Last time I said I would move on from worms today, but I thought that I should at least cover the creatures alive today that resemble those which made the fossil burrows I showed last time, and talk a bit about the links between flatworms, and the worms I will cover today, so one more post on worms (for now!)
This, is a Nemertea (also known as Ribbon Worms):
Not all of them are this pretty, a lot of them are dull coloured, and some look like this:
The species shown above, Lineus longissimus is one of the longest creatures in the world! It is usually 5-15m long, but has been reported up to 60m long! In contrast to its length, it is usually 5mm or so wide!
It is extremely common in the UK, and can be found wrapped around the bases of algae, or appearing to have tied itself in knots in shallow pools.
This is a video of one moving around, so you can see how they move differently to the flatworms we looked at before
So, why am I showing you pictures of a slightly gross looking creature? (I cannot decide whether I find them fascinating or gross…maybe they can be both)
Well, this phylum of organisms used to be thought to be directly related to the flatworms we covered previously, but now they are thought to have arisen separately, whereas flatworms and Cnidaria are thought to have evolved from one ancestral species, with one line of descendants becoming the radial corals and jellyfish, and another heading off to become the flatworms, with the bilateral symmetry.
The latest research, looking at both genetics, and how cells develop within an organism (cell lineage) appears to show that they are not related to flatworms, but nevertheless, they have some similar features, as well as some which are more evolved, and one very unique feature.
Lets start with what they have in common, and work out from there.
They have bilateral symmetry, and a triploblast body (the three levels of cell within the body) both of which we first saw in the flatworms. (See here for the diagrams from that post)
They do not have a body cavity as such, so they are usually classified as acoelomate, but they have a special structure, which we will get to shortly, and so there is discussion about whether this is a body cavity or not.
The image below is a diagram of a Nemertea, and hopefully you can see the similarities to the cross section of the flatworms here
Here is a cross-section of a flatworm for easier comparison:
Nemertea also have the protonephridia that flatworm have (the flame cells which act as a simple excretory system), and they are able to reproduce both asexually, and sexually, also like the flatworms.
So, they have a few features in common, what do they have different?
Firstly, they have an exit to their digestive system (you can see this at the bottom of the picture above). This means that solid waste does not exit through the same orifice as food goes in by. They therefore have a complete digestive system, although still a very simple digestive system, without the liver, kidneys and other organs which we have. Apart from the obvious yuk factor to only having one way in and out for food and waste, what is the advantage of having two ends to your digestive system?
Well, firstly, I do not think animals are bothered by the yuk factor in the way we are, but there is a very clear advantage to having both a mouth, and an anus. The advantage is, simply, that you can eat and excrete simultaneously, which means having a complete digestive system is more efficient.
Another important development is that Nemerteans have what is known as a closed loop blood-vascular system. In flatworms, oxygen and other nutrients are distributed by diffusion (remember how they had a digestive system spread all through their body). Nemerteans have dedicated blood vessels to transport nutrients around their systems.
Finally, I said that Nemerteans have a unique feature, and now I will see if I can explain this, because it is very weird, and very cool, and quite gross at the same time.
Nemerteans have something called an eversible proboscis. Eversible means something that can be turned inside out, and a proboscis is something which sticks out from a head, so an elephant has a proboscis, usually called its trunk. Butterflies have a proboscis, it is the straw-like organ similar to a tongue which they stick into flowers to suck up nectar.
So, Nemerteans have a body part which is turned inside out, and comes from their head….sounds weird, but what exactly does it mean? In the image below, the proboscis is shown inside the body, and is the dark line running the length of the body.
This proboscis lies within a cavity in the body, above the digestive cavity, known as the rhynchocoel. It is because of this cavity that the discussion arises about whether Nemerteans are acoelomate or coelomate.
The proboscis itself is a hollow tube of muscle, and the image below shows it being stored in the body, and after being everted (turned inside out)
So, how does the proboscis get from the inside to the outside, and what is the point of it?
There is fluid in the cavity where the proboscis lies, and when the animal runs into prey (quite literally sometimes, as some species are not very good at finding their way around), muscles at the back of the cavity contract, and this causes an increase in pressure in the cavity, which forces the proboscis out of the body. There is a spike on the end of the proboscis, and this is stabbed into the prey, whilst the proboscis itself coils around the prey.
The proboscis also has toxic slime (mucus) on it, and this goes into the prey. Some of these toxins are the same as in the puffer fish (tetrodotoxin), so very effective! The proboscis is then pulled back into the body, bringing the prey with it, this brings it towards the mouth (the opening under the head), and it is then swallowed whole.
I think the description might seem a bit confusing, so this video hopefully makes it clearer (also take a look at the length of the proboscis relative to the length of the animal).
I did say I thought they were both gross and fascinating…
Next post will be away from worms, I promise!
References and further reading:
Phylogenetic position of Nemertea derived from Phylogenomic Data -Struck & Fisse 2008: http://mbe.oxfordjournals.org/content/25/4/728.full
Progress in Nemertean Biology: Development and Biology – Turbeville 2002 http://intl-icb.oxfordjournals.org/content/42/3/692.full
Information on Cell Lineage – Chisholm 2001: http://bio.research.ucsc.edu/people/chisholm/lab%20PDFs/Cell%20Lineage%20(EoG).pdf