Abstract
Five species of the genus Spiochaetopterus: S. typicus SARS, S. bergensis GITAY, S. costarum (CLAPARx00C9;DE), S. oculatus WEBSTER, and S. solitarius (RIOJA) have been compared. These species can be divided into two groups: group A, with boreal biogeographic affinity, consisting of S. typicus and S. bergensis; group 8, with temperate biogeographic affinity, consisting of S. costarum, S. solitarius and S. oculatus. The two groups differ in the shape of the specialized setae of segment A4, the number of segments in region 8, and the structure of the tube. Representatives of group A have a middle region of two segments, modified setae of segment A4 are distally rhomboid and the tubes are without articulations. Representatives of group 8 have a middle region of many, always more than 2 segments; modified setae of A4 are distally cordate, and the tubes are with articulations. In each geographic unit of the Atlantic-Mediterranean area, the species are divided into two groups of different sizes. The usefulness of such systematic characters as the number of segments in region 8, ornamentation of the tubes and relative size of the pro- and peristomium is reexamined. New characters include the detailed morphology of the A4 setae, location of the secretory part of the ventral shield, and the structure of the secretory pores. Two diagnostic keys to species, one based on the tubes and the other on the A4 setae, are included.