Flatworms-Living without a heart

by Matthew Norton

The insides of many animals are made up of blood vessels, nerves, muscles and organs (and more) working together as one complicated machine with each small part performing specific tasks to keep this machine working properly. How easy it is to forget that not all animals are, or need to be, this complicated, with some weird animals groups able to survive without the major components that we need to live.

Flatworms (also called Platyhelminthes) are one such weird group of animals that come in a wide range of body forms, even though their basic biology is very simple, with some ‘vital’ organs completely missing. For example, they have no heart, gills, or circulation system for extracting oxygen from the environment and pumping it around their body. Instead, they simply rely on the oxygen to flow through their bodies, a process called diffusion, but this only really works over small distances, which is probably why flatworms are flat and/or small.

Flatworm article image 1
Flatworms are definitely a weird and varied group of animals with examples including (clockwise from top left) Turbellerians, Trematodes, Tapeworms and Monogeans.

Even the few organs that flatworms do possess are very simple in design with the digestive system being just a fleshy sack instead of a system of different compartments (stomach, small intestine, large intestine etc) for different stages of food digestion. Some species don’t even have an anus for getting rid of waste and have to vomit this waste back out through their mouths. Their nervous system is also very simple, although they do have a clump of nerves at one end of the body which could be a brain.

Clearly, flatworms have very little to work with and you wouldn’t expect them to do anything interesting. Many flatworm species cannot even function on their own and survive as parasites on animals that include fish, crabs, snails, frogs and even humans. Other species however, can live on their own and catch their own prey by waiting to ambush them, trapping them in nets of sticky mucus, or poisoning the water around them.

Flatworms also demonstrate some interesting social behaviours despite their primitive brain, if it even counts as a brain. This is well demonstrated by Pseudobiceros bedfordi, a species that meet in pairs and fence with their penises. Like many flatworms they are simultaneous hermaphrodites (which means each animal is both male and female at the same time), but each animal would prefer the other to ‘be the female’ and do the job of carrying the fertilised eggs. Neither animal develops a specific opening for the sperm so the ‘fencers’ will try to burn a hole in each other with acid, while trying to avoid being burnt themselves.

Flatworm article image 2
Both these Pseudobiceros bedfordi flatworms have the common goal of reproduction, but the one that carries the fertilised eggs will have to spend a lot more energy dragging them round.

A common misunderstanding with evolution is that it aims to produce some sort of perfect organism in the long term, but really it works to change a species one small step at a time and equip it with what it needs to survive, and nothing more. The complicated machinery that support many large animals is just one of many options to help animals survive in the natural world and it is an option that flatworms don’t need.

From a human perspective

Unfortunately, when it comes to the interactions between us and flatworms it is the parasitic species that stand out, and with good reason. The flatworm group includes some nasty parasites that can infect various parts of the body including (but not limited to) the blood, liver and intestines and can cause serious health problems. One particular case from 2017 where a 17 year Mexican teenager who had somehow got a tiny flatworm burrowing around inside his eye. The flatworm was successfully removed, but it left the teenager with a permanently damaged eyeball.

On a wider scale, several species of flatworm parasite are also responsible for Schistosomiasis (also known as snail fever) which is the world’s 2nd most devastating disease caused by a parasite after malaria. Symptoms are varied and include blood in the urine, pain in the genital area, stunted growth, infertility and reduced learning ability, all of which are harmful, but rarely cause death directly. Victims can become infected with Schistosoma flatworms from any contact with water that contains their larvae, whether they are drinking it, bathing in it, or washing their clothes in it. The water itself can become infectious from releases of flatworm larvae by infected snails, or the faeces of infected humans, which can contain flatworm eggs.

Flatworm article image 3
Without access to clean drinking water and good sanitation there is a serious risk of picking up a Schistosomiasis causing parasite.

Without access to clean drinking water and good sanitation there is a serious risk of picking up a Schistosomiasis causing parasite. Image source: Bb.braam, 2017 (right, CC BY-SA 4.0).Flatworms can also cause us serious economic problems by attacking other animals that we rely on. A number of flatworm parasites can infect livestock, farmed fish and shellfish, making the meat unsuitable for human consumption and costing the farmers considerable amounts of money. Even non-parasitic species can be a problem, for example some Bipalium flatworms, such as the New Zealand flatworms, have been massacring earthworms in Northwest Europe. This is bad because earthworms are an especially valuable species because they can be a protein rich food source for our livestock and a useful decomposer of food waste.

Flatworm article image 4
New Zealand flatworms (Arthurdendyus triangulates) and their relatives are invasive species in Northwest Europe.

Despite all these negative effects, some flatworms can help us to develop useful ideas to our problems. In particular, the ability of Planarian flatworms to clone themselves from tiny fragments the original body may hold insights for stem cell research. There has also been interest in using certain species planarian and turbellarian flatworm, and their consumption of insect larvae, as natural controls against mosquitos.

Survival in the natural world does not usually encourage niceness to other animals, especially towards different species, and in some cases we can get caught in the firing line. However, this is nothing compared to the impacts (e.g. climate change, overfishing, plastic pollution) that we have on the environment, both above and below the waves. No other species on earth manipulates the world around them like we do.

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Thanks for reading


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Dean. 2017. Where No Flatworm Has Gone Before: Doctors Find Wriggling Parasite in Teen’s Eyeball. https://www.sciencealert.com/where-no-flatworm-has-gone-before-doctors-find-wriggling-parasite-in-an-eyeball. Last accessed 08/04/2019

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Image sources

Hhgutierrez49. 2012 [CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0)%5D. https://commons.wikimedia.org/wiki/File:Faringe_planarias.jpg

Neely852. 2018 [CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0)%5D. https://commons.wikimedia.org/wiki/File:C._vesicaudus2.png

Jana Bulantová. 2018 [CC BY-SA 4.0 ()]. https://commons.wikimedia.org/wiki/File:15_Eudiplozoon_nipponicum.tif

Nico Michiels. 2004 [CC BY 2.5 (https://creativecommons.org/licenses/by-sa/4.0)%5D. https://commons.wikimedia.org/wiki/File:Mating_Pseudobiceros_bedfordi.png

Bb.braam. 2017 [CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0)%5D. https://commons.wikimedia.org/wiki/File:Male_and_female_schistosomiasis.jpg

Rae’s. 2014 [CC BY 2.0 (https://creativecommons.org/licenses/by/2.0)%5D. https://commons.wikimedia.org/wiki/File:New_Zealand_flatworm_4.jpg

All other images are public domain and do not require attribution

One thought on “Flatworms-Living without a heart”

  1. Fascinating insight to these types of worms regarding the differing bodily make up, ( amazing diagrams) and the pros and cons of their place in ecology.


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