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I'm trying to design a species that is largely aquatic and yet capable of walking / running (loosely) on land. I've already figured out a gill and lung type respiratory system to allow them to breathe in both air and in water, but I'm stumped on why a mostly aquatic being would have legs (or things that serve a similar function)... or ARMS for that matter.

The species is based mostly on octopi and squid. My plan was to give them the same basic features (very large eyes, beak, tentacles) but for the sake of my story they need to be able to move on land. I've also been thinking of the concept of their "legs" and "arms" having joints that deliberately dislocate to create a type of fluidity in the water so that those limbs aren't completely useless underwater, but I'm not really educated anatomically and I'm not even sure if that would work.

Tldr: basically put, how do I give squid-like mostly aquatic aliens arms and legs while remaining truthful to science?

Any ideas?

Edit: it's come to my attention that while octopi are very dexterous underwater they are rather clumsy on land and have a hard time lugging around their bodies. If I want to keep the general form of the octopi, how would I counteract this? Otherwise, I'm still searching for reasons and ideas on how to give them legs.

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    $\begingroup$ You might be interested in the Aquatic Ape Hypothesis. It's a theory, mostly disregarded, but nonetheless intriguing. The main proponent has a TED talk here. $\endgroup$
    – Mike Nichols
    Commented Aug 15, 2016 at 22:38

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I don't think you want bipedal. If they are octopus-like, they should not be limited to just 2 legs for locomotion. Octopuses are amazing and have no trouble moving on land for short periods (they're only limited by the fact that they can't really breathe air.) As for their limbs, octopi have no bones. They are unique, and I suggest you study the very specific and strange biology and movement of octopi--they have no joints at all, as result of having no bones.

Here's some great links: an octopus using armor.

how they move and manipulate their limbs.

they can open jars

they are smart and can escape almost anything.

I believe that they do have fine motor control, and have very good sense of touch. If you want to give them double tentacles at the end of two for a thumb-like structure, you can, but I would say that this is really a non-problem.

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    $\begingroup$ the problem is that the specific and delicate motor functions that we as humans have is lost between us and octopi. I'm hoping to create some sort of median between the two so that my aliens are capable of fine motor skills and yet also have the larger more fluid motion of them in the ocean. So we cut out them having legs maybe, what capability would a octopi have constructing a delicate instrument without the delicate motorskills of fingers and an opposable thumb? Otherwise I think you're very right. $\endgroup$
    – LOCHLAN
    Commented Aug 15, 2016 at 2:45
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    $\begingroup$ They are very, very adept at using tools and getting into practically anything. They are surprisingly flexible and have excellent control. If they had the motivation, they could construct a delicate instrument. They are amazing and have been known to use tools in the ocean. Simply put, they don't actually need thumbs. $\endgroup$
    – Erin Thursby
    Commented Aug 15, 2016 at 3:54
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    $\begingroup$ I added some helpful links in my answer. With eight appendages, they can really do anything we can. their brains are actually wired to use them in concert, as you'll see from the link there. There's no need for hands, legs or bones. $\endgroup$
    – Erin Thursby
    Commented Aug 15, 2016 at 4:10
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    $\begingroup$ The links have been very helpful and I think I'm going to go with no legs and arms then! Thanks a bunch $\endgroup$
    – LOCHLAN
    Commented Aug 15, 2016 at 7:29
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    $\begingroup$ you could use liquid skeletons $\endgroup$
    – Charon
    Commented Aug 15, 2016 at 9:28

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