I'll try to answer this sensibly: The 'tail' of Sligs is a pair of vestigial legs which have merged together. Normally, you'd think the anus would be from behind, which is generally suitable for the purposes of many animals. But due to how the Sligs crawl, it would seem liable that evolution moved it to the front of the animal, so all waste comes out the front, instead of the Slig crapping all over itself. This system would force the Slig to then crawl through it's own waste, so thoughts would be that maybe an extendable tube has formed so it can move it's waste to the side of itself. Otherwise, if they live in swamps, it may not really matter so much if their underside is in goo half the time anyway.
I hope your happy now. Oh, and the reason you can't see would be either A) it's retractable to prevent it sliding through mud and dirt and chances of the inside getting infected or B) the designers just didn't think of it and never bothered putting it there.
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