Most mollusc species producing the purple colour belong to the gastropod family Muricidae, as it was originally understood. Later, this family was split up into several families, into subfamilies, genera and subgenera. This nomenclature splitting has resulted in a situation that, for non-specialists, may seem rather confusing. For example, the names Murex trunculus, Hexaplex trunculus and Trunculariopsis trunculus all pertain to one and the same species, the banded dye-murex, nowadays generally called Phyllonotus trunculus. A very important point is that no animal of the purple-producing species contains any purple pigment as long as it is alive and well. Snails have near the periphery of the body, a cavity called the mantle cavity, which is in direct contact with the outer world, and in which some organs, for example the gills and the genital opening, are located. ln this mantle cavity, most of the marine snails also have a gland, called the hypobranchial gland, the location of which is indicated in the sketch of a dissected spiny dye-murex, Phyllonotus (Bolinus, Murex) brandaris (fig.12). ln this hypobranchial gland the species we are concerned with here produce a whiteish viscous secretion.