caddisworm

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun The aquatic wormlike larva of a caddisfly, enclosed in a cylindrical case covered with grains of sand, bits of wood or shell, or other debris.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun larva of the caddis fly; constructs a case of silk covered with sand or plant debris