I mean it's quite evident that their "shells" are just strapped to their backs, you can even see one of the Kojin with his shell off while he tries to repair it.

I'm not even sure where are they getting those shells from, probably real turtles. Sure enough those are not part of their bodies because turtle shells are basically made of hardened skin, real turtles really can't get out of those.

Why are they even carrying around those shells? I guess that's just one of their traditions or something. Maybe they serve as backpacks like those giant gobbiebags?

Their calling "people" shell-less is even more ironic than Ixals calling them "featherless".

Also are those their actual faces or are they wearing masks?