Proboscis sensory cells in Nemertea: comparative morphology and phylogenetic implications

Abstract

Abstract Analyses of molecular data have clarified the phylogenetic relations between classes and orders of the phylum Nemertea as a whole, but the ‘deficit’ of morphological synapomorphies characterizing main clades remains problematic. Characters identified with classic histological studies of nemerteans reveal a high level of homoplasy, thus complicating the search for synapomorphies. To identify more potential synapomorphies, sensory cells of the proboscis epithelium of 39 nemertean species were studied with electron and confocal laser-scanning microscopes. Three types of sensory cells were described: monociliated (found in nemerteans from all orders), multiciliated (found only in polystiliferous hoplonemerteans) and nonciliated (found in two species of monostiliferous hoplonemerteans) sensory cells. Monociliated sensory cells of the proboscis have a common structure, differing from monociliated sensory cells of the epidermis and cerebral organ canals. Each monociliated cell consists of a cilium with a bulb-like expanded tip surrounded by a cone-like collar of microvilli, an intra-epithelially located body (perikaryon) and a single basal process (axon). Some features of the monociliated sensory cell structure are thought to provide solid mechanical support. Specific features in the structure of the axial rootlets, cilia, microvillus collars and their microfilaments, considered synapomorphies/autapomorphies, were revealed in the representatives of some nemertean taxa.