External morphology of the posterior end, the "opisthosoma", of the beard worm Oligobrachia mashikoi (Pogonophora).

The entire length of the beard worm, Oligobrachia mashikoi (Pogonophora), including the posterior end, the "opisthosoma" was collected successfully. This species is exclusive to Tsukumo Bay in Ishikawa Prefecture, Japan. Although the portion preceding the opisthosoma was similar to a fine filament, it abruptly assumed a shape similar to a shovel and appeared to be composed of many segmental structures. The number of segments exceeded 50. The dorsal side of the opisthosoma differed from that of the ventral side in morphology. The opisthosoma was equipped with 4 lines of setae arranged longitudinally and a sucker on the tip. When considering the fact that the Family Oligobrachiidae is the most primitive group of pogonophores, the external morphology of the opisthosoma is interesting as it may be reminiscent of the ancestral condition. This is the first report of the opisthosoma in Oligobrachiidae.