Abstract
The detailed arrangement of the setae is more or less distinctive in nine British species of Spirorbis, including S. cuneatus sp. n.The genus, which now contains about sixty‐four species, haa been briefly reviewed. To judge from numbers of thoracic segments, method of incubation and larval characters, S. malardi and S. spirillum may be regarded as perhaps more primitive than most British forms andS. pagenstecheri aa more advanced. The abdominal setation is more asymmetrical in these apparently primitive species and least so in the advanced one.