REP: The Multilingulates

Albeit resembling brachiopods a lot in external appearance and lifestyle, the multilingulates had a completely different past, since they evolved from entirely vagile animals. Their ancestors included large, pelagial filter-feeders and even some carnivorous species that evolved their back plates into jaws. The larval forms of the recent species are nectonic and resemble their ancestors. Multilingulates can be found worldwide and adapted to live on all kinds of surfaces and biomes; some species live exclusively on large marine animals like squid whales.

Cannonball: a species with a round, robust shell and short tail which lives in tidal zones and other places with strong currents. Their tail suckers excrete acid to enforce the connection to the surface they are living on.
Mudtail: a suckerless multilingulate evolved for muddy environments. Albeit their tolerance for low oxygen levels, the adult stages are able to crawl over the ground if the oxygen level drops too low.