I have a concept for a special kind of jellyfish that somewhat bears a resemblance to the generic slimes of videogames. This jellyfish was of the Cubozoa class and existed roughly 300 million years ago during the Pennsylvanian period. Sporting the same kind of advanced rhopalium as present day box jelly fish and complex nervous systems, these jellyfish evolved to expand their oxygen storage capabilities for longer than the average 48 minutes a normal jellyfish can survive on land before death. They evolved a reinforced mesoglea as well, expanding on their Cubozoan capabilities of active movement through muscular contractions to slowly drag themselves across land. (Not sure how fast.) These adaptions allowed them to traverse the swamps of the Carboniferous for one to two hours.
Early land jellyfish were detritivores, dragging themselves across swampland in order to eat. They looked like relatively flat pancakes, with tentacles splayed. Their rhopalium was incredibly advanced, able to see colour, light, etc, similar to modern day box jelly fish but even better so that they could move across land. Later on tentacles were used as collectors, dragging along detritus and passively moving it towards the jellyfish's mouth.
These jellyfish became more and more terrestrial in nature, eventually being able to store oxygen for many hours like turtles. A species of these jellyfish emerged, with even more reinforced mesoglea, enough to make it stand somewhat upright and was the size of a small baby. They had three rhopalium on their 'face', with two large ones acting as its main 'eyes' and a smaller one beneath mostly dedicated to light perception and contracting the mesoglea to become very hard. This species leaped at prey, contracting and hardening their mesoglea so that the force of impact would be enough to stun them. Eventually either body trauma or brain damage killed their prey, where they would drag themselves over and eat the prey over a long period.
Is any of this plausible and realistic? What are some prevalent issues?