What if they had a sort of "selfish mutualism" to their evolution, with a carnivorous plant? (I mean, I want to get carnivorous plants in here SOMEwhere XD.) At least, it was selfish in their early history - some clever individuals worked out that they could "skim off" a sort of nutrient-rich protein-shake from the bucket of a pitcher-plant, and took to "feeding" the pitcher to encourage it to make more protein. Lepuuri can "chew", with a pair of radulae, but it's hard work and their diet is primarily liquid.
The partnership developed, and now the two individual species are quite reliant on each other - the plant relies on the lepuuri for a portion of its diet (they do lure insects, and can photosynthesise, but aren't very efficient at either) and the lepuuri use the plants to help them digest proteins, and as "guard dogs" (the snappy ones).
The plants themselves are still sessile - we're not talking Day of the Triffids, here, they can't get up on their root stumps and walk about - but capable of more elaborate movements (they have a sort of "hydraulic musculature", utilising a specialised elastic xylem/phloem), and have a well-developed nervous system. (I'm basing that part to a degree off the mimosa, which has always fascinated me by the way its leaves all fold up if you brush against them.) Smart plants, woo.