Allan A. MacFarlan, The boy’s book of outdoor discovery , New York, Galahad Books, 1974, p. 44.

Caddis worms busy on the bottom of ponds, make themselves suits of a armor from particles of sand, pieces of pebbles, tiny twigs, and oyther minute material to protect themselves from enemies.
This armor would be very uncomfortable if the caddis worm did not weave himself a silken undershir, like a tube, to wear next to his body. He spins this silken case with his mouth while holding the case in place with little hooks in his tail.