CLSM Analysis of the Phalloidin-stained Muscle System of the Nemertean Proboscis and Rhynchocoel

The proboscis and rhynchocoel musculature of 56 nemertean species was studied using phalloidin labelling and confocal laser scanning microscopy. Six types of muscle layers are found in the anterior proboscis of the nemerteans: inner circular, inner diagonal, inner longitudinal, outer diagonal, outer circular, and outer longitudinal. Only the inner circular and inner longitudinal muscle layers are present in all the nemerteans studied. Ten types of arrangement of the proboscis musculature are described. Three primary types ('palaeotype', 'heterotype', and 'hoplotype') correspond to the three nemertean supergroups (Palaeonemertea, Heteronemertea, and Hoplonemertea). The evolutionary transformations of the initial 'palaeotype' proboscis proceeded in two primary ways: increasing bilateral symmetry (Callinera, Hubrechtella, and most of Heteronemertea) and increasing polyradial symmetry (Baseodiscidae, Oxypolellinae, and Hoplonemertea). The musculature of the middle portion of the proboscis differs among the three groups with armature: Palaeonemertea (genus Callinera), Polystilifera, and Monostilifera. The musculature of the stylet apparatus of the monostiliferous nemerteans is more complicated than that of the polystiliferous nemerteans, and consists of four muscle components-basal and anterior sphincters, radial and longitudinal musculature. Among the studied monostiliferans, the different components of the stylet musculature are developed to varying degrees. In addition, data on the structure of the rhynchocoel with interwoven musculature are provided. The taxonomic significance and phylogenetic interpretation of the proboscis and rhynchocoel musculature is discussed.