Spirula spirula is a small, muscular species (45 mm ML) found in mesopelagic waters of the tropical open ocean. In the title photographs, the skin of the mantle is missing. The intact mantle is covered with regularly aligned collagen fibers that produce a silvery sheen, as seen on the head in the title photographs (Herring et al., 1981). Spirula carries an unusual internal shell that is calcareous and has the shape of a horn coiled in a single plane without the coils touching one another (open planispiral).(Eol)