The organization of musculature and the nervous system in the pygidial region of phyllodocid annelids

Abstract

The annelid body can be subdivided into three main regions: the prostomium, the body segments, and the pygidium. The prostomium and pygidium, originating from the episphere and the posterior part of the hyposphere of a trochophore larva, are usually mentioned as non-homologous to body segments. However, recent studies revealed that the pygidium of several annelid species is far more complex than previously mentioned and possesses some segment-like features. To assess the diversity of a pygidial organization, I describe the innervation and muscular system of the pygidium in 19 annelid species belonging to the order Phyllodocida using phalloidin labeling, immunohistochemistry and confocal scanning microscopy. The musculature of the pygidium varies between families and usually consists of the circular and/or horseshoe-shaped hindgut sphincter muscles. In several families it can be accompanied by small additional transversal or dorso-ventral muscles. In contrast to the variable musculature, the pygidial innervation is far more uniform and in general comprises two huge main longitudinal nerves, a terminal commissure between them, and paired circumpigidial nerves. The pygidial epithelium bears numerous receptor cell endings, suggesting that the pygidium may act as an important sensory organ. The obtained results are in accordance with the recent data and indicate that such muscular and nervous organization may be characteristic of the whole order Phyllodocida.