going on a limb to say niehara's true form would be something related to the earlier chameleons. itou points the detail that there werent that many in the first place, so the teacher could have been replaced by one while the rest sit waiting to replace more of the class in the tanks. would also explain why he say itou killed a 'subordinate' at the end, rather than killing a piece of himself, which was my original line of thought. also, the apparent parallel between their camouflage and the 'type' of monster he classified himself as.