The central hydrom strand in the seta of the sporogonium of most mosses has already been alluded to.
Besides this there is usually a living conducting tissue, sometimes differentiated as leptom, forming a mantle round the hydrom, and bounded externally by a more or less well-differentiated endodermis, abutting on an irregularly cylindrical lacuna; the latter separates the central conducting cylinder from the cortex of the seta, which, like the cortex of the gametophyte stem, is usually differentiated into an outer thick-walled stereom and an inner starchy parenchyma.
D, Seta of Lumbricus.
E, Seta of Criodrilus.
The external segments are usually definable by the setae; and if the setae are absent, as in the anterior segments a, Penial seta of Perichaeta ceylonica.
B, Extremity of penial seta of Acanthodrilus (after Horst).
C, Seta of Urochaeta (Perier).
In the vibraculum the part representing the zooecium is relatively smaller, and the mandible has become the "seta," an elongated chitinous lash which projects far beyond the zooecial portion of the structure.
There appears to be little either in the development or in the structure of the Haplodrili to warrant the view held by Hatschek and Fraipont that Polygordius and Protodrilus are exceedingly primitive forms, ancestral to the whole group of seta-bearing Annelids (Oligochaeta, Polychaeta, Hirudinea and Echiuroidea).
Tomopteris is remarkable for the fact that the hammer-shaped prostomium has paired ventral processes each with a single seta.