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This one might be a bit weird, but here it goes...

In designing the evolutionary history of my world, I've devised an evolutionary branch of animals adapting to terrestrial environments. Lacking rear fins, the lineage has instead modified their tails into an impromptu limb, making them tripods.* However, here's the problem.

Another idea I had for these creature is that in this evolutionary lineage, post-anal tails never evolved - like in arthropods, the anus is placed right at the end of the body. Now that the rear of the body has evolved into a limb. The question is this -

Is it feasible for an animal's digestive system to travel down a limb?

Any answers would be greatly appreciated.

*If this is hard to visualize, imagine similar animals to the Sheather888's Tribbetheres from their Serina project.

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    $\begingroup$ I’m almost certain nature already got there first. It usually does... $\endgroup$ – Joe Bloggs Nov 23 '18 at 13:42
  • $\begingroup$ Any answer to this question is purely oppinion-based. Why wouldn't it be feasible? What complications or limitations do you forsee? $\endgroup$ – Elmy Nov 23 '18 at 13:43
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    $\begingroup$ Elmy Things like contortion of the digestive tract in a jointed limb, could a limb even function if po... digestive waste was encased within it. I'll try and give some more examples in time. $\endgroup$ – N Francis Nov 23 '18 at 13:49
  • $\begingroup$ @N Francis You didn't ask this on biology SE, so I'm giving a rather broader range of possible hints as to the creature that might have evolved. I've taken into account the ambiguity of the question regarding limb/limbs and you will see in my answer - the bit about ribon worms, that the digestive system does indeed travel - perhaps the length of the whole body. I hope this fits your story. $\endgroup$ – Measure of despare. Nov 25 '18 at 1:49
  • $\begingroup$ @Duckisaduckisaduck yeah, I get what you mean. Still kinda learning the ropes, I could've been more specific. Still, thanks for answering. $\endgroup$ – N Francis Nov 26 '18 at 15:49
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Scorpions have their as... Er, anus at the tip of their tail, right next to the sting. The while tail is basically bowels. Which only makes a scorpion sting even more gross.

Scorpion anatomy

Also starfish. Each arm has bowels inside them as well. They don't have the anus there, but that's just how they evolved - they could have a hole in each arm and still be functional.

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    $\begingroup$ There's also a type of scorpion that can lose part of its tail. Except when it "heals" it just heals over the stump, which closes off the bowels. They eventually die because of fecal buildup (Although sometimes it causes another segment of the tail to break off, but there's only so many segments...) $\endgroup$ – Andon Nov 25 '18 at 1:03
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Pycnogonid sea spiders are so slight in build that their digestive systems have to extend into the limbs, and then double back to end at a conventionally positioned anus at the rear of the abdomen.

enter image description here

They don't suffer from contortion of the digestive tract as you mentioned, and I see no reason why a leg couldn't function should it have faeces in it - the worst it can do is add extra weight, but remember - non-human animals have no qualms about not "holding it", unless they're distressed/unnerved (as many a dog owner will know), so the faeces doesn't actually stay in the bowels for very long at all.

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I'll deal with the ambiguity in the question by providing options.

What if a creature such as you propose evolved characteristics found in more primitive Earth creatures, how about "What if: It can impale it's prey with a paralysing venomous barb that comes from the same orifice that it poop through."

For a "Digestive System Within Limbs"- (plural) your creature design might need a combination of features many of which are already found in nature.

  • Since it's not an amoeba you're going to need:

Extracellular Digestion.

Extracellular digestion is a process in which saprobionts feed by secreting enzymes through the cell membrane onto the food.

Sure, this is most commonly found in fungi and don't houseflies start digesting their food before they eat it?

Humans use extracellular digestion when they eat. Their teeth grind the food, enzymes and acid in the stomach liquefy it, and additional enzymes in the small intestine break the food down into parts their cells can use.

  • Well, but a digestive system in the hands and or feet, how would that look?

An Armed Proboscis

enter image description here

the proboscis exits from an ... orifice. A typical member of this class has a ... calcareous barb, with which the animal stabs the prey many times to inject toxins and digestive secretions, after ... digestion, its tissues are sucked in ... Either side of the active (barb) are sacs containing back-up (barbs) to replace the active one as ... an active one is lost.

Ribon Worms can do it and here's the phenominal speed of it. Annoyingly I'm not able to Embed a Youtube video, so here's the link:

https://www.youtube.com/watch?v=_AkeEIzwjBg

  • Hellava handshake! But your prey is not going to like that very much, they'll kick up a fuss unless:

Paralysing Venom

The ... tooth is loaded with venom from the venom bulb and ... is fired from the proboscis into the prey by a powerful muscular contraction. The venom paralyzes ... almost instantly

If each of your creature's fingers behaved like the guts of that thing, (IE. Digestive System Within Limbs - plural) then at a pleasant soiree it could just casually feed on hotdogs, burritos, crisps as it went around chatting, or equaly without a care continue a conversation whilst it fed on a person or defended itself from attack.

  • Alternativley sticking more closely to your chosen three limbed morphology, with the two fore-limbs and the foot, it could turn its (Single) gut inside out like a hydra:

It can impale its prey with a paralysing venomous barb that comes from the same orifice that you poop through.

**You can impale your prey with a venomous paralysing barb that comes from the same orifice that you poop through.**

Leaving your creation's mouth free for kissing/light opera/chat - whatever. While it's Anus digests it's paralysed prey at it's feet/foot. :)

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