Planarian

Planarian is a genus of planarians in the family planariidae. A single species represents it, ‘Planaria torva,’ which is observed in Europe. This is a unique species because it regenerates new individuals from each piece when it is cut into pieces. The planaria scientific name is Planariidae.

The Scientific Classification Planaria Phylum is as Given below – 

• Kingdom – Animalia

• Phylum – Platyhelminthes

• Class – Rhabditophora

• Clade – Adiaphanida

• Order – Tricladida, Lang 1884

• Suborder – Maricola, Cavernicola, and Continenticola

Body Mechanism of Planaria

Planaria receive oxygen and release carbon dioxide by diffusion. There are many flame cells in the excretory system made of many tubes. Flame cells remove the unwanted liquid from the body through ducts and excretory pores, and the waste is released on the dorsal of the planarian. Eyes and chemoreceptors on the anterior of the triclads sense organs. Some auricles protrude from the margins of the head and carry mechanical and chemical sensory receptors. 

Depending on the species, the number of eyes in the triclads varies where species like Dugesia or Microplane have two eyes, and others may have many distributed along the body. The subterranean species are mostly eyeless or blind. Ciliated epidermis containing rhabdites covers the body of the triclads. There is a parenchymatous tissue or mesenchyme that is always there between the epidermis and gastrodermis.

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Anatomy of Planaria

Nervous System

The head of the planarian evolved from cephalization, and there is a ganglion under the eyespots. The cerebral ganglion is a bi-lobed mass of nerve tissue that is called the planarian brain. It is known to exhibit spontaneous electrophysiological oscillations that are similar to the electroencephalographic activity of other animals. Two nerve cords extend along the length of the tail from the ganglion. The nervous system looks like a ladder as there are many transverse nerves connected to the nerve cords, enabling them to respond systematically. The planarian's black, brown, blue, grey, or white body is soft, flat, and wedge-shaped. Two ocelli are there on the blunt, triangular heads, and pigmented areas sensitive to light. There are two earlike projections at the base of the skull, which are sensitive to touch and the presence of certain chemicals. The mouth is covered with cilia (hair-like projections) and is located in the middle of the underside of the body. Circulatory or respiratory systems are absent, and oxygen entry and carbon dioxide exit occur from the body wall. 

The Reproductive System of Planarians

The sexual planarians are hermaphrodites and possess both testicles and ovaries. Both sexual and asexual planarians exist. The reproduction includes the combination of two gametes of the two planarians. There is the transport of secretion from planarian to planarian, and giving and receiving sperm takes place. 

Eggs in the shape of capsules are developed inside the body, and weeks later, the eggs hatch and grow into adults. In asexual reproduction, the tail part of the planarian is shed, and each half grows the lost part by regeneration. This is done by allowing the endoblasts (adult stem cells) to differentiate and divide, resulting in two worms. This whole process of asexual reproduction has remained a subject of research and debate. Some researchers opine that the products derived from bisecting planarian are the same as the products from planarian asexual reproduction. Some species of planaria are exclusively asexual, while some reproduce both through the sexual and asexual process. 

Biological and Biomedical Research on Planarian 

The reproduction system of planarians makes them unique and has been the subject of research. Their life history is a model system for research on many biological processes which can impact human health and disease. Scientists are studying the reproduction process of planarians worldwide and in molecular genetics technologies in particular. The gene function in these animals is a mystery that is being attended to solve. 

The planarians have a lot of cell types, tissues, and simple homologous organs. Planarians are also studied for aging research. The animals possess unlimited reproductive capability, and asexual animals maintain their telomerase levels for the whole life, making them ‘effectively immortal.’

Regeneration in the Planarians

Regeneration is a special Planaria characteristic. When the planarian is cut into two pieces, it can regenerate a whole individual from each piece. A blastema is proliferated at the place of the wound through cells that will differentiate into new tissues and regenerate the missing parts that are lost due to cutting into two pieces. ‘Immortal under the edges of a knife’ is the phrase they have earned due to this characteristic. It’s this fantastic feature that has attracted every genetic engineer for research. It is said that the 1/279th of the organism can regenerate into a complete organism in a few weeks. In simple words, if you cut one planarian into 279 parts, after a few weeks, there will be 279 entire organisms grown. 

The new tissues grow because of the pluripotent stem cells that can create all types of cells. These cells are called endoblasts and are present 20% or more of the cells in adult animals. The replacement of older cells is done by differentiating and proliferating. Additionally, the existing tissues are rearranged to maintain symmetry and proportion with the new planaria. 

The planaria phylum can regenerate itself even if it’s cut down by the head in the half down to its center. Each side can regenerate two heads and live. That means it is not required to cut it into two pieces, and even if one part is cut in half, it can regenerate into two of them and live. You must be wondering what is planaria, how it does so? Tufts University, US, researchers are working to determine the microgravity and the micro-geomagnetic fields impact the growth and regeneration of planarian worm, Dugesia japonica. It was discovered that one of the amputated segments sent to space was regenerated into a double-headed worm, but 95% of the amputated worms did not. Two-headed planaria can be induced by exposing the amputated parts to electrical fields, and it will lead to a planarian with two tails. It can be used with pharmacological agents that alter cyclic AMP, calcium, and protein kinase C activity in cells and interface RNA.

Experiments with Biochemical Memory

Based on the finding of regeneration ability of the planarian, if anyone asks about what is planaria, then the answer is that a lot of experiments were done to find out various impacts of external conditions on the planarian. In 1955, James V McConnell and Robert Thompson kept the planarian bright light in combination with an electric shock. This was repeated many times, after which they stopped electric shock and exposed the worms only to bright light. Finally, it was revealed that the worms reacted the same way to only bright light combined with an electric shock. 

Then they did another experiment by cutting the worm in half. Again, both the completely regenerated organisms showed the light-shock reaction like the earlier one. After this, in 1963, McConnell repeated the experiment, but instead of cutting the trained worms into parts, he grounded the worms and fed them to other flatworms. Again, he found that these flatworms quickly associate with the light-shock effect than those who were not provided with the trained worms.

The experiment had the purpose of finding the memory transfer process. It was revealed that memory transfer takes place chemically. The same experiment was carried out on fish, mice, and rats, but it did not give the same results even a single time. The conclusion came out as, rather than the memory transfer from animal to animal, and it is the hormones in the ingested animals that led to a change in the behaviour of the worms. McConnell named it Memory RNA based on the evidence of a chemical basis. His results are now included in the observer bias. 

Michael Levin and Tal Shomrat in 2012 proved that the planarian retrieving a new head after regeneration has long-term memory retrieval. 

Phylogenetic supertree can be described as 

• Tricladid

• Maricola – Cavernicola and Continenticola

• Continenticola – Planarioidea and Geoplanoidea

• Planarioidea – Planariidae, Kenkiidae, and Dendrocoelidae

• Geoplanoidea – Dugesiidae and Geoplanidae

Size Range

A flatworm or planarian worm will be less than five mm (0.2 inches) long, and many are microscopic. The planaria size is the largest in this class, about 0.5 meters (20 inches). Trematodes are usually 1 to 10 mm (0.04 to 0.4 inches). Members of some species may grow several centimetres long. The smallest cestodes are of length one mm. However, in a few species, one can see them grow as long as 15 meters (50 feet). The monogenean is from 0.5 to 30 mm (0.02 to 1.2inches) long, and aspidogastrea are from a few mm to 100 mm in length. 

Can the Planarian Species be Killed? 

The answer to this question is a Yes as well as a No. This is because not all forms or ways of killing small worms can work on planarians. However, if we take a close look at what planaria eat, we can get to know a lot about the life cycle and feeding process of planaria, and maybe then we can find an appropriate and effective way of killing it. So, the answers go as ‘Yes, planaria can be killed’ and ‘No, it is not that easy to kill planaria.’

If planaria, in some way, start cutting down on their food habits, then they are likely to weaken with each passing day and can also die soon. Another way to kill planaria is through burning. Hot water can kill planaria for sure, which is why this is one of the most confident ways to kill planaria. 

The most complicated ones among these can be through medications. Medications can be taken to kill planaria or at least start deteriorating its health and making it die slowly. 

Planarian is a kind of species that has a very restricted life. It does not harm any creature on the Earth, whether it is small or big, and that is why it is trustworthy and harmless. Therefore, being around this species will not give any trouble to people.

Also, even if people try to break this species in numerous parts and hurt it in the first place, it has a special self-defensive quality that can regenerate all its parts and that too in a very little time. So, we can understand that planarians on Earth are very particular and underestimated and that too, according to the numerous research papers and findings.