A Comparative Study of the Setae of Pogonophora and Polychaetous Annelida

Pogonophora are long thin tube-dwelling invertebrates similar in size to polychaetous annelids, but lacking both mouth and gut, and found mainly in the deep sea. Current discussion of their systematic position had revealed a need for more information about particular aspects of their morphology and embryology (Southward, 1971a). The earlier attribution of Pogonophora to the echinoderm-chordate group, Deuterostomia, based mainly on their apparently tricoelomate structure, has been questioned recently, following the discovery of the previously unknown posterior end (Webb, 1964a; Ivanov, 1965). This posterior end is a small and delicate structure composed of several coelomate segments separated by septa and provided with short bristles. It has such a strong resemblance to the posterior end of an annelid, though of course it does not contain a gut, that there has been much speculation about the possibility of a relationship between Pogonophora and Annelida and some reconsideration of the features thought to indicate affinity to the Deuterostomia (Livanov & Pornrieva, 1967; Webb, 1969b; Ivanov, 1970; Norrevang, 1970a, b; Southward, 1971a).