In a taxonomic review, nine previously defined subgenera are adopted and a tenth Marsipospira subgen. aov. is proposed for about four species, with S. striatus QuieVreux (1963) as the type. These all retain their embryos in a thoracic brood pouch, fixed to a modified dorsal tentacle, and also possess a translucent tube, often with transverse ridges, an opercular talon, with lateral lobes and/or lateral knobs, and vestiges of a fourth thoracic segment.
The remaining species, numbering more than 74, may be grouped in six subgenera with egg string incubation and three with opercular incubation. These three main methods of incubation are likely to have been derived independently, from ancestral serpulids spawning into their tubes.
The distribution of abdominal uncini helps to characterize subgenera. In Spirorbis, Spirorbella, Pileolaria, Leodora and Janua the abdominal tori are not very asymmetrical, those on the concave side being only slightly larger than the others. In Pileolaria the largest abdominal tori are usually found in the posterior half of the abdomen, but in other subgenera they usually lie in the anterior half of that region. Paralaeospira, Greets and Marsipospira have the abdominal tori very asymmetrically developed and usually absent from the convex side. This marked asymmetry is often associated with the tubes being translucent, smooth and presumably slippery on the inside. It may thus be related to the anchorage problem, during rapid withdrawal. The asymmetrically developed traces of a fourth thoracic segment, with which the abdominal asymmetry is often correlated, seem unlikely to be primitive survivals. They may rather be incidental expressions of genotypes evolved through selection pressure which was concerned primarily with anchorage problems in the abdominal region.
