Alright, B was the obvious, favorite choice. Hopefully, the votes move fast enough for two or more updates, today.
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Eventually, our creatures simply...move away from the threat, even though they move away from the primary food source of algae, however, down on the bottom, in the murky silt that had begun to build up over the millions of years, where waste and the carcasses of other creatures fell to be devoured, as well as "lesser" organisms that fed on the same things. Eyes grew fairly useless in the muck, and weakened a bit, but for the quality of eyesight that was lost, much was gained in the form of gaining two forward, weak, jointed arms with a pair of tiny little claws on each that were used to gently probe through the muck and grab bits of food that it could find.
The jets that used to be primary movement grew to be obsolete, and only one remained in front to function as a rudimentary mouth, while the rear ones were kept to function as a self-defense mechanism to stir up silt and debris to blind any Plakth that came bravely down to the dark, murky bottom. Population was kept at a fairly stable level, and grew slightly isolated from the rest of the biome.
One group fragments from the mainstream population by venturing far too close to a large aquifer, moving to a wide, if dramatically shallower pool. Thick green foliage is planted along the edges and even in the murky silt that was much more pronounced here than any other place, and was unsuitable to Plakth, as they couldn't see in the muck, and unsuitable for algae as just the bare top was uncovered with stirred silt from the rough current that was draining to other pools that held more of the thick flora. Here, our creatures flourish by devouring rotten plant material, and smaller organisms, growing much dramatically larger in size, now nearly four millimeters in general size, and nearly symbiotic with the plants, as the removal of old plant mass granted more space for new plants to grow.
Other organisms, albeit smaller ones, live on the dry surface, notably, a species of social arachnids that create a series of webs to form colonies among the small leaves of the primitive foliage, though they posed no threat since they were incapable of swimming or breathing in water. Other larger and smaller species of other indepentent arthropods lived on the rocky surface, where the soil was thin and fairly sandy.
A - Develop a form of amphibious lungs, and take advantage of decaying matter and small(er) animals on the shoreline.
B - Further develop the mouth, so as to begin to eat harder substances, and munch on some of the plants that grow here.
C - Begin using large towheads of plants as instinctive, communal nesting grounds.
D - Find some way to get at some of the other organisms that live in the plant "canopy."