The phasmadrill has replaced its ancestor the burysquid. The phasmadrill hasn't changed much from its ancestor, except for a few things. Its eyes are much higher compared to the burysquid; one pair is even on its tentacles. Thanks to this, they can also reach out for sinking corpses, like gilltail corpses. It has also aided in 'escaping' riverskippers. When a riverskipper is spotted, the phasmadrill holds its, now browner, tentacles against the ground, and avoids being spotted thanks to its new camouflage.
Their sac has gotten larger by 10 cm, shifting their proportions more towards the sac. The reason for this is that the phasmadrill will first let some of its kin grow up in the sac. Those that remain in the sac will always be female, while those that go in the fresh water always become male. This has to do with the temperature the eggs receive.
Another adaptation are vibrating setae on the phasmadrill's mantle when it starts digging. Once it starts digging, it will do the same as its ancestor, but due to the water sac filling and the setae being much deeper now, the setae vibrates once the water jet is released. This allows the phasmadrill to go much deeper, which it uses to sleep.