Not stated, but named 'polytrema' for the series of cell spaces in the tube, which Philippi comments on. Quote from the English translation of this article (in Annals & Magazine) "in this species only the septa as it were of the cells are developed, and the three keels perforated by the rows of their apertures are highly elegant in appearance." Polytrema is from Greek, meaning full of holes. Trema is a neuter noun, but polytrema must be an adjective agreeing with the genus. Polytremus is rarely used, but, regarding polytrema as now Latin, Pillai (2009) created Spirobranchus pseudopolytremus for a species similar to S. "polytrema" in what is a masculine genus. As S. polytrema is so widely used it seems best at the moment not to force a mandatory gender suffix change in WoRMS, until/unless adopted in print.
Philippi, A. (1844). Einige Bemerkungen über die Gattung Serpula, nebst Aufzählung der von mir im Mittelmeer mit dem Thier beobachteten Arten. Archiv für Naturgeschichte, Berlin. 10(1): 186-198, plate 6. available online athttp://biodiversitylibrary.org/page/13704089