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Split off from the horrorstar, the terrorstar has modified its mouth to better tear flesh. Previously, muscles pulled on the skin of the inside of the mouth to pull it inside-out, creating suction; in terrorstar, the muscles attach to cartilaginous support structures—the “pelvis” inherited from its ancestor and a new pair of upper and lower “mandibles”—to pull the mouth closed, creating what is almost a proper jaw. In order to ensure it closes completely, the upper and lower mandibles are pulled through a loop of cartilage—modified left and right “mandibles” from the radially symmetric embryonic stage—which forces them to close. Doing this also forces the paired keratinous mouth spikes to close over the mouth; this helps seal the mouth shut, as they similarly did in the ancestor. All of this combined allows it to tear flesh with far more efficiency, though without the benefit of suction it has lost the ability to feed on worms or crystal flora. Its mouth is also now directly connected to its intestine, and it has developed a primitive stomach with digestive enzymes derived from what was previously a waste collection area, so it no longer has to digest food inside its mouth.

The terrorstar has also begun to develop the beginnings of additional cartilage support structures in its arms and digits. These are small and many in number, and somewhat resemble vertebrae. With these, it can swim with far more strength than it could before and with more efficiency. Its eyes have also developed further into pinhole eyes, which allow it to see detailed though unfocused images, which in turn helps it to identify prey and avoid predators. As it is able to eat with its mouth with so much more efficiency, it has completely lost the filter baits on its limbs. To better distribute oxygen and nutrients between its cells, it has also now developed a closed circulatory system powered by the motion of its flipper-like left and right arms. Though swimming pumps its blood, it doesn’t technically need to swim constantly to survive, as it is capable of absorbing oxygen through its skin and doing some amount of cell-to-cell nutrients transfer. However, if it never swims at all, it will still eventually starve if even if it is fed constantly and has oxygen-rich water flowing into its face. Its gill openings have advanced and are greatly enlarged, allowing more room for water to pass through even more gill-baits to absorb as much oxygen as possible. It pumps water through its gills through rhythmic motion of the gills themselves, rather than using its mouth like a Terran fish. Its blood carries oxygen using hemerythrin, which gives its blood a magenta color when oxygenated.

The terrorstar’s tail has been modified into a more useful tail shape. It can make very fast turns, a must for evading predators. This is accomplished with membranes stretching between the anal fingers; to allow it to still hold on while mating, the membranes can be retracted, freeing up the fingers. The fingers on its left and right arms are also fused in adulthood to make a more stable flipper shape; all fingers remain free in larvae.

Terrorstar development, showing the transition from radial larva to a bilateral adult.
The terrorstar is otherwise pretty similar to its ancestor. It uses its front arm to grab sessile and slow-moving prey that latch onto rocks or flora, and the rest of its arms are used for swimming. It reproduces sexually, with two individuals linking their anal arms together using their anal fingers to hold on; it still cannot distinguish between male and female, so it just mates with any individual it comes across with a 50% success rate. After each successful mating, 15-25 baby terrorstars develop inside the mother until they are large enough to swim out on their own, at which point they are less than a millimeter long. They are born radially symmetric with four jaws; as they mature they become bilateral, with the left and right jaws shrinking down to hold the upper and lower jaws together when the mouth is closed and the arms differentiating for their various uses. Newborns aren’t especially good at hunting yet, so they follow their mothers around and eat leftovers from her meals until their jaws are more functional.