Vinagobble
The vinagobble descended from vinagoblets that further developed their burrowing abilities, so that they now lie entirely under the surface, aside from the gaping mouth.
As before, their larvae resemble smaller vinagobs: free-swimming organisms shaped like cups around a big stomach, with two rows of four fins, bioluminescence, and vents for sucking in and expelling air and water. They also grow thin digestive hairs within the stomach and on their fins.
In adulthood, however, they burrow into the soft silt or sand in a shallow area, which may include the tidal zone. They bury their entire body underground, the tail boring through the ground, with only their mouth facing upward. They lose bioluminescence. The four lower fins turn into hooks. The four upper fins grow upward as tentacles, breaking through the surface. (The tentacles split in two near the tip, but this is mostly a vestige of the original fins' shape.)
The thin digestive hairs on the tentacles are capable of absorbing and digesting passing microbes. However, these hairs are also highly sensitive to vibrations. (This is their main sense; they lack sight.) This alerts the vinagobble to passing organisms, and if such an organism is of an appropriate size, the tentacles will grab and ensnare it and feed it to the waiting mouth. The stomach also absorbs nutrition through its hairs, but it additionally contains ribbon-tailed detriti microbes, which break down trapped food until it is ready to be absorbed.
They can secrete a thin membrane across the mouth to resist desiccation... and to hide beneath, as it is liable to become covered in silt, meaning that an unwary creature crawling across the sea floor may break the membrane and fall through to its doom. And woe betide any interweb that thinks the vinagobble is a good place to stretch across, as it will soon find itself digested.
Vinagobbles retain their ancestors' system of air pockets, now distributed more widely through the body, but the pockets become filled with a series of adjustable fluids, as they no longer have access to surface air. In the larvae, some of these vents face outside the organism and some inside, but once the adult has buried itself, these vents only face into the stomach. Some produce a flow of water inward (useful for eating when the membrane is absent or broken), while others expel it. Vinagobbles often collect a layer of inedible debris when the membrane is broken; as this is undesirable, outward water flows can clean them out. Furthermore, they can set up a directional flow in which they take in water on one end of the organism, and eject in from the other side, producing a steady stream for filter-feeding.
Outward water flows are most useful, however, for reproduction. Vinagobbles have four reproductive organs, one at the base of each hook. These constantly produce eggs and sperm. Their ancestor ejected sperm outside of the body, but being underground made that impossible. Instead, they're redirected into the stomach, and shot up quickly enough to keep them from getting digested. They retain eggs in their body. Sometimes they will receive sperm and use it to fertilize their eggs. Either way, the eggs develop into larvae within a sac; most will be clones of their parent but not all. These larvae are likewise ejected into the stomach and out into the wider ocean.