Wormstem
When thornworms crawled their way eastward into the Dixon-Fermi Temperate Sea, they found a relatively barren environment. As a result, they came to rely increasingly on photosynthesis for nourishment. No longer needed for protection, their thorns grew into broader leaflike structures. A sessile lifestyle was useful for gathering sunlight. They would settle on a surface, using additional thorns as anchors (albeit not very firm ones), and grow upward to reach toward the sun. A segment near the tail would form the base, and they would curl upward on both sides. The former head would grow tallest, and retained a mouth, while the former tail curled up next to it, with an anus. The digestive tract would keep pumping water through for respiration and to provide supplemental filter-feeding. A mouthpart extends from the mouth, a muscular proboscis that pumps the water inward. The thorns grow even more, especially the ones at the tip of the head.
Thus, they developed a lifestyle in which juveniles keep up a mobile lifestyle, ranging across the sea, scooping up detritus, constantly growing by adding new segments. On finding a suitable location, they anchor themselves and curl into a J-shape, transitioning into an adult, a much lengthier stage of their life cycle. Adults often settle in patches near each other, growing upward to compete with each other for sunlight. They continue to grow by adding new segments near the base.
Each segment consists of a soft core (originating from the detritivorous line) and a hard green photosynthetic chitinous shell (originating from the photoedo line). These hard shells make them quite resistant to desiccation, and as a result they can be found well into the tidal zone or even poking their heads above the surface of the water. However, they still need water to be present at least some of the time to breathe and reproduce.
They reproduce by spawning. Juveniles begin producing sperm fairly early in the mobile stage, which they continue to be able to produce throughout adulthood. However, after adulthood, they also begin producing the more expensive egg cells. These "sperm" and "eggs" are all spawned in units of two connected cells: one to form the soft core and another to form the outer shell, as they are genetically distinct from one another. The egg cells come in capsules containing multiples of these pairings along with packets of nutrients. Thus, once fertilized, the juveniles come in a small group, with a ready boost of energy. In scarce conditions, the strongest juveniles may eat the rest; in more abundant conditions, they may migrate together and form a grove in their new home.
They have very little in the way of a nervous system, which atrophies upon adulthood. Juveniles have a range of muscles, anchored between segments and using the gaps between as joints, but adults only retain simple muscles at the base of each thorn, which they can use to track the motion of sunlight. Aside from this heliotropism, they have no sense of sight, but they do have some ability to sense chemicals and basic vibrations.