Ian Cox, The wild life around us and the story of the rocks, Londres, Allen and Unwin, 1939, p. 38.

Many of them, such as creepers, or the young stages of stone-flies, hide themselves under stones ; others, like worms and the larvae of gnats, bury themselves permanently in the mud ; caddis worm build houses for themselves out of sticks, gravel or empty shells.
Pursuing this matter of protection we might look first at the animal that are protected by houses they manufacture themselves ; that is the true