In addition to the sensory information we receive about the external world from our eyes, ears, noses, tongues, and the millions of nerve endings in our skin, there is a sixth, and most noble, sense that we all possess. Erica knows what I’m talking about; as a dancer, she wouldn’t be able to make one revolution around a room without falling over if it wasn’t for the wonder of proprioception: the sense that tells us where all the various flailing parts of our body are at any given moment even if we cannot see them, that lets us know how our muscles and joints and ligaments and tendons are connected to each other — how they bend and flex and when they stretch. It’s the sense that keeps us aware of internal pressure and distension and temperature, prevents us from continually having to check which parts of ourselves are moving and which are still. It gives our nervous system — the nervous system is the arbiter of proprioceptive information — a basic road map of the overall shape and architecture of our bodies. Proprioception is so inextricably part of our experience of being human and alive that it’s almost impossible to describe — but if you lost it, you’d definitely notice. It would be a little bit — not quite, but a little bit — like losing your skeleton.
”She could scarcely even sit up - her body ‘gave way’. Her face was oddly expressionless and slack, her jaw fell open, even her vocal posture was gone.
”Something awful’s happened ,” she mouthed, in a ghostly flat voice. “I can’t feel my body. I feel weird - disembodied.”
from Oliver Sacks - The Man Who Mistook His Wife for a Hat
I bring up proprioception only because I wanted to tell you a little bit about pycnogonida tonight, and I began to wonder (I hope you will wonder with me) what on earth the proprioceptive sense is like in other animals — particularly animals that, like arthropods, lack endoskeletons and have nervous systems that are so different from ours.
— from asbjorn.hansen - (?)
We’ll never know the answer to that question, unless the Hindus are right about reincarnation, but I’ll tell you a little bit more about pycnogonida anyway and then you can let me know what your best guesses are.
Pycnogonids, also known as pantopoda (”all legs”), are arthropods. That makes them related to (in the sense that they share a common ancestor with) both crustaceans and arachnids, so it only makes sense that the tremendously cool looking pycnogonid in that wonderful picture above resembles nothing so much as a cross between a lobster and a spider. In fact, as you might already know, pycnogonids
a) are commonly known as “sea spiders,”
b) usually have 8 legs, like true spiders (but sometimes have as many as 10 or 12), and
c) were classified until recently by most scientists as members of the subphylum chelicerata, to which belong such other long-legged dancers as spiders, scorpions, and horseshoe crabs. These days, though, many progressive comparative biologists think they actually constitute their own, separate, subphylum.
Re: point c) — because sea spiders have essentially no fossil record, it’s very difficult to know exactly how old they are and what kinds of beasts their closest ancestors were. As a result, and also because their morphology is generally so strange and fascinating, there has been some scientific dispute (thankfully nothing too ugly) over the question of how sea spiders are related to other arthropods, and whether they do in fact deserve the distinction of being their own odd little category of creature.
I think they do. But what I think doesn’t really matter, so lets forge on. Sea spiders are bottom-dwellers. They spend most of their lives crawling upon the ocean floor; some of them can be found wandering slowly about at fathoms as deep as 7000 meters. A few pycnogonida species are good, if ungraceful, swimmers, but most are strictly perambulators (when you’ve got gams like they have, it would really be a pity if you didn’t stroll a lot). Their jointed legs often end in claws that they use to grasp the tiny aquatic organisms they feed on. Tinier than themselves, I should say, for sea spiders tend to be slight: the smallest have limbs as short as a few millimeters, though some species are said to lumber around on ungainly stilts that are almost a meter long when unfolded.
No matter how far their toes may stretch, however, sea spiders all have relatively small bodies (scrawny pebbles, mostly providing a place for all those legs to attach). A pycnogonid has a minute abdomen, from which protrudes a comparatively impressive proboscis that may have teeth or spine lining its interior walls. Once a sea spider has ground up its prey, the food particles get sucked into its gut — which branches out into long cavities in the creature’s walking legs, because really — in an animal this spindly, where else is there going to be room for digestion to take place?
Sea spiders don’t appear to have any excretory organs (although maybe we just haven’t been looking closely enough). Scientists (trying hard, and mostly succeeding, not to sound as if they’re making things up) think that pycnogonida simply excrete — and also respire — through their exoskeletons. Somehow.
So, my dears, take a moment now to close your eyes and note the way it feels to be inside your own body. Pay attention to the strange and inescapable sensation of having jointed arms and legs; straining muscles; a sturdy skeleton, creaky though it may be. Remember the delicious dinner you undoubtedly had, the remains of which you may be able to feel still churning in your gut. Take in a couple of deep lungfuls of sweet air. Proprioceive as much as you can. And then attempt, if you will — I am still doing so! — to imagine what it would be like to walk a few meters on the silty sea floor in the shoes of a sea spider.