A comparative study of body wall structures of a pogonophore and an annelid from a phylogenetic viewpoint

Pogonophores are tube worms that live in reducing deep-sea waters where sunlight does not penetrate. They are highly adapted for their special habitat in lacking guts and possessing endosymbiotic chemosynthetic bacteria. Because of these peculiar characteristics, it is not yet clear whether they should be classified as annelids or not. Electron-microscopic observations of sections of a Japanese pogonophore (Oligobrachia mashikoi) show that the body wall has circular and longitudinal muscular systems. These muscular systems, however, differ from the annelid (Branchiura sowerbyi) in these ways: (1) The outer circular muscle of the pogonophore was constructed of smooth muscle cells. In contrast, that of the annelid was composed of obliquely-striated muscle cells, even though the cells were small and bore undeveloped characteristics. (2) The inner longitudinal muscle of the pogonophore was constructed of undeveloped obliquely-striated muscle cells, whereas that of the annelid was composed of well-developed ones. These observations suggest that this pogonophore can not be classified as an annelid, although many previous studies have placed pogonophores in that phylum.