It has been concluded from studies of the elemental composition of the jaws of four Glycera species that a high concentration of copper is a structural feature of the glycerid jaw (Gibbs & Bryan, 1980). In Glycera gigantea Quatrefages, for example, the whole jaw when dried contains about 1–5% of copper, whilst zinc, calcium and magnesium, in roughly equal amounts, account for a further 1%. Copper is mainly confined to the distal half of the jaw and a concentration of 13% is present in the extreme tip; the other metals are more evenly distributed. Of non-metals, chlorine is concentrated in the distal tip along with the copper but sulphur is fairly evenly distributed and probably accounts for several percent of the dry weight.

In view of these findings, it was decided to investigate the Family Goniadidae, a group of worms that are closely related to the Glyceridae (and often included in that family) but which are readily separated by the different structure of the jaw apparatus and also of the parapodia (see Hartman, 1950). In goniadids the jaw apparatus consists of a pair of larger toothed jaws (macrognaths) plus dorsal and ventral arcs of smaller, H- and Y-shaped jaws (micrognaths), the whole forming a circle around the tip of the proboscis. The present observations were made on Goniada maculata Oersted, the only goniadid species present in the Plymouth area, and a total of four specimens, all about 5 cm in length, from different areas (Salcombe, Rame Grounds and Cawsand Bay: see Marine Biological Association, 1957) were examined.