Myzostomida

Image: Pbsouthwood

Parasitic worms... on stilts!

Myzostomida is an order containing more than 150 species of small, marine worms who are remarkably lacking in body parts. The heads, shoulders, knees and toes song is not an exhaustive list when it comes to humans, but it pretty much would be for a Myzostomid. And they don't even have knees...

Image: Pbsouthwood

Most Myzostomids can be found clinging to the arms of crinoids. IF you can find them! These things are often just a few millimetres long, remarkably flat and adorned with all the colours necessary for good camouflage against their host.

Oh! You remember crinoids, don't you? They're the echinoderms that look like a bouquet of feathers sticking out of a tiny cup. The feathers are like elaborate gutters filled with sticky mucus and cilia. Tiny bits of plankton get stuck to the mucus while the cilia constantly beat back and forth so that the mucus flows down into the mouth.

Mystomids are mostly ectocommensal, meaning they live on the outside of their host, eat their host's food but don't otherwise cause harm. They just plunge their mouth into those gutters and consume the plankton that comes their way. The mouth is at the end of a tube that can be extended or retracted. It's called an eversible pharynx, and it means they can plunge their gob right down into the gutter. Yum!

Video: LilCookieMonsterJnr
On its back. Look at those table legs!

They keep hold of their host using five pairs of parapodia, or false legs. Each one ends in a hook and is kept stiff by a long, internal bristle called an aciculum. This bristle acts like a bone and means that the parapods can't bend. They're like stilts!

Next to the parapods are a total of eight "lateral organs". They look like suckers and are thought to be sensory. Around the edge of the Myzostomid's body are ten pairs of short, sensory appendages called cirri. That seems to be all the sensory organs they have!

Video: LilCookieMonsterJnr
A closer look than anyone wanted

The other body parts are all things that wouldn't get into the heads, shoulders song. There's an anus which lies at the end of a little tube of its own on the other side of the body from the mouth. The presence of an anus is highly relevant when you're talking about a worm like this! If you're lucky, you might even be able to see the digestive tract through the worm's thin skin. You would notice that it splits up into lots of channels to bring food to all parts of the body. A lot like many flatworms, in fact.

Finally, there are both male and female gonads, since Myzostomids are hermaphrodite. They start out life as minute, trocophore larvae. These things are almost spherical and swim around using cilia before they have to settle on a host.

Image: Ria Tan

So that's most Myzostomids, but some are even more lacking in body parts! There are some that don't care to walk around on their host and instead prefer to pick a spot and stick to it. Some of them induce their host to create a nasty, bulbous growth called a gall, that surrounds and protects their parasite.

There are also Myzostomids that are proper, internal parasites of various echinoderms like brittle stars and sea cucumbers. They hang around in the digestive tract, gonads or in the body cavity.

These Myzostomids have no need to get around and its a bit pointless having any senses at all, so they start losing what few body parts they have left. The biggest Myzosomid known is one of these internal parasites. It reaches a little over 3 cm (an inch) long, so I guess you give a little, you get a little.

Image: Ria Tan

This perhaps is why Myzostomids were initially thought to be trematode flatworms, or flukes. They have almost no body parts, too. But Myzostomids would be extremely strange flukes, especially since most of them would be flatworms on stilts. Thus came a later suggestion that they might be crustaceans... which is even more bizarre! Later came the idea that they were related to water bears.

Today, after 200 years of argument and whole lot of genetic analysis, the evidence suggests that they're annelid worms. Polychaetes, no less. Research is still ongoing, because the main thing that Myzostomids are, is weird. They are bizarre and weird no matter what they are!

Why are they so strange and so... different?

Image: Peter Southwood

It turns out Myzostomids have been around for a looooong time. There are fossil crinoids dating back over 300 million years that bear the marks of Myzostomid activity. The relationship between crinoids and their worms is an ancient one, and those worms have become heavily adapted to a truly unique habitat.

I still like to think of them as flatworms on stilts, though.