Fine structure of the pharyngeal apparatus of the pelagosphera larva in Phascolosoma agassizii (Sipuncula) and its phylogenetic significanceстатья
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Аннотация:Sipuncula is a small taxon of worm-like marine
organisms of still uncertain phylogenetic position. Sipunculans
are characterized by an unsegmented body
composed of a trunk into which the anterior part, the
introvert, can be withdrawn. The group has been placed
at various positions within Metazoa; currently, it is either
seen as sister group of a clade comprising Mollusca
and Annelida or as sister to each of these. An in-group
position in either Mollusca or Annelida has usually been
precluded till now due to the lack of so-called annelid or
molluscan ‘‘key-characters’’ such as segmentation and
chaetae or the radula. In the development of certain taxa
the trochophore stage is followed by a planktonic larva,
the pelagosphera, which might exhibit phylogenetically
important structures. Among these is the buccal organ,
which has been considered homologous either to the
ventral pharyngeal organ present in many sedentary
polychaetes or to the radular apparatus of molluscs. In
the present paper, the ventral pharynx of the pelagosphera
larva of Phascolosoma agassizii is investigated by
transmission electron microscopy. The pharynx comprises
dorsolateral ciliary folds, a muscle bulb formed by
transverse muscle fibres with large intercellular spaces,
and an investing muscle. A tongue-like organ is lacking.
These results show great structural correspondences to
the ventral pharynx of polychaetes, especially to that of
the flabelligerid Diplocirrus longisetosus. In contrast,
there are no signs of structural similarities to the corresponding
structures of molluscs. Thus evidence increases
that Sipuncula are closely related to annelids;
moreover, an in-group position of Sipuncula within
Annelida, as suggested by recent molecular studies, is
not precluded by the present data. Instead these studies
find additional support. Hence the lack of segmentation
and chitinous chaetae in Sipuncula would be a secondary
rather than a primary situation, as has recently been
shown for Echiura and Pogonophora.