Arsnoot
The Arsnoot split off from the Starcrusher and developed adaptations for a benthic durophagous lifestyle. Its face is narrow, useful for snatching passing prey while allowing room to use its lateral mandibles which have become stronger and thicker for the purpose of crushing shells. This allows it to have hard-shelled organisms as its sole diet. Nautstars are especially important to its diet, as they are abundant across its entire range, though it also consumes worms and crystal flora. As this inevitably means it has a lot of mineral content in its diet, especially calcium, it deposits this in its skeleton for storage and later use. In combination with other materials this creates hydroxyapatite, which turns its skeleton into true bone. It has similarly calcified the keratin in its teeth and jaw spikes, strengthening them for its new diet. Its eyes have advanced, developing into complete camera eyes with a lens that can be focused and a nictitating membrane to clean and protect it; a ring of bones set inside the eyes helps hold their shape, as they are not perfectly spherical and their sockets are not sufficient for that purpose.
The Arsnoot has also developed new bones for support and muscle attachment. Derived from ribs and similar processes, it has developed “scapular ribs” and “sterni” associated with all of its arms. The purpose of the scapular ribs is to reduce the force needed to move its lateral arms up, down, and forwards by serving as levers for additional muscle attachments, while the sterni, which are directly attached to the pelvic ring with a fixed joint, provide both muscle attachments for moving the lateral arms backwards more effectively and support for the sockets of all limbs to reduce the chances of them being dislocated. Due to homology and its radially symmetric larval stage, these bones are present in all four of the Arsnoot's arms, even though the scapular ribs are useless in its tail-like anal arm.
The Arsnoot retains its raptorial arm for similar use to its ancestors—grabbing food. However, it also uses its raptorial arm as a leg when it is resting on the ocean floor in a characteristic “sitting” pose (pictured), which keeps it at a slightly upward angle at rest and allows it to rise to the surface from its resting position far more quickly than it would laying flat. It has developed a pair of hearts near its lateral arms, derived from repurposed musculature that ran under the pelvic ring, which allows it to pump blood without moving constantly; this grants it greater stealth in the presence of predators. To support this position, the vertebrae in its raptorial arm are leg-like and only 6 in number; the first two are short, granting high flexibility, while the remaining four are longer for strength and leverage.
The Arsnoot is otherwise much like its ancestor. It has magenta blood, a skull derived from vertebrae, four lungs set in its pelvic ring, its eyes bulge when it bites down hard, and it gives live birth to radially symmetric offspring. Its parental care instinct has advanced so that a female will keep track of her own offspring to protect them from predators in addition to leaving behind scraps for them to eat.