Ultrastructure of the nerve cells and sensilla of Geocentrophora baltica (Platyhelminthes, Lecithoepitheliata) and the surface sensilla in the Geocentrophora group

Abstract

Two types of nerve cell could be distinguished ultrastructurally in the central nervous system of Geocentrophora baltica (Prorhynchida, Lecithoepitheliata) . Both show invaginations in the plasma membrane, but they differ in the character of the cytoplasm (light or densely stained) and the distribution of the neuronal vesicles (evenly or in groups) . Different kinds of vesicles and neuronal release sites are observed . Special features of the synapses are pronounced local thickenings of the presynaptic membrane connected to paramembranous densities . In G. baltica and five endemic Geocentrophora spp . from Lake Baikal six types of surface sensillum were observed at the epidermal surface : 1 . those with a long thin rootlet ; 2 . a short, balloon-shaped cilium with an aberrant axoneme and a reduced rootlet ; 3 . a rootlet branching into many striated bundles ; 4 . a thick rootlet ; 5 . a reduced rootlet and numerous neurotubules ; and 6 . collared sensilla each with one cilium in a deep pit surrounded by a collar of 11 to 12 microvilli . The variable number of microvilli in the collared sensillum is considered plesiomorphic relative to the stable number of eight microvilli known in sensilla of the Prolecithophora, Proseriata, and Rhabdocoela . The ultrastructure of the collar sensillum indicates that the Lecithoepitheliata is only distantly related to the Prolecithophora and higher turbellarians . 183

The phylogenetic position of the Prolecithophora (Rhabditophora, ‘Platyhelminthes’)

“…This, and the supported monophyly of the Prolecithophora regardless of the alignment and the studied gene, contradicts Timoshkin (1991), who hypothesized that the Lecithoepitheliata have arisen from prolecithophoran stock similar to the ‘primitive’ prolecithophoran Protomonotresis centrophora Reisinger, 1923. The basal placement of the Lecithoepitheliata among neoophorans is supported by numerous other studies such as Böckerman et al . (1995), Campos et al .…”

Ultrastructural and cytochemical studies of the female gonad of Prorhynchus sp. (Platyhelminthes, Lecithoepitheliata)

“…Inclusions in the oocytes and vitellocytes of prolecithophorans studied to date are quite similar to homologous inclusions of the Rhabdocoela (Gremigni, 1988 ;Lucchesi et al, 1995) . These ultrastructural features do not support the speculation that Lecithoepitheliata (or at least Prorhynchidae) are closely related to Prolecithophora (Timoshkin, 1991), but correlate with the recent statement of Bockerman et al (1995) that there is no close relationship between these two groups .…”

First description of collar receptors in Temnocephalidae (Platyhelminthes)

“…Thus, the cilium can be very shortened and thickened (Reuter 1975; Schockaert and Bedini 1977; Rohde et al . 1988; Böckermann et al . 1995), or more than one cilium can be present (Bedini et al .…”