Homology of the lophophore and its evolution within lophophorates

Abstract

: The lophophore is a tentaculated organ — a specificity of Lophophorata. Since molecular phylogeny has yielded contradictory data on the monophyly of lophophor-ates, results of comparative morphology are used here to shed light on the question about the homology of the lophophore in different groups of lophophorates. We compare the morphology of the lophophore and the organization of its coelomic, nervous, and muscular systems in three different phyla of lophophorates: phoronids, bryozoans, and brachiopods. The morphology of the lophophore and the structure of all lophophoral organ systems are examined in respect to the standard criteria of homology. The comparative analysis supports the homology of the lophophore and suggests the monophyly of the lophophorates. We present a hypothetical scenario of lophophore evolution based on our analyses. Accordingly, the ancestral lophophore of lophophorates had a horseshoe-shaped brachial axis and was innervated by supraenteric and subenteric nerve centers, which were connected by circumenteric connectives and a circumoral nerve. This ancestral lophophore then evolved in three different phyla by two pathways: (i) by complication and (ii) by simplification.