por What are planarians?
The planarians or turbellarians are a group of animals that belong to the edge of flatworms. They are flat worms that can measure up to approximately 5 cm. This subphylum was first described in 1831 by the German zoologist Christian Ehrenberg.
Planarians are a group of animals that require abundant moisture conditions. This is why they live, either in bodies of water or in terrestrial environments where there is enough of this element. It covers a large number of species, approximately 3,000, and many of them are characterized by their color patterns.
Characteristics of planarians
Planarians are multicellular eukaryotic organisms, which means that they have a structure called the cell nucleus, within which the DNA is found, forming the chromosomes. Likewise, they are made up of various types of cells, each one specialized in a specific function.
These animals are triblastic because during their embryonic development they present the three germ layers: ectoderm, endoderm and mesoderm. From these layers the different organs and structures that will make up the adult organism are formed.
They are also acoelomates, because they lack the internal cavity known as a coelom. They have bilateral symmetry, since they are made up of two exactly equal halves, separated by an imaginary line in the longitudinal axis.
They are hermaphrodites, that is, they have both male and female reproductive organs. Its reproduction is asexual and sexual. Regarding the latter, fertilization is internal and development in most species is direct. Only a few have an indirect development with larval stages.
This is a group of animals found in both aquatic ecosystems and terrestrial ecosystems. Some are adapted to live in freshwater environments and others, most, in brackish water environments. In the following video you can see a planaria swimming:
taxonomy
The taxonomic classification of planarians is as follows:
Domain: Eukarya
Animalia Kingdom
Phylum: Platyhelminthes
Subphylum: Turbellaria
Morphology
external anatomy
Planarians do not have the typical shape of a worm, since their bodies are flattened dorsoventrally. Its size is varied; there are species that measure as little as 1 cm, to others that can exceed 5 cm.
Some species show obvious cephalization. In some, the differentiated cephalic region of the body can be seen, since it has a characteristic triangular shape. In this region you can distinguish small extensions called auricles.
Also in the cephalic region there are small spots known as ocelli and that function as organs of vision.
In the ventral region of the turbellarians several holes can be seen: the first corresponds to the mouth, through which the pharynx can exit; the rest of the orifices, in a variable number (between 1 and 3), correspond to genital orifices.
internal anatomy
body wall
The body wall of planarians is made up of several layers:
Epithelium: it is the outermost layer and contains a wide variety of cells -glandular, epithelial, sensory and cells with rabdites-.
Basement membrane: It is located immediately below the epithelium.
Muscle Layers: Beneath the basement membrane are three muscle layers. The first one is made up of circular musculature, the intermediate one by longitudinal musculature and the last one by diagonal musculature.
Nerve plexus: a nerve network that is located between the muscle layer and the parenchyma.
Parenchyma: it is a type of tissue that is made up of cells, among which there are some spaces that are known as endolymphatic systems or spaces.
Digestive system
It’s pretty simple. It is made up of the mouth, which is on the ventral surface of the animal. Following the mouth is the pharynx, which can have different morphologies (simple, bulbous, folded), depending on the species.
The pharynx opens into the intestine, which is blind and branched. There is no exact number of branches. Turbellarians do not have anal orifice.
Nervous system
These animals have a cerebral ganglion, from which arise two lateral nerve cords. Both are connected by nerve fibers that go from one to the other.
In addition to this, planarians have some sensory organs such as ocelli (visual) and statocysts (balance). They also have cells that function as receptors, allowing them to perceive external stimuli. These are chemoreceptors, tangoreceptors, and reoreceptors.
Excretory system
The excretory system of planarians is made up of a system of structures known as protonephridia. These are blind tubules that open to the outside on the surface of the animal’s body through an opening called a nephrostome.
Respiratory system
They do not have a proper respiratory system, the respiration of planarians is cutaneous. This means that the gas exchange occurs through the skin.
Habitat and distribution
From the point of view of distribution, planarians are animals that are widely distributed throughout all regions of the world.
However, due to their anatomical and physiological characteristics, as well as their requirements, planarians must live in humid places, where there is a wide availability of water.
There are planaria that are purely aquatic, while there are others that can be found in terrestrial habitats.
Regarding those that live in aquatic environments, there are some that have managed to colonize brackish water ecosystems, which is why they are generally found as part of the biodiversity of coral reefs.
On the contrary, there are others that have adapted to living in freshwater environments. Due to this, it is common to find them in bodies of fresh water that have little flow.
Likewise, the planaria found in terrestrial ecosystems are located mainly in places with high humidity and where direct sunlight does not reach them. Among those places we can mention cracks, tree trunks or they can be found on the substrate, covered by remains of dead leaves.
Reproduction
The two types of reproduction that exist are observed in turbellarians: asexual and sexual.
Asexual reproduction
This type of reproduction does not involve the fusion of sexual gametes. Therefore, the descendants that are obtained will be exactly the same as the parent that originated them.
Planarians can reproduce asexually through two processes:
Fragmentation
It is the most frequent type of asexual reproduction among turbellarians. It consists of the development of an adult individual from small fragments of another animal. This can happen if the planaria suffers some trauma that causes it to lose a piece of its body.
Reproduction by fragmentation is possible thanks to the totipotency of the cells that make up planarians.
Parthenogenesis
It is a type of reproduction that consists of the development of an individual from unfertilized ovules of virgin females. Parthenogenesis is generally present when different populations go through periods of stress, such as the absence of individuals of the opposite sex.
sexual reproduction
Sexual reproduction involves the union or fusion of female gametes (ova) and male gametes (sperm).
Fertilization
Fertilization in turberlaria is internal, since it occurs inside the body. Although it is known that these are hermaphrodite animals, self-fertilization does not exist in them. Instead, fertilization can be of two types: crossed and by hypodermic impregnation.
In the case of cross-fertilization, two individuals mate and copulation occurs. Here there is an exchange of sperm between both specimens. The sperm is stored in a structure called the copulatrix sac.
On the other hand, hypodermic impregnation consists of the mutual perforation of the body wall to introduce the sperm. Here is the mating between two planarians:
Development
Once fertilization occurs, the egg or zygote is formed. Depending on the species, two types of eggs are observed:
Ectolecite: yolk (nutritive substances that nourish the embryo) is found in the so-called vitalogenic cells.
Endolecyte: the yolk is found inside the egg.
Embryonic development involves a segmentation process, in which cells divide through mitosis, expanding the number of cells contained in the embryo, so that they can then begin to specialize.
In the turbellarian embryo, the type of cleavage is spiral and, in most cases, development is direct. This means that an individual emerges from the egg with the characteristics of an adult individual. On the contrary, there is a small proportion of species that present larval stages.
Feeding
Turbellarians belong to the group of animals considered carnivorous. This means that they feed on other animals.
The main prey of turbellarians are small invertebrates such as crustaceans, insects, molluscs, and other worms.
Digestion
The way it feeds is as follows: through different mechanisms, depending on the species of turbellarian, it catches its prey and puts it in its mouth. There are some species that wrap the prey in a mucus-like substance, making it impossible to move, and there are others that directly inoculate digestive enzymes.
The mouth continues with a pharynx that is quite resistant and with great capacity, which is why it can assimilate prey of considerable size, compared to the size of the turbellarian.
Immediately after the pharynx is the intestine, which is blind and branched. The number of branches depends on the species. This is where the process of digestion and absorption of nutrients takes place.
However, it is important to note that digestion can be intracellular or extracellular. In the first case, it occurs thanks to the presence of a digestive vacuole, which secretes digestive enzymes (exopeptidases, lipases).
On the other hand, extracellular digestion occurs due to the action of some enzymes secreted in the pharynx, as well as thanks to specialized endopectidases.
This video shows how a planarian catches a snail:
Examples of planarian species
Pseudoceros dimidiatus
This species belongs to the Pseudocerotidae family. It is a planaria adapted to living in freshwater environments, which is why it is found mainly in the Indian Ocean, specifically in the area that goes from the Red Sea to the coasts of Australia.
This planaria is characterized by the bright colors that adorn its body, which allows it to be easily identified in coral reefs. On the anterior margin of their body they present…