Annelida: Sipuncula

Abstract Sipuncula, commonly known as peanut worms, constitutes a small marine invertebrate taxon of about 150 species that is widespread and at all depths throughout the oceans. Adult sipunculans exhibit an unsegmented body, subdivided into a posterior trunk and a retractable anterior introvert. The central nervous system of adult sipunculans consists of a dorsally positioned brain at the base of the tentacles from where circumoesophageal connectives are forming an unpaired ventromedian nerve cord that extends the length of the body. The ventral nerve cord forms a single longitudinal neurite bundle without ganglia. The somata of the ventral cord are arranged ventrally, while the neurites, as well as the glial cells, are on the dorsal side. The innervation of the tentacles, nuchal organ, retractor muscles, digestive system, and ventral nerve cord originates at the brain, while sipunculan eyes, bipolar photoreceptor cells, lie dorsolaterally on each side of the bifurcated brain. Lateral nerves, irregularly arranged along most of the ventral nerve cord, innervate various structures such as the body wall, digestive system, and sensory organs on the body surface (e.g. cilia, glands, papillae), and thereby form the peripheral nervous system. Immunocytochemical studies of the nervous system in larval sipunculans have had a critical impact on phylogenetic considerations, since they have revealed numerous features that are shared between sipunculans and annelids, including the formation of the nervous system; thus supporting their close affiliation. For instance, in the ventral nervous system, serotonergic somata and lateral nerves are arranged in a rope-ladder-like pattern which is lost at metamorphosis.