John Woodside Ritchie & Chapin Walker Day & Margaret Ritchie, Biology and Human Affairs, New York, World Book Company, 1946, p. 218.

An artificial shell.

Nature gives to mant the protection of a shell. Other animals build for themselves out of sticks or small stones a case that gives them  same security. The larva of caddis-fly is one of these.

This larva lives  in streams and ponds. It is a plant eater and it needs a defense against the many hungry carnivores that live in the same waters. The larvae of some species secure this by building an armor for thei bodies out of sticks which they tie together by threads of silk. Others build a case of small stones cemented so as to form a perfect covering. To move about, the little animal thrust its head and the forepart of the body that  a microscopic animal (Difflugia) and an insect larva(caddis fly) that build themselves artificial protective coverings.