Woodlice & Water Slaters
The Malacostraca is one of a handful of Classes that make up the Crustaceans. The Malaostraca itself includes many Orders, most of which are marine animals, but also includes the Order Isopoda which includes the terrestrial Woodlice and fresh water Water Slaters.
In the UK there are around 40 species of woodlice and water slaters. It is almost never heard of to have both of the two species of water slater found in the UK, so by having one (the common Water Slater) it is unlikely we will ever see the other.