Although part of the annelid radiation, Myzostomida exhibit a highly specialized body plan that lacks many typical annelid characters. Their annelid ancestry is evident from their trochophora-like larvae, adult myoanatomy and parts of the nervous system, whereas segmentation is considered at best to be incomplete in myzostomids. We analyzed the morphology of two myzostomid species, the ectocommensal Myzostoma cirriferum and the endoparasitic Mesomyzostoma cf. katoi using a broad set of fluorescent markers to reveal the degree of segmentation in myzostomids. We used immunocytochemical and classical fluorescent staining methods combined with confocal laser-scanning microscopy to visualize tissues labeled with antibodies directed against classical invertebrate neurotransmitters (serotonin, dopamine, FMRFamide), synapsin, which labels nerve cell terminals, and the marker phalloidin–rhodamine which binds F-actin in muscle. Our data provide a broad body of additional evidence for the segmented origin of Myzostomida. It becomes apparent that the adult nervous system of M. cirriferum exhibits signs of pseudoradial symmetry with repetitive patterns of putative FMRFamide, serotonin and dopamine-like immunoreactivity. An analysis of the staining patterns in juvenile M. cirriferum yielded evidence for positional changes, as well as additions and reductions of neuronal structures during development. Interestingly, the neuroanatomy and myoanatomy of Mesomyzostoma cf. katoi indicate further reductions of neuronal and myoanatomical patterns in this species. Notably this taxon shows a presumably secondarily evolved cylindrical and strictly bilateral morphology, which is supposed to have evolved from a flat, disk-shaped Myzostoma-like ancestor with an underlying pseudoradial symmetry.
