More Turbellaria: Moving, eating, mating

After being a bit technical last post, today will be more visual I hope!  I am hoping that I can show you some very interesting things today about Turbellaria, and hopefully something you had no idea about, but will find fascinating.  Apologies for not posting this yesterday, I was trying to track down some good videos and images.

So, lets start out with moving around.  As I mentioned in my last post, Turbellaria are mobile organisms.  Although jellyfish and medusa stages of the Cnidaria are not sessile (stationary), they do not really move, they are more carried around by currents within the water (This is called passive locomotion).  So, another step forward that Turbellaria make is being more in control of their movements (Active locomotion).

Being more mobile may have led to the cephalization that I mentioned last time, as in order to move, you need to see, or sense, where you are going, and so sensory organs became located at the front end of the organisms. Many species of Turbellaria have light sensitive spots on their front end (these are called ocelli), and some have small flaps sticking out of the side of their heads, which look like little ears, but are usually chemo-receptors (in us, we have our taste buds and our sense of smell as chemo-receptor)

A Turbellarian (Sabussowia ronaldi) with the eyespots visible (the dark spots). Image from wikipedia

So, now they can see, at least a little bit, where they are going, how do they move where they want to go?

Many Turbellarians move by moving their muscles (and in smaller species, cilia) in an undulating motion (like ripples moving along their body), and sliding over mucus which they secrete (A bit like snails, but more wiggly!).  There are some species however, which swim, and they do this in the same manner as the ones which wriggle along the sea floor or ground, by waves of muscle contraction.  I think this looks really fun, and beautiful, so here is a video of a marine flatworm swimming.

Now, to get a little bit stranger, once they get where they want to go, how do they eat, and what do they do with their food?

As I mentioned last post, Turbellaria have blind guts, that is, there is only one entrance and exit.  This does not mean that absolutely everything which is taken in and not used comes back out the same way, although the majority of it does, as we shall see shortly.  Turbellaria, and their phylum Platyhelminthes, are the first organisms we encounter which have excretory organs.  In humans, these would be things like kidneys, which filter out waste products from our blood.

Turbellaria have nothing as complex as our kidneys, but they do have what are known as protonephridia.  These are effectively tubes which are closed at one end, and have small holes which draw fluids in from the body. They have special cells called flame cells, which have flagella attached to them (flagella are structures extending from a cell, which often move in a whip-like motion, which is where they get their name.  Flagellum is Latin for whip. These are often used for movement in bacteria) These beat, and move fluids down into the open end of the tube, where they are moved along with the help of cilia, and the tube opens out at the edge of the body.  These protonephridia are used for something called osmoregulation which is mostly removing excess fluids from the organism, although small amounts of ammonia are also removed.

Protonephrida. The blue arrows indicate where fluids enter near the flame cells, which beat and move the fluids down the tubes so they can leave the body at the surface.  Image from Biology 8th Edition: Campbell & Reece via College of DuPage

Turbellaria are carnivorous, and feed on things such as small crustaceans (crabs are crustaceans), insects, and very small organisms called rotifers.  The cool thing about Turbellarian feeding is not what they eat, but how they eat it.

The image below is the general anatomy of a Turbellarian, it shows some of the structures I have already mentioned, including the eyespot, and the brain-like collection of nerves at the front end, and the protonephridia, and some parts that we will come to in a bit, however, the part we are interested in now is the tube sticking out on the underside, labelled as “pharynx”.

General anatomy of a Turbellarian. Image from

The image below more clearly illustrates the location of the pharynx.  In humans, our pharynx is the tube which runs from our mouth down towards our stomach, behind our larynx (the tube which we breath through).

Diagram of a Turbellarian, showing the pharynx, and gastrovascular cavity. In humans, the gastrovascular cavity is our stomach and intestines. Image from DuPage college, and Biology 8th Edition: Campbell and Reece

So, the pharynx in turbellaria is a tube which can be pushed out of a hole in the underside.  As these do not have teeth, or a mouth for holding food and starting digestion as we do (food begins to be digested within our mouths, before we swallow it), turbellaria instead have digestive juices come directly out of their pharynx onto their food, which begins to digest it, and it is then sucked back up into the pharynx, where it goes to the cavities within the organism for digestion to be finished.  Undigested food is excreted back through the mouth.

The video below shows a pharynx extended from a turbellarian ready for feeding (at around 0.27 seconds), as well as some of the other features I have mentioned in this post.

I said at the beginning of the post that I would show you some things that you probably didn’t know, and this is where I do that, I hope you find this next section as interesting and weird as I do!

Turbellaria are able to reproduce both sexually, and asexually.  When they reproduce asexually, they usually divide themselves in two behind their pharynx, then each new animal regrows the parts that it is missing, so one part grows a new head end, and one grows a new tail end, some divide themselves lengthwise to reproduce asexually.  This in itself is fairly cool, as asexual reproduction is usually confined to much smaller organisms.

However, the really cool part is when they reproduce sexually.  If you look at the detailed diagram of the anatomy above, you can see that it has various genital organs.  The image below shows this in clearer detail

General body plan of a Turbellarian.  Image from Southwest Tennessee Community College

In the image above, you can see that the animal has both a penis, and a genital pore, as well as testes, oviducts and ovaries.  This means that Turbellarians are hermaphrodites, having both male and female genitals. This is useful from an evolutionary perspective, as it means that if you encounter another of your species, you are able to reproduce.  In humans, in order to reproduce, we need someone of the opposite sex, whereas Turbellarians just need to bump into another of their own species.

So, when they bump into each other, do they both fertilise each other, or how do they decide which one is going to be receiving the sperm?

Well…this is where it gets weird.  Flatworms do something called penis fencing, yes…you read that right.. (This is part of the reason I was delayed making this post, I had to find a video which clearly showed this occuring, and internet searching “penis fencing” can lead to some…strange results!)

Basically, both flatworm extend their penises, and they have a fight with them, and try to insert theirs into their opponents genital pore.  Unfortunately, they are not always very good with their aim, and tend to stab each other quite a bit in the process!  I think the video below illustrates this much better than I can with words.

I hope you found this as interesting as I did researching this!

Next post will be moving a bit further up the evolutionary tree.


5 thoughts on “More Turbellaria: Moving, eating, mating

    • Some species are indeed kept as pets, I ran into a lot of sites while researching this post which were about these in aquaria. It seems quite a few turn up unannounced, both the colourful marine ones, and the ones I think are cute with the eyes and ears, this seems to be due to their larva being inadvertently transferred when buying other marine species.
      Some are kept on purpose, for tank cleaning, or just because they are beautiful.

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