Me: "This is going to be a story done for fun, no extensive worldbuilding."
Also me:

The sporobiont is a sessile predator/filter feeder, later some species developed a symbiosis with algae and became autotropic.
The gametobiont is a detritivore and sifts through sediment for edible particles. It moves around mostly by walking, but is able to swim with its tail. Later, the first three limb pairs developed into mouthparts, leaving only two hind pairs for locomotion. The antennae are ordinary sensory organs, as any metaphysical abilities only evolved in a land-dwelling clade much later.