Platyhelminthes: Flatworms and Body Plans

As promised, today is back onto moving through the history of life on Earth.

These next few posts are about a phylum known as Platyhelminthes (from the Greek platys meaning flat, and helmins meaning worm), and specifically about a class called Turbellaria.  For anyone reading this who already knows about the order in which the various phyla and classes are thought to have originated, yes, I know I missed Ctenophores, but I will cover them in future posts.  Also, I know that both Platyhelminthes and Turbellaria,  are debated within the taxonomic community, but, as far as I know, they are the names commonly used at present, and so that is why I have chosen to use them.

First of all, what are Platyhelminthes, and why am I writing about them just after Cnidaria?

Well, Cnidarians are radially symmetrical, this means that they are a bit like a pie, in that if you divided them up into slices, each piece would look almost the same.  They do not have a left or right side, as we do, they have a top (dorsal, like a sharks dorsal fin is its top fin), and a bottom (ventral) side only. The image below illustrates this type of symmetry.

Radial Symmetry and Bilateral Symmetry. Image from Glasgow University Animal Diversity Wiki

The image above shows a second type of symmetry, bilateral symmetry where there is a clear left and right side.  This type of symmetry is common to most animals, as well as plant leaves.  Plathyminthes are the simplest organisms with this symmetry, they are also the first truly mobile organisms I have covered, as Cnidaria are either sessile, or move mainly with currents within the oceans.

Another difference between Cnidaria and this group of organisms is something called cephalization. Cephalization is when sense organs, and nerve control becomes centralised at one end of a body. In more complex organisms, such as ourselves, this is our head, but with Turbellaria, it is not immediately obvious that there is a head.

Before I dive into more technical stuff, I had better show you what on earth a Turbellarian is.

A flatworm (Turbellarian) Pseudoceros dimidiatus. Image from Wikipedia

See? They are very weird creatures!  Here are some more pictures (because I think they are extremely beautiful, and the name flatworm makes them sound a bit boring and gross)

Also Pseudoceros cf dimidiatus. A subspecies of the previous image. Image from

And one more, because they look so cool!

Prostheceraeus giesbrechtii Image from wikimedia

As you can see, apart from looking really amazing, these are externally very simple organisms, but a little closer to us than Cnidaria.  As mentioned above, these have bilateral symmetry, and cephalization. I think some of them look quite cute, but then, as you will discover as we move through these posts, I have a strange idea of what is cute! The cephalization is not clear in the Turbellaria shown above, but I think the image below makes it more obvious, as well as looking really cute.

Dugesia subtentaculata. Image from wikipedia

So, apart from having symmetry similar to us, and a primitive form of head (These still have no brain as we would recognise one), what else do these have that separates them from Cnidaria?   This is where it can get a bit technical, but I will try to keep it clear, it is one of those cases where I know what I want to write, but am not sure if I can get across what I mean simply enough.

There are two further important differences between Cnidaria and Platyhelminthes:

First, they are what is known as triploblasts.  This means that when they are developing, very shortly after fertilisation, the group of cells forms what is known as a blastula. This is essentially a ball of cells, with a hole in the middle, and they look a bit like this:

Blastula. Image from University of Wisconsin

Now, this is where it gets a bit cool….The cells of the developing embryo then folds inwards at one end, and continues until the folding in of the cells effectively splits the cavity in two.

Diagram showing Gastrulation. Image from University of Wisconsin

Pictures of gastrulation occurring. Image from University of Wisconsin

This happens in Cnidaria as well as with Platyhelminthes, right the way up to us.  In us, the opening which you see here becomes the exit of our digestive passage, and another hole is formed at the opposite end which becomes the mouth (We are deuterostomes, meaning second mouth). Some organisms form the mouth where the original opening is, and the exit at the opposite end, and they are called protostomes (first mouth).  Turbellaria have only the one opening, in common with Cnidaria.  This is called a “blind gut”.

So, after a bit of a detour, what does this have to do with triploblasts?  Well, in organisms such as Cnidarians, there are 2 layers of cells, the ectoderm which is the outside cells, like skin, and the endoderm, which forms the lining of the gut. Flatworms, like us, have these two layers, and an additional layer, the mesoderm.  In mammals, the ectoderm forms the skin, the endoderm forms the lungs, the digestive system, and liver, and the mesoderm forms muscles, cartilage and blood vessels, among other things.

Comparison of diploblasts and triploblasts. Image from Kenyon College

Finally, Turbellaria are acoelomate. This means that they have no body cavity,as we do, containing their organs.  This cavity, often fluid filled, serves to protect the organs from crushing. This sounds more complicated than it is, as these images illustrate.  The first shows an acoelomate body, where the organs are in contact with the epidermis (skin layer) through the mesoderm.

Acoelomate body structure. Image from University of Illinois Chicago


The second image shows a coelomate body diagram, and it can be seen that the organs are suspended in a cavity, which allows for compression without affecting the organs.  If an acoelomate has pressure exerted on its epidermis, this squashes the entire area underneath, including any organs which are there, whereas in a coelomate, the cavity compresses before the organs are compressed, adding an additional layer of protection.

Coelomate body structure. Image from University of Illinois Chicago


The next few posts will be on how Turbellaria feed, how they move, and how they reproduce.



14 thoughts on “Platyhelminthes: Flatworms and Body Plans

  1. Pingback: More Turbellaria: Moving, eating, mating « skepticalsquirrel

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  6. Wonderful post! I believe that the brightly colored ones are marine species called polyclads. I use some of the fresh water species in my own research in pharmacology and neurobiology. They are simply fascinating? Did you know that they can get addicted (yep, that’s what I wrote) to most of the same chemicals that humans abuse, like nicotine?

    Excellent introduction to the flatworms; thanks!!!

  7. Wow. How on earth did I miss this great post until now? It allows me to start understanding a question which was (allegedly) in an Oxford University Finals exam paper: ‘Are we just updgraded flatworms?’

    I agree with you about these animals’ beauty and cuteness.

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