The majority of Desmoscolecoidea are characterized by ring-shaped accumulations of mineral particles on the cuticle resulting in the impression of a peculiar body articulation. Live observations made during the extraction of desmoscolecoids from mud sampled in the Swedish Gullmar fjord suggested new ideas pertinent to the particle accretion and the likely functional significance of the prominent rings. Undisturbed desmoscolecoids were observed to perform perpetual, vigorous, stationary undulations with their body. In our opinion these undulations affect the distribution of particles adhering to the cuticle by pushing them from soft, pliable cuticular sections to comparably stiff rings thus generating distinct concretion rings and naked interzones. The prominent concretion rings are assumed to increase positive mechanical effects of the undulations on the productivity of ubiquitous microbial populations residing on ambient sediment particles. The microbial production boosted by this kind of bioturbation may be of nutritional importance for the desmoscolecoids. An electron-microscopic investigation (TEM) of Tricoma sp. from the Gullmar fjord revealed the concretion rings to consist of stacks of clay mineral platelets in the submicrometer size range. A perusal through contributions of other authors suggests that such clay mineral aggregates are the basic component (granular component sensu Timm 1970) in the concretion rings of all desmoscolecoids. In a number of species these aggregates contain a specifically determined admixture ofconspicuous other mineral grains.