Alister Hardy, The living Stream: a restatement of évolution theory and its relation to the Spirit of Man, Londres, Collins, 1965, p. 229.

Others, however, instead of producing their own shell by secreting lime, build around themselves grains of sand or other particles, to form a shelter like those houses constructed by the aquatic larvae of caddis-flies. Many make just a rough covering of of sand, but some-and to these I particularly want to call attention-build houses which are little short of marvels of engineering and constructional skill. I use the word « skill » advisedly. With the late Dr E. Heron-Allen who made a special study of these forms, I share the view that the buiding of these devices cannot be simply a matter of physico-chemical mechanism alone.
Let us briefly consider the « house » built by just one or two different species of these remarkables microscopic animals..