Board Thread:Warhammer 40k General Discussion/@comment-6589306-20130228113457/@comment-3258762-20130302095821

You can see it pretty clearly on the drones (the Xenomorphs in question) that they have an endoskeleton. If you look at them, you can see that the bones are covered in oily skin the way a very malnourished creature would have it. If anything, this would be a liability, as if the skin is damaged, there is little chance of it mending. A Tyranid, as you can clearly see, has an exo and endoskeleton, giving them two fronts of protection.

Plasteel exists in 40k, it's a much cheaper variant of the common metals used such as Adamantium and is not very tough at all, it can not come close to compare to Ceramite.