The arachnotetrapods are a type of animal in my worlds with an anatomical setup that is hard to figure out

Specifically, it has an arachnid-like intestine, with many branches going out into the limbs, but the limbs have an internal skeleton with a series of long bones, as in tetrapods

This presents a problem for their anatomy: In arachnids, the intestinal branches go down the middle of the limb. However, in tetrapodal limbs normally have bones and muscles running there, with no internal spaces for the intestines

Is there any way that these intestines could fit into these limbs?

The answers must relate to the combination of the arachnid intestine, which extends into the limbs, and the tetrapod limbs, which are supported by internal long bones. Do not mention other organs or structures besides the limbs and the branches of the intestine. This question is specifically about the structures described above (limbs and intestine), nothing else

  • $\begingroup$ you can but you will suffer the same problem the spine has, (which also has an organ running down the center), it will drastically limit the flexibility of the joints. $\endgroup$
    – John
    Commented Feb 9, 2022 at 21:44
  • $\begingroup$ Arachnids' gut ceca only just barely extend into the first section of the legs (think the hips/shoulders). They don't have these legfuls of intestines you seem to think they have, I'm afraid $\endgroup$ Commented Feb 9, 2022 at 22:06

3 Answers 3


Ribs & shoulders ?

Q: "Is there any way that these intestines could fit into these limbs?"

A little frame challenge

I always thought spiders had their abdomen and intestinal tract in the anterior side of the body. What would be the purpose of pumping fluids down and up spider legs ? A spider needs to weave its silk with spinneret glands in its anterior (back), these useful excrements certainly don't travel via the legs.

This is only a little frame challenge. When I google around for physiology, I do see extensions from the stomach toward the leg, like illustrated like this,

enter image description here http://what-when-how.com/animal-life/subclass-arachnida/

Another diagram elsewhere shows these "gut secum" loops as part of the stomach itself,


.. but these intestinal "loops" are quite short and very near to the pumping stomach. Make some room inside..

Ribs and shoulders

You may be able to widen the posterior (front) and give it a suitable bone structure (or chitine plate) that sits below - or contains - these stomach extensions. The pumping stomach will rest on a rib cage, like in tetrapod mammals and it could have 2 pairs of shoulders, that provide room for the extensions. You may need to add muscle anyway, when the legs have bones.


You can do it by using hollow bones, with the intestines developing along a spiral enveloped inside the hollow space, with the core being bone marrow, if needed.

Something mimicking a glass condenser, but with the spiral going back once it reaches the end:

enter image description here

  • the outer part would be the bone
  • the inner spiral would be the intestine

You would need only two holes for letting the intestines in and out go from the abdomen to the leg.

If you instead opt for the single way route, you could have your creatures defecating from the far end of their legs.


Easy Peasy

Tetrapods already have an analogue exactly what you're looking for: blood and lymph vessels.

Your arachnotetrapods will develop digestive invaginations that grow along with the developing limbs. Their arachnid-like intestine, with its many branches going out into the limbs will simply run parallel to the circulatory and lymph systems, if present.

If you want a high pressure system, their digestive fluids could be pumped through the body by a gastric heart.

If you want a low pressure system, those fluids can be managed by tissue pressure and the vessels can be unidirectionally valved so everything flows the right way.


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