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Several early authors had pointed to the similarities between the ‘trochophora’ larvae of several main bilaterian groups, and Hatschek (1891) elaborated this in his trochophora theory, which proposed that this larval type was present in the common ancestor of the protostomes (his Zygoneura). Molecular phylogenies can serve as hypotheses for the relationships between clades, but only morphology can give information about evolution from the early ancestors to the living groups. Also, the ‘D-quadrant cleavage’ ( Nielsen 2012) with the postero-dorsal blastomere of the 4-cell stage giving rise to the germ cells, and in most cases to the mesoderm, seems to be a protostome apomorphy. The monophyly of the group is well supported by characters from adult morphology, embryology and Hox genes ( Nielsen 2012) and from phylogenomic analyses and microRNAs ( Erwin et al. Its most conspicuous characteristic (apomorphy) is the paired ventral nerve cord, which is reflected both in Hatschek’s name (meaning paired nerve) and in alternative names, such as Hypogastrica ( Goette 1902), Hyponeuralia ( Cuénot 1940) and Gastroneuralia ( Ulrich 1951). The name Protostomia was introduced by Grobben (1908) for the group called Zygoneura by Hatschek (1888). Alternative ‘intercalation theories’ propose that planktotrophic larvae evolved many times from direct-developing ancestors, but this finds no support from considerations of adaptation. The predicted presence of a perioral loop of the blastoporal nerve ring has now been demonstrated in two annelids. Almost all new information about morphology and embryology fits the trochaea theory. The circumblastoporal nerve became differentiated into a pair of ventral nerve cords with loops around mouth (the anterior part of the blastopore) and anus. Paired cerebral ganglia developed from the first micromere quartet. The apical ganglion was lost in the adult, as in all neuralians. The archaeotroch became differentiated as prototroch, metatroch and telotroch in the (trochophora) larva, but was lost in the adult. The pelago-benthic life cycle evolved through the addition of a benthic adult stage, with lateral blastopore closure creating a tube-shaped gut. According to the trochaea theory, protostomes are derived from a holoplanktonic gastraea with a circumblastoporal ring of downstream-collecting compound cilia (archaeotroch) and a nervous system comprising an apical ganglion and a circumblastoporal nerve ring. The origin and radiation of the major metazoan groups can be elucidated by phylogenomic studies, but morphological evolution must be inferred from embryology and morphology of living organisms.
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