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As far as I have been able to determine, the majority of life reuses the same plumbing for reproductive and digestive/excretory systems. In fact, the anus is believed to be derived from the male gonopore.

This is seen in all vertebrates and arthropods, as the anus and reproductive organs are derived from the same region. In cnidarians, where the mouth and anus are the same orifice, gametes are released from inside this blind gut. Only in planarians is the gonopore separate from the anus (which is also the mouth).

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    $\begingroup$ This really feels more like a biology question than a world-building one. $\endgroup$ – Ummdustry Jun 25 '18 at 18:55
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    $\begingroup$ Life could evolve differently, but having as few holes in your body as possible is a very useful trait and the fact that you keep it relatively clean by excreting is a bonus. $\endgroup$ – Demigan Jun 25 '18 at 19:17
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    $\begingroup$ @Demigan "you keep it relatively clean by excreting" the toilet paper industry disagrees. $\endgroup$ – Renan Jun 25 '18 at 19:19
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    $\begingroup$ @Renan I was talking about the penis. Men have far lower chance on inflamed bladders than women as peeing pushes any bacteria etc out before it reaches the bladder (or reproductive organs), which is also why men with inflamed bladders are treated with more concern as whatever made it is far more persistent than most infections. This feature has the happy advantage that with sex you arent first spraying a mass of bacteria before the children seeds. So Yeah its pretty clean! Aside from the piss. $\endgroup$ – Demigan Jun 25 '18 at 19:36
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    $\begingroup$ As the old joke tells us: "God is an architect. 'Cause only an architect could have put toiled plumbing pipes straight through a recreational zone!" $\endgroup$ – Oleg Lobachev Jun 25 '18 at 19:36
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Yes, of course.

In your examples you are only including a very minor portion of all life that exists. Granted, by having the arthropods there, you've included insects which alone may count for more than one million species, with no other class in the animal kingdom having more variety.

But don't forget that plants and fungi have no anus. Their reproductive systems are not necessarily linked to their excretory ones. Plants mostly excrete through their leaves, and flower petals are modified leaves, but many plants do not have flowers.

And then there are microorganisms. I think those speak by themselves.

Last but not least: virii. Whether they are alive or not is debatable, but supposing you count them as living, remember they have no excretion by definition.

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    $\begingroup$ The singular of “virii” is “virius”. ;-) Or, put differently, the plural of “virus” is “viruses”. But at any rate they have neither secretory nor complete reproductive system so I don’t see how they’re relevant here. $\endgroup$ – Konrad Rudolph Jun 26 '18 at 11:05
  • $\begingroup$ @KonradRudolph I see their capsides as part of their reproductive system. They do reproduce after all. $\endgroup$ – Renan Jun 26 '18 at 11:58
  • $\begingroup$ That’s a bit like saying that your skin is part of your reproductive system. $\endgroup$ – Konrad Rudolph Jun 26 '18 at 12:25
  • $\begingroup$ Plants and fungi do not have digestive systems, or any organ systems for that matter. Microorganisms are typically so simple they lack tissues. I do not think they are a good example, since my question relates to organ systems. $\endgroup$ – Anonymous Jun 27 '18 at 13:16
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Molluscs have separate excretory and reproductive systems. Among the hermaphrodite land snails, the reproductive system opens onto a genital pore on the right side of the head. Male squid pass spermatophores into the female's mantle cavity around her mouth.

There is some sense in using the "get stuff out of the body" system for both gametes and waste, but not all animals do this, and there is no reason to require it in any new creatures that you design.

Remember the Centauri of Babylon 5. They look humanoid, but the females have six slits on their back and the males have six corresponding prehensile penises, quite separate from their excretory system.

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    $\begingroup$ Used for cheating at cards $\endgroup$ – Separatrix Jun 25 '18 at 20:53
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Octopuses: Penis in a hand

You should take a look at the reproductive system of the octopuses. The male has the reproductive conduct inside (along) one of his tentacles (called hectocotylus), which by all means works like any other tentacle. When mating, he inserts that specific tentacle inside the female. You can see more detailed information here:
https://www.quora.com/How-do-octopuses-reproduce

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    $\begingroup$ You need to be careful when googling octopus sex by the way. The results are not always accurate. $\endgroup$ – pipe Jun 26 '18 at 7:21
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The fact that the same approach is used across several species to implement the same functionality (the functionality being excrete the wastes and excrete the seminal cells) is a prove of how effective this implementation is.

If you look at the implementation, it makes more sense to slightly modify an existing system rather than inventing a new one. The existing system being the tubing used for excreting the discards.

In this way the existing structures are used more effectively than the case where the reproductive system was completely separate. Having two separate lines, one for wastes and one for semen, would cost more in terms of energy, and would also pose the challenge of how implementing the complementary part for the receiving side (the female).

Not that another implementation would be impossible, rather it would have to be extremely more competitive to be successful in the present world. We cannot exclude that alternative implementation were present but did not survive through the mass extinctions happened in the past.

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  • $\begingroup$ “is a prove of how effective this implementation is.” — not really: it’s merely a proof that it’s ancient (and thus a common precursor). It could be woefully sub-optimal if it’s enough of an evolutionary one-way street to be hard to evolve out of. $\endgroup$ – Konrad Rudolph Jun 26 '18 at 11:07
  • $\begingroup$ @KonradRudolph, I am not so sure. We have had plenty of mass extinctions to reset the board, if no alternative model has emerged there must be some good reason. $\endgroup$ – L.Dutch - Reinstate Monica Jun 26 '18 at 11:11
  • $\begingroup$ Right but alternative approaches have evolved (see Renan’s answer). Just not in animals, which is where my comment comes in. In fact, I’m convinced that this solution is pretty bad (e.g. lots of diseases caused by it) and hard to evolve out of (due to being part of the fundamental body plan, which is highly evolutionarily conserved due to the fact that single mutations are almost invariably fatal). $\endgroup$ – Konrad Rudolph Jun 26 '18 at 11:12

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