They appear to be very dark, overly large worms - perhaps a handspan in length and half as thick as a finger. That and their matt, dull skin marks them as something other than a typical earthworm.
Chekov's Spell -- This spell has a two-fold effect. The first is to make the target item almost-unnoticeable to anyone not specifically looking for it. The second is that someone who has seen the item will remember it when they encounter a situation that would be perfectly solved by use of said item.