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alphynix · 1 year

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Strange Symmetries #07: Gastropods Do The Twist

Gastropods – snails and slugs – are a group of molluscs that originated sometime in the Cambrian Period, with the earliest definite stem-gastropods known from around 510 million years ago and the first true gastropods turning up in the early Ordovician.

The spiral-coiled shells of snails are their most familiar feature, giving them obvious external asymmetry, but gastropods are also defined by a specific type of internal asymmetry known as torsion.

Torsion is an anatomical process that occurs during larval development, and involves rotating their internal organs, mantle, and shell a full 180° relative to their head and muscular foot. This twists their gut into a U-shape, knots up their nervous system, and brings their respiratory organs and anus up close to their head.

And we still don't really know why they do it.

One idea (the "rotation hypothesis") is that it originated as a defensive function after early gastropods began developing their spiral shells. The shell opening may have originally been positioned at early gastropods' rears, meaning they retracted their bodies back-end-first leaving their heads and sensory structures still vulnerable – but twisting the shell around would allow them to pull their front end in faster instead.

A competing idea (the "asymmetry hypothesis") instead proposes that the shape of the coiled shell restricted the gills of early gastropods, which may have originally been positioned in mantle cavities on each side of their bodies. In response to this they developed a single larger gill cavity on just one side of their body, and then gradually expanded and rotated this asymmetric feature around to the front for better aeration.

In either case this resulted in some of the rest of their anatomy "coming along for the ride". And regardless of whatever the original evolutionary advantage of torsion actually was, it made gastropods incredibly successful – they're a massively diverse group, second only to the insects in terms of sheer number of species, and today they're found all over the world in almost every habitat from deep sea trenches to high mountain elevations.

Spinyplatyceras arkonense lived in what is now Ontario during the mid-Devonian, about 391-385 million years ago. Around 5cm long (2"), it was part of a group of Paleozoic marine snails known as platyceratids, which were probably related to either modern limpets or neritomorphs.

Platyceratids seem to have had a unique parasitic relationship with crinoids, attaching themselves to the top of the host's body and using their radula to drill into them, either robbing food directly from the crinoid's gut or feeding on its other internal organs.

The long spines on Spinyplatyceras' shell probably helped to deter predators. In an interesting case of coevolution the crinoid hosts of some platyceratids developed their own defensive spines, too – and it seems this wasn't to prevent the snails from infesting them, but to also discourage the snails' predators. These crinoids may have been frequently indirectly injured during snail-eating predators' attacks, and it might have actually "cost" them less to keep enduring an infestation than to deal with the collateral damage of the snails being removed.

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#science illustration #strange symmetries #paleontology #paleoart #palaeoblr #spinyplatyceras #platyceras #platyceratidae #gastropod #mollusc #invertebrate #art #name checks out; that platyceras sure is spiny

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ivory--raven · 3 months

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day 7! angelfish femslash thingy. Continuing directly on from yesterday but this time more in Dagon's head.

“Let me take care of you,” says Michael, reaching out. Dagon falls into her, desperate, aching, needing. It’s been too long. Has she always needed her like this?

“Do you have a… living space, somewhere?” Michael asks.

Dagon lifts her face from where it’s been buried in the angel’s neck and gestures to the back wall, which is almost entirely made up of cabinets. It’s not really the wall, though - there’s a gap, and she maneuvers them both through it, still clinging to Michael like a limpet. “It’s a bit - sorry.”

Michael slides both arms around her waist and holds her. “Dagon, my own, this is terrible.”

Behind the false wall of cabinets is the pocket of room Dagon lives in, when she bothers to stop working, the part she’s barely left lately. There are clawlike marks gouged into the floor. Papers are scattered everywhere, all destroyed, and the moth eaten blanket she’d been curled up in looks filthy now she’s abandoned it. It’s picked up all the crumbs and grease from the floor.

“This won’t do at all,” says Michael decisively. “Listen. Jeanne has that house, now. I know these aren’t ideal circumstances, but how about you come there now?”

“Here’s fine,” says Dagon, even though it’s not, she sees that it’s not. She wants to come.

“It’s really not. Need anything?”

Dagon shakes her head, clings closer. You, she doesn’t need to say. She needs Michael. She has Michael.

“Okay. I’m going to just…” Michael snaps her fingers. They emerge with a gust of cold wind on a field, some sort of wild meadow with a pond and a house, its slanting roof covered with moss. “Jeanne said no nepotism, and she filed everything herself - you’ll be proud - but if they were incentivized to hurry her things along, well…”

“Of course you did,” says Dagon. She doesn’t expect any less of her.

“I’ve seen it once,” says Michael as they reach the door, which is painted a rusty red, “there are some things I think you’ll like.”

The house seems bigger on the inside, but maybe that’s just how everything is laid out. There’s a spiral staircase, twisting up and around like a snail shell, elegant and wooden with iron railings. It doesn’t look much like Hell. The colours are vaguely familiar - if Hell was cleaner, kinder, home - but it’s not. Is Heaven like this? Dagon remembers being young, being new - and then the feeling is gone, like it was gone then, the fight for justice, the war, the Fall, plunging into something new and strange and scary and dark, the bleeding, broken bodies, still alive because death was more mercy than they deserved.

(Michael won’t -)

The fear is an urchin, gnawing on the holdfast of her heartstrings. It’s got teeth and claws and spines, but Dagon’s teeth are sharper and scarier and stronger.

There are sweet-smelling rushes on the floor. That’s a very human thing, Dagon thinks, Hell never cared to keep the place nice and she can’t imagine Heaven, now, as anything but the cold sterility that clings to Michael’s clothes. Jeanne is human. So perhaps the rest of the house is human, too. Earthly. It’s Earthly.

“Jeanne?” calls Michael. There is no response. “She must be out.”

Michael sounds unhappy. Jeanne is a competent girl, and she’s a good leader, a strategist, a speaker, she’s very aware of her surroundings, but she is these things because she was a soldier in the thick of battle and then she was a prisoner and then she was dead. Martyred. Michael knew her for six dangerous years of her dangerous life, and now she has to let her go.

Jeanne is nineteen, has been nineteen for some time. She’s old enough to handle herself. She’s capable of handling herself. She’ll come back soon. Dagon trusts her.

Of course Michael doesn’t.

“Nevermind,” says Michael, shakes herself out of it. “She’ll be pleased to see you when she comes home. In the meantime, come upstairs.”

They do, together, because Dagon isn’t ready to let go. “Jeanne’s room, she means to bring her sword collection,” says Michael, “a sitting area by this window, in the sun, and here - this one’s for us.”

It’s a beautiful room. It’s very elaborate. The walls are paneled, the bedposts are carved, and there’s a canopy hanging like an umbrella over the bed. It’s really more a sequence of rooms - Michael ignores the bed, which Dagon would very much like to collapse onto.

Then she sees it.

It’s not a swimming pool, and it’s not a fishtank, and it’s not a bath, but it reminds her of all those things. There are stairs leading into it, it’s full of water, and it’s big enough to stretch all the way out in and not touch either side.

She loves it.

Michael snaps her fingers, and it’s warm, has Dagon’s body always been this achey, this needy? “Get in,” says Michael, and then as Dagon takes a haltering step towards it, transfixed by the lights reflecting on the surface, “take your clothes off first! Honestly.”

Dagon obediently peels her clothes off, looking back at Michael. Michael narrows her eyes and makes a displeased humming noise, which is entirely the wrong reaction. Dagon must be making a face, because Michael raises her eyebrows. “The scales on your back are in a horrible condition. Get in the water and wait for me.”

Dagon slides into the water. It’s warm and welcoming and it fairly sings to her, so she dips her head underwater and lies on the bottom. The only thing missing is Michael. She trusts Michael will come back soon. Michael is okay. Closing her eyes, she can feel her hair is snarled in a tangle behind her head. She cracks her neck and feels the gill slits she has there opening.

A hand brushes her shoulder. Michael. She surfaces and opens her eyes. Michael has with her an assorted collection of creamy looking things in jars and brushes and combs and rough-looking tools. Michael is also mostly naked.

“Be good,” says Michael, as if Dagon isn’t a demon, as if she can. Nothing she does is ever good by definition. Still, she’ll behave because Michael is asking. For now.

Michael scrapes the peeling, flaky bits off Dagon’s scales, rubs the dry skin around them with careful fingers. Dagon is in silent bliss. Michael’s touch feels right. She knows her body, knows how to make it stop hurting in places Dagon didn’t realize there was pain. Mostly, she thinks, it’s good that Michael is here. Michael is okay. They’re both okay. They’re all okay.

#femslash omens #angelfish #ineffable administrators #femslashfeb2024

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