Rainbowt
The rainbowt derives from rainbowhedrons that lost their top plates. This resulted in the shell having a cup-like shape. The inner chambers joined together and expanded so that the soft core shrank to a network shape across the inner surface, meaning that most of the inside shell was now exposed to sunlight. The cup shape was highly stable, with a heavier bottom plate, which was useful as the rainbowt slightly increased in size. The interior contains a small layer of water over the base, but the interior water level is kept lower than outside, thus maintaining buoyancy.
The rainbowts keep a consistent lifestyle of floating at the ocean surface rather than rising and falling like their ancestors did. They most readily took to high-lattitude waters because their geometry was well-suited to the sun's low position in the sky; the low sun could illuminate one of the side plates. The inside of the shell and the upper portions of its plates were bright green, the usual crystal flora color for high sunlight. The bottom plate would rarely be exposed to light, however, and used their stark dark red pigmentation, with a transition through the spectrum between these two zones.
Like its ancestor, the shell is covered in microscopic pores, which let in food particles and can adjust the amount of water or air on the inside. The pores open and close using tissues from the soft core on the inside. These pores allow some filter-feeding but are small enough that they more often take in particles from dead organisms than living cells. The inner core secretes a thin layer of digestive enzymes, which breaks down any food particles that settle on its surface. Further away from the core, however, is more hospitable, and a lucky enough cell may survive to escape.
They use the same pores for reproduction. The shell and core both spawn gametes, separately, which can swim in through another rainbowt's pores. (Self-fertilization is also common.) The core rejects the gametes as food, so they will float off in the more survivable parts of the interior water layer. Once a rainbowt has been fertilized by both types of gametes, it will grow a tiny rainbowt inside it, little more than a few cells of core with a shell along the outside. This exits through the pores. They are only about 30 micrometers at birth. If they live long enough, they'll grow a hundredfold, but the ocean is full of the more common and much smaller young.