With this object, the front pads fill out and provide support, while those behind shrink and leave free scope for their segments to contract.
With the double support of its back and belly, with alternate puffings and shrinkings, the animal easily advances or retreats along its gallery, a sort of mould which the contents fill without a gap.

But if the locomotory pads grip only on one side progress becomes impossible.
When placed on the smooth wood of my table, the animal wriggles slowly; it lengthens and shortens without advancing by a hair's-breadth.

Laid on the surface of a piece of split oak, a rough, uneven surface, due to the gash made by the wedge, it twists and writhes, moves the front part of its body very slowly from left to right and right to left, lifts it a little, lowers it and begins again.

These are the most extensive movements made.

The vestigial legs remain inert and absolutely useless.
Then why are they there?
It were better to lose them altogether, if it be true that crawling inside the oak has deprived the animal of the good legs with which it started.