FLATWORMS: CHARACTERISTICS, REPRODUCTION, FEEDING, SPECIES - BIOLOGY - 2024

The flatworms constitute a phylum of invertebrate animals is composed of approximately 20,000 species. Due to their morphology, they are also known as "flatworms."

This group was described for the first time by the North American naturalist Charles Sedgwick Minot in 1876. It is made up of two subphiles -Turbellaria and Neodermata-, which are integrated into five classes: Catenulida, Rhabditophora, Cestoda, Trematoda and Monogenea.

Platelminto specimen. Source: LiCheng Shih / CC BY (https://creativecommons.org/licenses/by/2.0)

Many of the best known flatworms are causal agents of certain diseases that affect humans, such as Schistosoma mansoni, Fasciola hepatica and those of the genus Taenia.

Many of these diseases can cause a progressive and chronic deterioration of human health conditions. For this reason, it is important to study and characterize each of the species that are part of this phylum, in order to be able to face these pathologies.

General characteristics

Flatworms are considered multicellular eukaryotic organisms. This implies that in their cells they have a cellular nucleus, in which the DNA is contained, structuring the chromosomes. Likewise, they are made up of several types of cells, each one specialized in a specific function.

These types of animals present bilateral symmetry, that is, they are made up of two exactly equal halves, which are joined in the longitudinal plane.

They are triblastic, since during their embryonic development the three germ layers appear: ectoderm, mesoderm and endoderm. From them the different organs of the animal develop.

They are hermaphrodites because they have both male and female reproductive organs. They reproduce both sexually and asexually. Fertilization is internal and can have direct or indirect development.

Most flatworms are parasitic, that is, they need to live within the organism of a host, while a few are free-living.

Taxonomy

The taxonomic classification of flatworms is as follows:

• Domain: Eukarya
• Animalia Kingdom
• Subkingdom: Eumetazoa
• Super edge: Spiralia
• Phylum: Platyhelminthes

Morphology

Flatworms have a flattened body in a dorsoventral direction. Its length can be variable, depending on the species. For example, peatlands are approximately 5 cm in length, while members of the cestode class can exceed 10 meters.

Likewise, most have undivided bodies, while cestodes have their bodies divided into fragments known as proglottids. Their body is solid and they are cellophane, that is, they do not have a general cavity.

Those that live a life of parasites have structures such as suction cups, fixing hooks and grapples that allow them to adhere effectively to their host.

Classification

The phylum Platyhelminthes encompasses two subphiles: Turbellaria and Neodermata.

Subphylum Turbellaria

Turbellarian in its natural habitat. Source: MDC Seamarc Maldives / CC BY-SA (https://creativecommons.org/licenses/by-sa/4.0)

This subphylum is made up of the well-known planarians. They are animals of short length (up to 6 cm) and are characterized by having a free life. They mainly inhabit places of high humidity, such as fresh and brackish water ecosystems, as well as humid terrestrial environments.

The cells of the planarians still retain totipotency, a property that allows them to differentiate into any type of cell. This is important because it gives the animal the opportunity to regenerate an adult individual from any fragment of its body.

Subphylum Neodermata

This is a group of flatworms that are characterized mainly by being parasites of other animals. This means that during their life cycle, they must necessarily be inside another organism to take advantage of it and thus be able to develop.

Its type of reproduction is mainly sexual, with direct and indirect development. They also have structures known as suckers, which allow it to attach itself to its host, and in this way, feed on it.

The Neodermata subphylum includes three classes: Cestoda, Trematoda, and Monogenea.

Cestoda class

Graphic representation of Taenia saginata. Source: Servier Medical Art / CC BY (https://creativecommons.org/licenses/by/2.0)

It is a class that encompasses approximately 3,500 species. Most of them are of great length, even exceeding 15 meters. They are obligate endoparasites, focusing exclusively on the digestive tract of mammals, including humans.

Their life cycles are quite complex, including intermediate hosts and a definitive host. They present indirect development, which means that they have some intermediate larval stage until the adult individual develops.

Likewise, they have a body region called "scolex", which corresponds to the head and in which they have, apart from the suction cups, hooks that help them to attach to the host more efficiently. The well-known tapeworms belong to this class.

Class Trematoda

It is the one that includes the largest number of species, with approximately 9000. They are also known as "staves". They are short, reaching just a few centimeters. They feature specialized structures such as suction cups and fixation discs, which allow it to adhere to its host.

During their biological cycle they have several larval stages, which develop in different hosts. In most cases the intermediate hosts are members of the gastropod class (snails). Sometimes its definitive host is man.

Many of the species in this class are of health importance because they are the causal agents of some diseases in humans. Among these we can mention the trematodes of the genus Schistosoma, which cause schistosomiasis (formerly known as bilharziasis) or Fasciola hepatica, responsible for fasciolosis.

This class is divided into two subclasses: Digenea and Aspidogastrea.

Monogene class

It is the least diverse class, with only 1000 species. They are ectoparasitic organisms of vertebrates such as fish, reptiles and amphibians. Its size is very small, and can barely reach 2 cm in length. Its body is flattened, like that of all flatworms, and it is fixed to its host by means of a fixation organ that is located at its posterior end.

It differs from the other flatworms because in its biological cycle it only requires one host. They reproduce mainly by cross-fertilization, even when they are hermaphrodites, and their development is direct.

Despite not being causative agents of any disease in humans, flatworms of this class can be responsible for great economic losses when parasitizing other animals of commercial interest, such as certain fish.

Digestive system

The digestive system of flatworms is very rudimentary and there are even some, such as cestodes, that lack it.

It has a single hole, which is the mouth, which is used both to ingest food and to release waste. Immediately after the mouth, there is the pharynx, which communicates with the intestine. This is blind and can sometimes present several sacs or blind.

Circulatory system

They lack a structured circulatory system as such. Because of this they do not have specialized structures like a heart or blood vessels.

However, the circulation of certain substances is established between its cells. This is done thanks to the diffusion process. Substances pass from one cell to another through this process.

This does not apply to all flatworms, since in some species of bog and digenae there is a certain organization and some very small conducting vessels known as the endolymphatic system, which forms a kind of plexus in the parenchyma.

Respiratory system

Flatworms also do not have a respiratory system, due to the simplicity of their anatomy. However, they must carry out the gaseous exchange with the environment, at least those species that are free-living.

In this sense, the type of respiration that flatworms have is cutaneous. This means that the gases diffuse through the animal's skin.

However, those that are endoparasites of vertebrates have an anaerobic mechanism, since they develop in an environment in which oxygen is practically absent.

Reproduction

Two types of reproduction can be observed in flatworms: asexual and sexual.

Asexual reproduction

This type of reproduction is characterized by the fact that there is no fusion of sexual gametes. Descendants originate directly from one parent.

Asexual reproduction occurs through two processes: fragmentation and parthenogenesis.

In the case of fragmentation, from fragments of an animal, an adult individual can be generated. This type of reproduction is especially characteristic of peatlands (planarians).

On the other hand, parthenogenesis consists of an adult individual developing from unfertilized ovules of virgin females.

Sexual reproduction

Flatworms are hermaphroditic organisms. Despite this, there is no self-fertilization. To reproduce, the intervention of two individuals is necessary, one acts as a female and the other as a male.

In the individual who has the role of female, the ovules mature and are transported and deposited in a place known as an ootype. Later they reach the uterus, where they join the sperm, which were previously deposited there by the male animal. In this way, fertilization occurs, which of course is internal.

Regarding the type of development, among flatworms, both direct and indirect development can be observed. The peat bogs and monogeneans have direct development, while the trematodes and cestodes have larval stages, so their development is indirect.

Examples of species

Taenia saginata

It is a flatworm that belongs to the class Cestoda. It is of great length, sometimes even exceeding 12 meters. They present the scolex in the cephalic region, where four suction cups can be seen, through which it is attached to the intestine of its host.

It is also known as the famous "tapeworm." It fixes in the first portions of the small intestine and there it feeds on the nutrients that the host ingests.

It is worth noting that in their biological cycle the intermediate host is a mammal, generally cattle, and they pass to humans through food.

Taenia solium

Like the Taenia saginata, Taenia solium is a member of the Cestoda class. It does not reach the same length, since it can measure up to approximately 5 meters. Its adult form is responsible for taeniasis, while its larval form can cause a pathology known as cysticercosis.

It presents a scolex in which, apart from the four characteristic suction cups, it has a rostellum that has two hook crowns. These structures facilitate attachment to the host's intestine.

This parasite passes to humans through the ingestion of cysticerci, its larval form.

Fasciola hepatica

Fasciola hepatica. Source: Adam Cuerden / Public domain

It is known as "stave" and belongs to the Trematoda class. It has been identified as the causal agent of a parasitic disease called fasciolosis that is widespread throughout the world, but is more frequent in places where hygienic conditions are precarious.

It is a flat worm, measuring approximately 3-3.5 cm in length and brown in color. In its biological cycle it presents several larval stages. Their hosts are generally mammals such as goats, sheep, horses, and even rodents.

Humans can become infected by ingesting one of its larval forms, the metacercariae. Inside the body it is housed in the bile ducts. From there they cause symptoms that are mainly reflected in the liver

Schistosoma mansoni

It is a flatworm that belongs to the class Trematoda. It consists of an endoparasite, responsible for a disease known as schistosomiasis.

As with all flatworms, its body is flattened. They are dioecious, that is, the sexes are separated. This constitutes one of its distinctive elements. They also have a certain sexual dimorphism, at least in terms of size, since the female is longer than the male.

In their biological cycle they have an intermediate host, a snail and their definitive host is the human being. It is a very widespread parasite throughout the American continent, especially in rural areas, where hygienic conditions are not optimal.

Pseudorhabdosynochus morrhua

This is a flatworm belonging to the monogene class. It is very small, since it measures only 0.48 mm in length. It is an endoparasite of a fish, the Epinephelus morrhua, a grouper.

The distribution of this parasite is restricted, since it has only been found in an archipelago of islands known as New Caledonia in the Pacific Ocean.

Schistosoma japonicum

This is an endoparasite that falls into the class Trematoda. It bears many similarities to Schistosoma mansoni. It is found in the Asian continent, specifically in China, Sri Lanka, and the Philippines.

Its intermediate host is also a snail, mainly of the genus Oncomelania. Its definitive host is a vertebrate, such as humans. In the organism of this, the parasite fixes in the mesenteric blood vessels (veins), where they reproduce.

This is the most infectious species of the genus Schistosoma and causes a disease called schistosomiasis japonica.

References

1. Almón, B., Pérez, J. and Noreña, C. (2018). Phylum Platyhelminthes. Chapter in book: Inventory of the marine biodiversity of Galicia.
2. Brusca, RC & Brusca, GJ, (2005). Invertebrates, 2nd edition. McGraw-Hill-Interamericana, Madrid
3. Curtis, H., Barnes,, Schneck, A. and Massarini, A. (2008). Biology. Editorial Médica Panamericana. 7th edition.
4. Hickman, CP, Roberts, LS, Larson, A., Ober, WC, & Garrison, C. (2001). Integrated principles of zoology (Vol. 15). McGraw-Hill.
5. Margulis, L. and Schwartz, K. (1998). Five Kingdoms: an illustrated guide to the Phyla of life on earth. 3rd edition. Freeman
6. Negrete,. and Damborenea, C. (2017). Phylum Platihelminthes. Book chapter: Macroparasites: Diversity and Biology. Chair Books.