Myzostomids are tiny, flattened worms commonly found on crinoids.
Most of them cling to the crinoid’s arms and consume the planktonic food that comes their way. There’ll be quite a lot, since crinoids use those arms to catch plankton and shuffle it down to their hungry mouth. Now there’s another hungry mouth standing in the way!
Many Myzostomids are free to walk around on their host’s body, but there are some who stay in one place and may even cause the crinoid to grow a protective gall around them. There are also a few Myzostomids who are internal parasites.
It has taken a long time to work out what these things actually are. At first, some 200 years ago, they were thought to be flatworms. Then they were thought to be crustaceans, and then a separate group related to water bears.
These days it looks like they’re highly modified polychaete worms. There are fossil crinoids more than 300 million years old that bear the tell-tale scars of Myzostomid activity. It looks like these worms have had a very long time to adapt to their unique habitat!
…Images: Pbsouthwood/Ria Tan