The chambered urchip have split from their ancestors, the shelled urchip. Although their ancestors had lost their ability to swim, the chambered urchip have regained the ability by creating hollow chambers within their shell, which can be filled with gas to allow for buoyancy. At the same time, the shell has become larger and the various segments of it have fused together, to make it stronger despite these hollows.
This new chambered shell allows them to float between the various crystal shrubite, however they have to be moved by the currents to take full advantage of this ability, as they have no way to swim otherwise. To compensate for this new ability, their eyes have strengthened, in order to see distant shrubites.
The entrance to their shell has two small flanges, which evolved specifically to help protect their eggs from predation. In a show of sexual dimorphism, two of the female's tentacles have once again become lengthened, in order to place the eggs into the flanges for safe keeping, where they are kept safe until the eggs hatch, at which time the young chambered urchips leave their mothers to fend for themselves.