Structure of the oral tentacles of early ontogeny stage in brachiopod Hemithiris psittacea (Rhynchonelliformea, Rhynchonellida).

Brachiopods have the most complex lophophore in comparison with other lophophorates, i.e., phoronids and bryozoans. However, at early ontogenetic stages, brachiopods have a lophophore of simple morphology, which consists of the oral tentacles. Data on the ultrastructure of the oral tentacles is mostly missing. Nonetheless, it has recently been suggested that the structure of oral tentacles is ancestral for all lophophorates in general, and for brachiopods in particular. The fine structure of the oral tentacles in the brachiopod Hemithiris psittacea is studied using light microscopy, transmission and scanning electron microscopy, cytochemistry and confocal laser scanning microscopy. The oral tentacles have a round shape in transverse section, and four ciliary zones, i.e., one frontal, two lateral, and one abfrontal. Latero-frontal sensory cells occur among the frontal epithelium. Four basiepithelial nerves in the ciliary epithelium are colocalized with ciliary zones. Lophophores of simple morphology in phoronids and brachiopods are characterized by non-specified round forms of tentacles. In phoronids and bryozoans, tentacles have additional latero-frontal ciliary zones that function as a sieve during filtration. In most brachiopods, lateral cilia are involved in the capture of food particles, whereas latero-frontal cells are retained in the frontal zone as sensory elements. The oral tentacles of H. psittacea contain a coelomic canal and have distinct frontal and abfrontal longitudinal muscles, which are separated from each other by peritoneal cells. A similar structure of tentacle muscles occurs in all bryozoans, whereas in phoronids, the frontal and abfrontal tentacle muscles are not separated by peritoneal cells. We suggest that the lophophorates' ancestor had tentacles, which were similar to the tentacles of some phoronids with lophophore of simple morphology. We also assume that the structure of the oral tentacles is ancestral for all brachiopods and the specialization of brachiopod tentacles correlates with the appearance of the double row of tentacles.