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The animal kingdom is a wonderfully diverse category of organisms. How, though, did we get from the simplest, single-celled organisms all the way to humans, the most complex? One stop along the way are the platyhelminthes.

Classification

Platyhelminthes refer to the phylum of the animal kingdom that includes the flatworms. The name ‘platyhelminthes’ literally means ‘flat worm’ (‘platy’ meaning flat and ‘helminthe’ meaning worm). A phylum is the first classification down from kingdom, the largest possible classification, so this includes a very broad classification of worms.

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While flatworms are in fact flat, they have a few specific features that distinguish them from other worms, including their digestive capabilities. There are three parasitic groups of platyhelminthes and one non-parasitic group. Parasitic platyhelminthes live in other animals and get their nourishment through them. Non-parasitic platyhelminthes eat their own food (more on that later).

Bilateral Bodies

All platyhelminthes are bilateral, meaning that the left and right hemispheres of the body are exactly alike. You could cut a flatworm lengthwise and all of the features would be the same on either side. Meanwhile, the top and bottom of the animal will look distinctly different.

Imagine a flatworm as being like a snake: The head half is awfully different from the tail half, but drawing a line down the middle will create two symmetrical halves.Unlike humans, platyhelminthes lack any real circulatory and respiratory systems. They have no body cavity, the space inside of a body, like a chest cavity, classifying them as acoelomates. Because they don’t have any of these organs or spaces, flatworms tend to stay microscopic.

Digestive System

Despite their lack of circulatory and respiratory systems, flatworms do have long, interconnecting digestive systems which break down food and distribute nutrients all around the flatworm’s body.Each flatworm is different in terms of food consumption, but most platyhelminthes absorb nutrients by consuming them through a mouth. Food travels into a gut-type structure that holds and digests it. Once the food is broken down, the digestive system passes it all through the body. This allows nutrients to disperse evenly across the organism.

The digestive tract branches out, sort of like a tree.Similarly to humans, the food travels through their digestive tract, where it gets moved out into the body where needed. Unlike humans, however, most platyhelminthes don’t have an anus, so after they’ve digested what they can, the unused food matter comes back out through their mouths.

Eating

We just looked at how food is digested; now let’s look at some specific ways that flatworms eat.

As we mentioned previously, there are three parasitic groups of platyhelminthes, and one non-parasitic one. The non-parasitic group eats in a manner similar to humans. When they encounter food, they use a mouth-like structure to eat it. As we saw, the food travels through a network of digestive tracts, scattering out into the animal to be used.Parasitic flatworms eat in a similar way, with some slight changes. All three of the parasitic groups have what are called holdfasts, which resemble jagged, toothed suckers. Parasitic flatworms enter the bodies of other animals, called hosts.

They then use their holdfasts to attach to the host’s digestive tract, where they can absorb and consume what the host eats. Each subgroup of parasites has a slightly different mechanism for this, but the overall action is the same.The parasites in the class cestoda are tapeworms, creatures that many humans are very familiar with. These organisms attach to a human’s intestine and absorb nutrients from the food that a person eats. They are tough to get rid of and can be a real issue in places with minimal healthcare.

Lesson Summary

Platyhelminthes are a phylum of worms that are collectively called flatworms. The phylum includes three categories of parasitic flatworms and one category of non-parasitic worms. These organisms are bilateral, meaning they are identical on the left and right sides. They can be microscopic, and they can lack a chest cavity or circulatory system.

Because they don’t have a body cavity, flatworms are considered acoelomates. They have simple digestive systems, with mouths to take in food and long digestive tracts to diffuse it around the body. Most flatworms take in food via their mouth, then move it into a digestive gut that attaches to the digestive structures. The food then breaks down and is absorbed out into the rest of the organism. Some have an anus to remove wastes, but most don’t and will simply excrete unused food through the mouth.

Parasitic flatworms attach to their hosts via sucker-like mechanisms called holdfasts.

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