups many peatlands but also the classes Monogenea, Cestoda and Trematoda under the superclass Neodermata. Catenulids are free-living flatworms in freshwater.

General characteristics

Flatworms are the simplest animals that have bilateral symmetry, that is, they have a defined polarity between the front and rear ends. In other words, their symmetry is like a reflected mirror if we split them in half, in a sagittal plane.

They have a flattened body dorsoventrally (hence they are called flatworms), and they have both oral and genital openings that are usually found in the ventral part (the part below the animal).


The flatworm-like epidermis can be cellular (that is, it has individual cells) or syncytial (that is, it has syncytia that are cells with several nuclei derived from the fusion of several cells). In some cases it may be a ciliated syncytium.

In the case of peat bogs, rhabdites may appear in the epidermis, which are a subcellular epidermal structure that forms in the rod-shaped parenchyma that serves to produce mucus to prevent desiccation or also with defensive functions.

In the case of monogenates, cestodes and trematodes, the epidermis is a syncytial integument.

Flatworms vary in size from one millimeter or less to many meters in length, as is the case with some species of tapeworms.

Despite the fact that, as we said, they are the simplest animals, it can be said that they have reached the level of organization of organs and systems since a greater specialization can be found and the presence of mesoderm allows the formation of more organs.

Flatworms lack circulatory and respiratory systems , although they do have a simple excretory system: protonephridia, a muscular, nervous, and digestive system.

Flatworms respiration is carried out by the passage of oxygen through the integument or epidermis by means of diffusion since they do not have a respiratory system as we have mentioned.

Flatworms nervous system

The nervous system consists of a pair of anterior ganglia with longitudinal nerve cords connected by transverse nerves and located in the mesenchyme.

The anterior pair of ganglia supposes the cephalization of the animal, so there is a centralization of the nervous system that extends posteriorly through the longitudinal nerve cords and the transverse nerves.

In some cases it is also similar to that of cnidarians .

The sensory organs are simple although they may have eyes in some cases.

Muscular system

The muscular system is usually sheath-shaped, with layers of circular, longitudinal, and sometimes oblique fibers below the epidermis. Its origin is mesodermal.

Digestive system

The digestive system consists of the mouth, pharynx, and intestine, with the anus usually absent. There are some species that can have one anus and others several. Depending on the class of flatworms, the digestive system may be more or less developed.

When it is present and developed it has ramifications since lacking a circulatory system is the way to get everywhere. In the case of parasitic species, they may not have a digestive tract.

Excretory system in flatworms

As previously explained, they have a rudimentary excretory system formed by two channels that lead to the protonephridia, which are groups of flamboyant cells.

Its main function is osmoregulation, although there may be excretion of nitrogenous products through the protonephridiums, the usual thing is that they are excreted through the integument.

The flame cells are large cells ciliated cells connecting a body with the outside through a small conduit. They have a part where the nucleus is and in their shape they have a cup-shaped projection. The area of ​​the mouth of the cup is ciliated, and when the cilia move they resemble a flame, hence the name. Attached to the cup is a tube-shaped cell that is also lined with cilia that help move the fluid through the tube, which ends in a nephropor.

Thus, the extracellular fluid enters the flaming cell which, thanks to the beating of the cilia, creates a pressure gradient from which urine is produced and, in turn, is pushed towards the tubular cell, where the cilia push the residue towards the nephropor which is at the end of the tube. In this way, they maintain the concentration of internal fluids.

Reproduction and life cycle of flatworms

Reproduction of flatworms can be asexual and sexual. Surprisingly, they have a highly developed reproductive system with gonads, ducts, and accessory organs; highly variable depending on the groups.

Most are hermaphrodites and internal fertilization always occurs, for this reason, they have copulatory organs. Turbellarians and trematodes always have cross-fertilization, and in the case of cestodes it is the preferred one, but if it is not possible they can self-fertilize.

Their life cycle ranges from the simplest for free-living flatworms to the most complex for parasitic flatworms that have multiple hosts. An example is the Dicrocoelium dendriticum trematode, which is a common parasite of ruminants but can occasionally pass to humans. This species has two previous hosts (snails and hormingas) before reaching the last one, which would be ruminants.

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