The midnight gill diverged from the midnight star by increasing to an even greater size, now a comparatively lofty 20 centimeters. Its changes are mainly adaptations to this growth.
The midnight gill retains the typical asterzalian body plan of four arms diverging from the central body, one of which contains an anus. The opposite arm has increasingly specialized as the front, now held in more of a forward position than a downward one. The two middle arms are stronger than before, specializing as paddles for propulsion. To better distribute nutrients through the larger body, they have developed blood, using the pinkish-violet pigment hemerythrin, which is integrated with a hydrostatic skeleton. The midnight gill maintains its internal pressure at the same level as the external water pressure, which can vary considerably throughout their vertical range; they are well-adapted to adjust internal fluid pressures as part of their overall hydrostatic system.
The biggest challenge of the new size was respiration; absorbing oxygen through the skin was no longer sufficient. In the midnight star, long bristles extend between the arms for filter-feeding. In the midnight gill, this is still the case between the front arm and middle arms, but the bristles between the middles arm and back one have become gills. They now branch into a luxuriant display, its large surface area ideal for extracting oxygen from the oxygen-poor ocean depths. (Hemerythrin gives the gills a pinkish color.)
The remaining filter-feeding bristles collect the tiny organisms the midnight gill feeds on. The digestive system has increased in complexity to better nourish the larger body. The bristles' openings serve as mouths, feeding into digestive tubes in each of the three frontmost arms, leading to a newly developed digestive pouch in the central body. Two small pumps (one left, one right) maintain the flow of blood to the digestive pouch and around the body. One small pump in back pumps blood in and out to the gills.
The front arm is developing some features of a head. The four original chemoreceptive patches all face in its direction, and four more chemoreceptive patches have developed on the front arm itself. Furthermore, a series of bristles grows along this arm's upper edge, derived from the filter-feeding bristles, but softer and filled with nerves. These sense vibrations in the water. The brain has moved closer to these sense organs and is now at the base of the front arm. This vibration sense combines with the chemical sense to detect disturbances in the water and others of its species.
With their increasing complexity, fragmentation was no longer a viable way to reproduce. The midnight gill instead forms buds on the ends of its arms. Once these buds are a centimeter or two in size, they detach as free-swimming larvae. They also have the beginnings of sexual exchange. Midnight gills, after sensing the presence of their own kind, can excrete gametes: undifferentiated cells with a tough encasement. Once taken up by a filter-feeding bristle, these gametes work their way through the body, and the host will use them to contribute genes to the next bud.