The nervous system of the tube‐worm Chaetopterus variopedatus (Polychaeta)

Abstract

Silver impregnation of serial histological sections of the tubeworm Chaetopterus variopedatus revealed the presence of a subepidermal nervous system. The anterior nervous system is delimited by the first 11 segments and comprises (1) two dorsolateral cerebral ganglia and lateral instead of ventral nerve cords which are widely separated and thus connected by unusually long commissures, (2) a pharyngeal ganglion in the fourth segment which is connected to the cerebral ganglia by pharyngeal nerves and constitutes along with the pharyngeal plexus a stomatogastric or enteric nervous system, and (3) small, presumably segmental ganglionic swellings along the lateral nerve cords from which emerge commissures and parapodial nerves. No subesophageal ganglion or periesophageal connective could be identified. The lateral nerve cords converge toward the midline in the 12th segment to form the posterior nervous system comprising a pair of ventromedian nerve cords with their repetitive segmental ganglia from which emerge numerous short commissures and three segmental nerves coursing toward the dorsal and ventral regions of parapods and toward the neuropod. Light and electron microscopic investigations of cerebral and segmental ganglia showed an arrangement of inner neuropile and of unipolar neuron somata at the periphery. The neuropile comprises numerous neurites ranging in diameter from 0.5 to 10 μm and making polarized or symmetrical synaptic junctions with each other. The pharyngeal ganglion consists of a similar neuropile and of a large mass of cell bodies which is traversed by an elaborate network of sinuses and harbors three types of neurosecretory cells in addition to the conventional neuron somata. These findings are interpreted in the framework of the highly specialized morphological features and habits of Chaetopterus, and the welldeveloped stomatogastric system is considered to be related to control of the feeding activities.

Systematics, evolution and phylogeny of Annelida – a morphological perspective

“…However, as already discussed (Bullock and Horridge, 1965;Martin and Anctil, 1984;Purschke, 2002;Orrhage and Müller, 2005) a basiepidermal position is more common than formerly thought. Interestingly, species with a subepidermal position of the nervous system in adults may have a basiepidermal position in juveniles (e.g., Scoloplos armiger; Purschke, unpubl.…”

“…2014). However, histological investigations of Chaetopterus variopedatus indicate that adult eyes are present (Martin and Anctil, 1984). Probably there are parallel events of miniaturisations and progressive reductions or losses of adult (and larval) eyes, one of which is characteristic for the lineage comprising most sedentary groups including Clitellata (Döring et al, 2013).…”

“…(see Lehmacher et al, 2014). Although ganglia and connectives are reported to occur in Chaetopterus variopedatus (see Martin and Anctil, 1984), annelid species regarded to be part of the basal radiation should be reinvestigated for this character.…”

Morphology of the nervous system of Polychaeta (Annelida)

“…The majority of nerve fibers pass to the contralateral side, thus forming well differentiated commissures (Smith, 1957;Golding, 1992). One (Platynereis dumerilii) up to seven (Chaetopterus variopedatus, Martin & Anctil, 1984;Saccocirrus papillocercus, Mü ller, 1999b) commissures can be counted, of which one can be termed the 'main' commissure because it is broader than the others (Mu¨ller & Westheide, 2000. (Fig.…”

Ultrastructure of pigmented eyes in Dorvilleidae (Annelida, Errantia, Eunicida) and their importance for understanding the evolution of eyes in polychaetes

“…Although being regarded as rudimentary, and ultrastructural data are still lacking, the same seems to apply to the eyes of Chaetopterus variopedatus (Renier, 1804) (see Fig. in Martin and Anctil ), which likewise is member of the basal annelid radiation (Struck et al . ).…”