The wall of a subcylindrical cavity

The Encyclopedia Britannica, eleventh edition, New York, The Encyclopedia Britannica Company, Cambridge University Press, 1910, p. 927.

Caddis-Fly and Caddis-Worm.

[…] The larvae know as caddis-worms are aquatic. The mature females lay their eggs in the water, and the newly-hatched larvae provide themselves with cases made of various particles such as grains of sand, pieces of wood or leaves stuck together with silk secreted fromthe salivary glands of the insect. These case differ greatly in structure and shape. Those of Phryganea consist of bits of twigs or leaves cut to a suitable length and laid side by side in a long spirally-coiled band, forming the wall of a subcylindrical cavity. The cavity of the tube of Helicopsyche, composed of grains of sand, is itself spirally coiled, so that the case exactly resembles a small snail-shell in shape. One species of Limnophilus uses small but entire leaves; another, the shells of the pond-snail. Planorbis; another, pieces of stick arranged transverseley with reference to the long axis of the tube.