A rude polygonal figure

Harland Coultas, The Home Naturalist; with practical instructions for collecting, arranging, and preserving natural objects, Londres, The Religious Tract Society, 1875, p. 185.

The caddis worms, or larvae of Phryganidae, have the remarkable habit of constructing tubular portable case out of gravels, small pieces of wood, leaves, aquaric shells, and even the pith of plants. The larva arranges the materials of the case about its body, and attaches them to itself by means of a glutinous silk, with which it lines the tube. These creature are to be seen crawling about the bottom of almost every pond and stream during spring. The head and feet of the larva are protruded from one end of the case, but, on the slightest alarm, they are quickly drawn within it.
Larvae cases differ very considerably, not only in their materials, but also in their mode of formation. Some are made of numerous little piece of grass and stems of aquatic plants, cut into lengths and placed crossways, forming a rude polygonal figure ; while in other instances the pieces are arranged longitudinally, or parallel with the body of the larva. But, whatever the material employed, the little builders usually contrive to make the cases of the same specific gravity as the water, so that they may be carried with ease.