The other day I finally decided to get a good look at the underside of a chiton. I meet the little molluscs all the time; they live in the same rock pools my experimental animals come from. Usually, I just find them sitting on a rock being cryptic like these two I found on their Wikipedia page (photo: Hans Hillewaert):
They live in my head as these fascinating living fossils (even though if you ask me I’ll say that the whole concept of living fossils is stupid), strange beasties whose anatomy preserves relics of the time when molluscs were segmented animals. Now, I’m not sure molluscs ever actually were segmented animals, at least not to the same extent as, say, a centipede or a ragworm obviously is. (But “segmented” is a complicated property, and I don’t want to digress too far that way.)
Either way, I pried one of them off the stone I’d picked up and stuck it under the microscope, because biologists just can’t leave poor innocent creatures alone. I wanted to look for those signs of segmentation, which, by the way, are pretty much limited to the repetition of shell valves and gills unless you take the animal apart, in which case there are also muscles. I don’t know what I expected, really, but what I found was that the belly side of a chiton is mostly… boring.
(Unless it’s the size of this gumboot chiton, courtesy of Prof. Douglas Eernisse via Wikipedia. Then it’s pretty impressive and kind of scary.)
For one thing, most of it is covered by that fleshy foot that looks like a dog’s tongue that spent too much time in formalin. You are at the mercy of the chiton to even catch a glimpse of the gills, because the foot can very nicely spread out and cover them. And the gills themselves are sort of, well, anticlimactic. In fact, the entire underside of the poor chiton I abused (who, unlike the monster above, was barely the size of my pinkie nail) was just the same bland, wet shade of beige. At least Mr Gumboot above has brown gills.
It wasn’t a complete disappointment, though. For one thing, I’m oddly fascinated by that plump kissy mouth. It’s so… ugdorable. And then there is the girdle – the soft-tissue bit around the edges of the shell – which I totally didn’t know was covered in little projections. I was expecting something smooth and fleshy and maybe a bit slimy, like most things mollusc, and then bam, I zoomed in on it and it was all warty. In a pretty way. Wikipedia tells me that sometimes they have calcareous scales or spikes on the girdle. I didn’t prod my chiton enough to find out whether its warts were hard, but anything that involves biominerals is pretty interesting to me!
And don’t worry, the little chiton went back to its rock unharmed aside from the stress of getting turned on its back and prodded by a big scary creature. I’m not quite heartless enough to keep an animal I don’t need for my experiments. Even though I heart molluscs of all kinds* and I don’t care if their boring bellies disappointed me.
*Well, with the possible exception of the evil nudibranchs who also live in my rock pools and once ended up sliming all over my dish. Yuck.