Current knowledge is that Schmarda (1861) combined different but related intertidal species from two places in his description of Placostegus caeruleus, namely Cape of Good Hope, South Africa and New Zealand. The type material is believed lost (Glasby & Read, 1998). The Cape of Good Hope material was what is now Spirobranchus kraussii (Baird, 1865), and the New Zealand material was what is now Spirobranchus cariniferus (Gray, 1843). Schmarda's colour plate of tube and worm is probably more like the South African species than the New Zealand species, but his name cannot be assigned as a synonym to just one of them as he didn't indicate which he based the description on, presumably finding the specimens indistinguishable. His name is junior to both taxa, so there is currently no nomenclature issue arising from the uncertainty of identification.
Schmarda, L. K. (1861). Neue Wirbellose Thiere: Beobachted und Gesammelt auf einer Reise um die Erdr 1853 bis 1857. <em>In Turbellarien, Rotatorien und Anneliden. Leipzig, Verlag von Wilhelm Engelmann.</em> Erster Band, Zweite Hälfte.available online athttp://www.biodiversitylibrary.org/ia/neuewirbelloseth21861schm
Glasby, C.J. and Read, Geoffrey B. 1998. A chronological review of polychaete taxonomy in New Zealand. Journal of the Royal Society of New Zealand 28(3): 347-374.