These are the organs of locomotion, the ambulacra.

When the larva wishes to move forwards, it expands its hinder ambulacra, those on the back as well as those on the belly, and contracts its front ones.

Fixed to the side of the narrow gallery by their ridges, the hind-pads give the grub a purchase.

The flattening of the fore-pads, by decreasing the diameter, allows it to slip forward and to take half a step.

To complete the step the hind-quarters have to be brought up the same distance.