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