Photoreceptors in species of the Macrostomida (Plathelminthes): ultrastructural findings and phylogenetic implications

Abstract

The submicroscopic anatomy of intracerebral and pericerebral photoreceptors in six species of the Macrostomida is described. Cylindromacrostomum notandum, Paramyozonaria simplex and Macrostomum hystricinum marinum possess two rhabdomeric intracerebral photoreceptors each consisting of two pigmented cup cells and three (C. notandum and P. simplex) or two sensory cells (M. hystricinum marinum). In C. notandum and P. simplex two of the sensory cells are equal in size, while the third one is much smaller. This organisation is hypothesised as an autapomorphy of the Dolichomacrostomidae. Photoreceptors with two mantle cells are also known for Microstomum spiculifer. Since only one cup cell exists in representatives of nearly all other highranked taxa of the Rhabditophora, it is concluded that the characteristic "two cup cells in rhabdomeric photoreceptors" has evolved in the stem lineage of the taxon Macrostomida or Macrostomorpha, respectively. In Myozona purpurea and Psammomacrostomum turbanelloides rhabdomeric intracerebral photoreceptors of a special type were encountered. These light-sensing organs consist of numerous cells forming an ellipsoid. The surface membranes of these cells are elongated to form filiform extensions which are tightly intertwined with each other. Pericerebral ciliary aggregations consisting of cells with an internal cavity into which axonemata of modified cilia project were observed in all species mentioned above and in Bradynectes sterreri as well. Such putative lightperceiving organs are widespread within taxa of the Plathelminthes Rhabditophora and have been hypothesised either as homologous characteristics or as analogous ones. With increasing examples being described it becomes likely that pericerebral ciliary aggregations are an apomorphic ground pattern characteristic of the Rhabditophora.

Neurobiology of the basal platyhelminth Macrostomum lignano: map and digital 3D model of the juvenile brain neuropile

“…Simple pigment-cup eyes, consisting of a pair of rhabdomeric photoreceptors ensheathed by pigment cells, are embedded in the dorsoposterior cortex of the brain. In addition to these lightsensing organs, a pair of ciliary receptors with presumed light-sensing function have been described (Sopott-Ehlers et al 2001) at a position anterior to the brain. The brain is formed by an outer cortex of neural somata surrounding a fibrous neuropile.…”

Ultrastructure of phaosomous photoreceptors in Stylaria lacustris (Naididae, 'Oligochaeta', Clitellata) and their importance for the position of the Clitellata in the phylogenetic system of the Annelida

“…In other taxa of Spiralia, eyes basically have the same structure as in polychaetous annelids. This applies to Sipuncula (Hermans and Eakin 1969), larvae and adults in Mollusca (Rosen et al 1979; Bartolomaeus 1992b; Blumer 1994, 1995; Sturrock and Baxter 1995) and Platyhelminthes (Sopott‐Ehlers 1991; Sopott‐Ehlers et al 2001). The multicellular eyes of Mollusca usually do not form a pigment cup and arrangement of cells is similar to that described above for polychaetes.…”

Spermiogenesis and submicroscopic features of spermatozoa inTyphloplana viridata(Plathelminthes, Rhabdocoela, Typhloplaninae)

“…Fixation, dehydration, embedding and staining followed conventional steps (see Sopott-Ehlers et al, 2001). A complete series of ultrathin sections were examined using a Zeiss EM 900 electron microscope.…”