Anatomie en ultrastructuur van de proboscis van eukalyptorhynchia (platyhelminthes, rhabdocola)

Abstract

All genera and species of the taxon Eukalyptorhynchia are characterized by a proboscis with terminal pore. The proboscis is an organ situated in front end of the animals and is used to capture preys. The morphology of this structure has been investigated ultrastructurally to gain better insight in the anatomy of this characteristic organ. In spite of the extensive amount of information on the genital system, the relationships within the Eukalyptorhynchia have been unclear up to now. From our study useful ultrastructural characters have been obtained to establish a sound phylogenetic system. The proboscis epithelia are formed by four or five circumferential belts. Two or three belts constitute the sheath epithelium. Three sheath epithelial belts are present in Polycystididae, Cystiplanidae and Koinocystididae. The presence of two belts in cone epithelium is considered the plesiomorphic state in Eukalyptorhynchia. The proboscis epithelia are generally void of cilia and covered by microvilli. The belts in the sheath epithelium in species with a bipartite sheath epithelium are mostly cellular. Syncytial belts are present in many species with tripartite sheath epithelia. The position of the nuclei varies from intra-epithelial to insunk in the parenchyma or intrabulbar. The sheath epithelium is characterized by infoldings of the basal plasma membrane. The cone epithelium in all species is bipartite. In some species a basement membrane is lacking in the cone. The basal belt is generally a syncytium. Only three species possess a cellular basal cone epithelium. The apical cone epithelium is in most species syncytial or formed by a single cell. Different types of glands are present in the epithelia of the proboscis. Some types can be homologized in all investigated species. Type g9 gland necks are homologous in all Eukalyptorhynchia and contain small (120-200 nm) ovoid, electron dense secretion granules. The terminal parts of the gland necks are reinforced by peripheral microtubules. Two types of gland necks (type g6 and g7) in basal cone epithelium can be homologized in all species as well. The sensory cells associated with the proboscis include uni- and multiciliary receptors. Uniciliary receptors in the sheath epithelium are characterized by primary and secondary rootlets. Those in the cone epithelium have blunt ciliary shafts and only primary rootlets. Multiciliary receptors associated with the distal belt of the sheath epithelium appear in different forms; intra-epithelial with fingerlike ciliary processes or with ciliary processes forming concentric lamellae in spherical sensory organs. In several species two insunk spherical invaginations of the distal belt of the sheath epithelium contain the multiciliary receptors. In these insunk sensory organs the ciliary shafts can either be fingerlike or flat sheetlike. The muscles of the proboscis show cytological differences. A peripheric sarcoplasmic reticulum is not present in Toia, Nannorhynchides and Floriane/la. In Polycystididae and Cystiplanidae the muscles show a cross-striation without H-zones. 1be outer musculature of the proboscis is formed by outer circular and longitudinal muscles of the sheath and motional muscles (protractors, fixators, proboscis and integument retractors), which enable the positioning, pro- and retraction of the proboscis. Only in Placorhynchidae outer circular muscles are lacking. In many species these muscles only appear beneath the proximal belt of the sheath epithelium. Gnathorhynchidae, Placorhynchidae, Psammorhynchus and a number of genera of the Cicerinidae are characterized by the presence of two sets of proboscis retractors, while fixator muscles are lacking. The inner musculature within the bulbar septum comprises inner circular and longitudinal muscles. The inner circular muscles are found from the nodus upto the junction or even in the cone. The inner longitudinal muscles can either form a thigh muscular block or are loosely arranged. In many species a division in central and peripheral longitudinal muscles can be made. Intra-epithelial muscles in the cone epithelium are present in Cystiplanidae and Polycystididae. The family Cicerinidae Meixner, 1928 appears to be a grouping of a number of genera based on plesiomorphic characters (germovitellaria, glandular proboscis). This family falls apart in three monophyletic families Nannorhynchidae, Zonorhynchidae and Cicerinidae (s.s.). The Cicerinidae (s.s.), Psammorhynchus, Cytocystis, Placorhynchidae and Gnathorhynchidae form a monophyletic taxon based on the presence of two sets of proboscis retractors. The position of Florianella (Bertiliellidae), of which the proboscis is characterized by a number of plesiomorphic characters and autapomorphic features, is doubtful. The families Koinocystididae, including Mesorhynchus, Cystiplanidae and Polycystididae form a monophytletic taxon based on the presence of a third belt in the sheath epithelium. Cystiplanidae and Polycystididae are characterized by intra-epithelial muscles in the cone.