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Octopuses are cephalopods, which have separate anal and oral openings. Indeed, descriptions of the cephalopod GI tract clearly depict an anal opening.

However, I am very confused about how this applies to the octopus. I don't think I've ever seen the anus depicted - where is it? What does it look like? Does anything much come out?

My first instinct would be to simply do a google image search for this. Unfortunately, those results are flooded with unrelated hits (apparently the octopus is a popular motif for tattoos... in a certain part of the body).

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The anus of Octopus is channeled into its siphon. enter image description here Image taken from Carina M. Gsottbauer

Note:

Siphon is a tube that leads from the mantle to the outside. Octopuses use their siphon to force water out in jets for propulsion and to flush waste products from the anus.

From Encyclopedia of the Aquatic World, Volume 6 By Marshall Cavendish Corporation

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    $\begingroup$ And here I was thinking that the siphon just ejects clean, harmless water... I'll keep this in mind next time I encounter an octopus! $\endgroup$ – Superbest Aug 16 '16 at 20:57
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    $\begingroup$ I would imagine that you'd probably have to worry more about that poison gland than the highly-diluted waste that is ejected from the siphon. $\endgroup$ – Robert Harvey Aug 16 '16 at 21:22
  • $\begingroup$ @RobertHarvey Isn't the poison of Octopus injected through a bite? thecephalopodpage.org/octobite.php $\endgroup$ – Tyto alba Aug 17 '16 at 6:10
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    $\begingroup$ @SanjuktaGhosh please add citations to your answer on where the image and quote originated. If you use images and/or text that are not your own words, you need to cite the sources. $\endgroup$ – MattDMo Aug 17 '16 at 16:24

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