Alright, the Futank is the big fat one in the back.
The Irgise is the long skinny one in the front.
When that drawing was made, the Irgise was about a foot long.
The Irgise has compound eyes, allowing for perfect vision, as well as sensing movement and smells through the water using those things around it's mouth. Obviously, it has a lot of teeth; it also has a low-power sonic screech ability, and eats rocks. It has that segmented spiky armored shell (It's ancestors used to use a technique called the SPIKED BALL O' DOOM!!! before they got too long to roll up into a ball), and in fact it's whole body is segmented within the shell.
The Futank was even more heavily armored than the Irgise, with a wide enough mouth that it could swallow an Irgise whole. It could suck in water to pull us towards it. It also had that mottled bumpy exterior that looked like rock until it started moving. It had developed legs, allowing it to chase the Irgise; these long slender legs did not have anything on the end to grasp with, though. It had four eyes, and four nostrils, developed to track prey.
As for behavior, the Irgise at this point was a territorial builder. It would use it's tongue and teeth to move rocks about to build large underwater towers, and chase away any predators; it would give refuge to harmless creatures, especially if they could be exploited to defend it's towers. The Futank was definitely a predator, stalking faster prey through the river networks, disguising itself until slower prey wandered past.
That's how I remember it.