This short note is to record a specimen of Rostraria
, which was found in the catch of the Dragonet II, an opening-closing quantitative trawl, trawled on the 50 m deep sand bottom about 1.2 km off Cape Setozaki on June 13, 1966. The specimen was found by Dr. R. Bieri in the upper-net sample which was, however, much contaminated with the upper water during the drawing up of the Dragonet. Thus, the exact level at which the present Rostraria
was caught is quite uncertain. At the first sight, the outline of the specimen fixed and preserved in 4% formaldehyde in sea water reminded me of something like Thliptodon
, a gymnosomatous pteropod. Further examination under the microscope, however, revealed immediately that the specimen was a polychaete larva. The existence of a pair of stout tentacles showed a close resemblance to some larvae of the family Spionidae. But, in the larvae of Spionidae many segments and then parapodia are completed very early as noted for instance in Hannerz' paper (1956: Zoologiska Bidrag fr. Uppsala, 31, pp. 1-204), and the tentacles may be contracted but never in a coiled state. The very slow development of segments and parapodia and a strongly coiled state of contracted tentacles, caused by uneven distribution of muscles in them, together with a prominent forward stretch of the prostomium and the existence of a pair of bundles of filamentous setae on the first segment, which are all the important characters to define the larvae belonging to Rostraria
, are applicable to the present specimen.
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