Planarians and other flatworms: the class Turbellaria

The flatworms are fascinating organisms, in recent years studies have been done on them have shown an immortal flatworm that can be multiplied countless times and that some species are a potential hazard as invasive species .

What type of organism is the planarian?

Planarians are organisms that are classified within the phyllum of flatworms , which are flatworms and traditionally within the class Turbellaria. So we could say that a planarian is a flatworm or flatworm. Like the rest of flatworms, they do not have a circulatory system and breathe by diffusion through the epidermis.

If we delve into the classification, traditionally, free-living flatworms have been classified as maelstroms whether they were related or not, regardless of phylogeny. This classification is still traditionally used to encompass free-living flatworms, but today the classification is different.

Flatworms are divided into two classes Catenulida and Rabditophora. Free-living planarians can be found in both classes, while parasitic flatworms (Cestoda, Monogenea and Trematoda) are also found in the second. For this article and to facilitate we will treat them as mobsters.

Where do planarians live?

Planarians live in aquatic environments or with very high humidity . The peat bogs are usually benthic, that is, they live on the bottom of either seas and oceans or in freshwater ecosystems such as lakes, ponds, streams, springs … They live on the bottom or are part of the interstitial fauna. There are some cases of pelagic peat bogs but it is not very frequent.

When they are terrestrial, they are in high humidity environments living under the leaf litter or on the ground and only going out at night to hunt when the sun has gone down.

Morphology of peatlands

As we said, the planarians are free-living and there are more marine than sweet. Their size is highly variable and they can measure from 5 mm to 50 cm.

Its appearance is foliaceous, that is, flattened and widened, in the form of an oval or lanceolate leaf, although some, such as the hammerhead planarian, have particular structures. Marine shapes have bright colors.

Planarians and other flatworms
Planarians and other flatworms

Planarians have few external characteristics: in the ventral part they have the mouth and in the dorsal part they have eyes that are simple ocelli that only receive light and do not process images. They have a ciliated body totally or ventrally.

Internal characteristics of peatlands

The epidermis is made up of cubic hair cells that have characteristics that allow adherence to the substrate: glands that secrete a viscous substance. In addition, they have a liberating gland and depending on the state of excitement they use one or the other.

Its movement is by cilia and it is an animal movement.

In the epidermis they also have a defensive gland called the rhabdoid cell . They have sulfurous and phosphate proteins called rhabdoids. When a planarian needs to defend itself, it projects them outwards (black color) and when they come into contact with water they swell, become gelatinous and surround the animal’s body.

They have, also in the epidermis, sensitive structures, nerve endings. Most groups have a basement membrane that limits the epidermis below. Beneath it are arrangements of mucular layers (smooth circular, intermediate oblique, longitudinal).

As it is organized in layers, the contraction is antagonistic, when the circular ones contract, the longitudinal ones lengthen and vice versa. It moves thanks to the muscles and the cilia.

Some have suckers, especially parasites, but they are few.

Movement of the planarians

Locomotion in planarians, in general, can occur in several ways.

On the one hand, by the movement of the cilia of the epidermis on the surfaces that usually occurs in the smallest bogs. On the other hand, due to muscular movements such as undulations, peristaltic movements, extension and retraction … It can also occur by sliding on a mucous surface that they themselves secrete.

Digestive system of peatlands

The peat bogs have a digestive tract with a mouth, pharynx and intestine but not anus (like the rest of flatworms). The mouth is in the ventral part. The animal moves forward, stands on top of the food and absorbs it.

The mouth gives rise to the pharynx, followed by the intestine of ectodermal origin.

The pharynx can be very simple, tube-like as is the case with the Acoella order. It can also be shaped like a bulb and can evaginate it to take it out as a trunk and suck the food. It can also be fully folded and project out.

The intestine can be simple as in the Rabdhocoella orders, and Allocoella with only the mouth and a blind tube or also not have as in the case of the Acoella Order.

In other cases the intestine may have ramifications since they need to absorb a large amount of water with food. They can also be more complex and have three blinds as in the case of the Tricladida order.

In the order Polycladida they have many branches. The larger the surface, the greater the absorption, since there is more time to absorb it. Excretion is through the mouth.

What do planarians feed on?

Planarians can be carnivorous or ghoulish, although most are carnivorous. They feed on small crustaceans, worms, insects, snails …

Planarians have a muscular pharynx that can be retracted and everted for ingestion and evacuation of food.

Excretory system of the peat bogs

The excretory system is a protonephridium. As there is no circulatory system, there is no transport of metabolites, thus a system that collects all the waste products in each organ is necessary.

They are numerous units made up of two cells, one flamboyant and the other tubular. They branch and end in the excretory pore, which are usually few. For more on flaming cells you can see it here .

The beating of the cilia (reminiscent of a flickering flame) provides negative pressure to draw fluid through the delicate interconnections between the flaming cell and the tubular cell.

Nervous system of peatlands

Planarians and other flatworms
Planarians and other flatworms

As in the rest of the flatworms, the organization of the nervous system is variable. However, it always consists of a nerve plexus below the muscle layer and in the simplest structure this nerve plexus is accompanied by an accumulation of neurons in the head.

In the more complex peatlands, the nervous system may be bilateral with a well-developed cerebroid ganglion and longitudinal nerve cords joined at regular intervals by lateral commissures. forming a ladder nervous system.

Sensory organs of the planarians

The sense organs of the planarians are focused on movement, the capture of light and food and balance.

Mechanoreceptors or mechanoreceptors

They are the receptors of touch. They are usually sensory silks as extensions of the epidermis that serve to orient themselves towards the substrate in the case of benthic bogs. They can appear ventrally, dorsally and around the epidermis.


They are the organs of taste and are used for food. They are usually concentrated in the front of the body, especially on the sides of the head.


They are the organs of balance. They do not appear in all peatlands only in those that are not benthic and therefore cannot use the sense of touch to orient themselves. When it comes to swimming, they need to orient themselves with respect to gravity and do it to the statocysts that are usually in the area of ​​the cerebroid ganglia. They are also present in cnidarians.

Photoreceptor organs

They are the ocelli and are usually in the anterior part. They are like a camera open in two parts, the light comes indirectly so they only capture the amount of light. To prevent it from being lost there is a pigmented layer, an inverted ocellus, the light reaches the reverse of the position they have. They are important to them as they are free-living animals.

Reproduction of planarians

Planarians can reproduce sexually and asexually.

Asexual reproduction of planarians

Thanks to asexual reproduction they are able to regenerate a complete individual when they are cut in half transversely towards the middle of the body. On the other hand, if a small end of the posterior part is cut off, the front large part can regenerate the cut part but the small part is unable to regenerate a complete individual.

In a natural way, peat bogs reproduce asexually or sexually depending on the photoperiod (length of day) and temperature . For example, many bogies reproduce asexually during the summer and sexually during the fall.

The way to reproduce naturally is through buds, by transverse fission at the height of the pharynx, each of the parts regenerating a complete individual. A few species of terrestrial planarians are capable of reproducing by architomy where an individual is fragmented into several pieces that give rise to cysts from which new individuals emerge.

Sexual reproduction

The peat bogs are usually monoecious , have male and female organs in the same individual and, in addition, they are protandric animals: the male gonad matures before the female. The fertilization is internal and crossed transmitting the spermatozoa, reason why there is copulation.

The gonads are highly variable . Regarding the male genitalia, depending on the species, they can have from one to multiple testicles. Each of them ends in a spermiduct. In addition, they also have a seminal vesicle and penis which can also be variable.

The female genitalia are made up of paired ovaries, which lead through oviducts to the gonopore, which is the equivalent of the vagina. In addition, they also have a copulatory bag and a seminal receptacle where sperm are stored.

When two individuals copulate, the sperm go to the seminal receptacle, where they wait for the ovum to arrive. When it arrives, the sperm fertilize the ovum and then one or more fertilized eggs and some yolk cells are enclosed in a small cocoon that are attached by small peduncles to the inside of stones or plants. There are also viviparous species.

In some cases of the Acoella, Rhabdocoella and Polycladida, the fertilization is traumatic, the copulation is not carried out through the genital orifice but through any part of the body, the male penetrates the female and the sperm advance through the tissues until the atrium.

The egg can lead to a ciliated larva that by metamorphosis gives an individual equal to the adult, and can give an individual equal to the adult but smaller.

Archphores and Neophores

According to their reproductive system, peatlands are classified as archphores or neophores.

The arcophore bogs are the most primitive. They have an endocytic egg with spiral segmentation. The larvae are sometimes free-living, although generally they tend to have direct development.

Neophoric peatlands have a more complex system. They have ectolecyte eggs (because their ovaries are more complex), without spiral segmentation, and development is always direct.

Curiosities and examples of planarians

The hammerhead planarian

The hammerhead planarian, especially Bipallium kewense, is an invasive species almost everywhere in the world. Originally from Indochina, this flatworm can measure up to 50 cm and attacks native snail populations. It is problematic in France, the United States, and also Spain.

What happens when a planarian is cut vertically?

As explained in asexual reproduction, planarians are capable of regenerating new individuals when cross sections occur unless it is a very small fragment of individual.

Despite what it may seem, planarians are also capable of regenerating a complete individual from a longitudinal section (that is, vertically, along the individual).

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