Members of the genus Phascolion Théel are probably the most familiar of sipunculans because most species inhabit the empty shells of molluscs and other similar structures and are therefore easily collected in dredge samples. Although the genus is represented in most seas and depths, few species have been investigated in detail and much of our knowledge is based on studies of the widespread Atlantic species P. strombi (see Åkesson, 1958; Hylleberg, 1975; Kristensen, 1970; Wesenberg-Lund, 1929). P. strombi is known to be remarkably plastic in many of its characters, particularly external, to the extent that Gerould (1913) was able to distinguish seven varieties in the western Atlantic region alone. Much of the variation in the size and colour of P. strombi, and in the form of the skin papillae, especially those generally termed ‘holdfasts’ or ‘adhesive papillae’, may be attributable to the size and type of shelter that is inhabited but experimental evidence is lacking. It follows that a similar degree of external plasticity to that recognised in P. strombi must be assumed for most, if not all, Phascolion, presently totalling around 45 species. Many Phascolion species have been erected on single or just a few specimens showing only slight differences of dubious systematic significance.
Taxonomically, the genus is very difficult and no satisfactory key to the species has yet been devised. As a first step, there is a need for definitions of subgenera, preferably based on internal characters which are not subject to environmental influences. To this end, the retractor systems of the north-east Atlantic species described below have been studied and compared with the type specimens of species from other areas.