Amorphous

Not every entity is constrained by a rigid skeleton; some organisms possess a malleable shape, or are made of a host of smaller beings, and can alter their forms as desired or due to changing environmental conditions.
A creature with this trait may increase or decrease its Size trait value by 1 as a Free Action (though doing so does not change its movement values). A creature with this trait also has the Crawler trait.
Dark Heresy - Core Rulebook - 134