William Blacker, Catechism of fly making angling and dyeing,Londres, Publié par l’auteur,   1842, p. 76-77.

The Cadis-worm, or Cor-bait. – These worm, incased like the snail, creep and roll about the bottom of gravelly streams for a length of time, previous to their being metamorphosed into a fly. They are seen in an oblong sheath, curiously wrought, and incrustated on the outside with small gravel or shells, or in two semi-cylindrical pieces of hollow bark cemented together, having an orifice at each end ; they walk on six legs, some have less, according to their kind, with a sort of helmet on their heads ; these appear when seeking their food- and are drawn in a twill when suddenly surprised. When I first observed these curious long rough substances in the gravela t the bottom. I could scarcely believe they contained a reptile ; but on a closer examination, breaking the case or sheath, I behed to my astonishment,  aliving creature, endowed with instinct ( by the Great Author of nature), to form itself a covering, to secure it from the inroads ot its enemies at the very bottom of the water, and obtaining its subsistence in the most obscure solitude.