One of the neat things that have come from advances in molecular biology is our ability to use DNA technology to tease out evolutionary relationships – especially those that aren't immediately obvious (such as the subject of an earlier post). Now here's another example – an animal that looks superficially like a worm – but turns out to be most closely related to sea anemones, jellyfish, & other radially-symmetrical animals.
You're probably familiar with the idea that you can classify most animals into two main groups: the radially-symmetrical animals (eg the phylum Cnidaria, which includes sea anemones & jellyfish) and the bilaterally-symmetrical animals (including chordates and vertebrates, arthropods, molluscs, and 'worms'). However, there are apparent exceptions, such as the myxozoans, which are tiny endoparasites with a relatively simple structure & no obvious close relations among the other phyla. One of the myxozoans is Buddenbrockia plumatellae, which moves like a worm, looks like a worm – but turns out to be most closely related to cnidaria. I don't know about you, but I think that's pretty neat!
Cnidarians have a fairly simple body plan – 2 layers of 'true' tissue, with a jelly-like layer, the mesoglea, between them (the group includes jellyfish, after all!). They have a simple, sac-like gut with only a single opening, and have a unique feature – nematocysts, or stinging organelles.
In comparison, most myxozoans are tiny organisms that live as endoparasites on other aquatic animals. They have bodies that are either sac-like, or something like a plasmodium; with the exception of Buddenbrockia, the 'worm', they don't look very much like any other animal group. However, they do form spores with structures that are similar to cnidarians' nematocysts, which suggests a relationship to that group.
Buddenbrockia's resemblance to a worm is more than skin-deep. Although it lacks a nervous system and obvious sense organs, once it has left its host (an organism called a bryozoan) it uses four blocks of longitudinal muscle to writhe and wriggle just like a worm. However, analyses of some of its protein-coding genes (Jiminez-Guir et al. 2007) suggest that Buddenbrockia is actually a cnidarian – and so, by extension, are the other myxozoans. (Other scientists have already shown that this strange 'worm' is closely related to the other tiny members of this phylum.)
If you're keen to find out more, here's a link to a post by an excellent science blogger (not Orac, this time!) which goes into the story in a lot more detail.
E. Jiminez-Guri, H. Philippe, B. Okamura & P. Holland (2007) Buddenbrockia is a cnidarian worm. Science 317: 116-118