Loss of nervous system complexity – Morphological analyses shed light on the neuronal evolution in Myzostomida (Annelida)

Myzostomida are putatively nested within the errant Annelida and exhibit a unique morphology. The latter fact might possibly be related to their long host‐dependent radiation. Hence, an incomplete segmentation, lack of prominent sensory structures in adults and a dorso‐ventrally flattened body are just some examples. Although numerous investigations of the nervous system exist for myzostomids, detailed ultrastructural as well as histological examinations of neuronal structures are lacking so far. Therefore, we investigate the nervous system of Myzostoma cirriferum Leuckart, 1836 using a comparative approach including paraffin histology, serial semi‐thin sections, immunohistochemistry and ultrastructural investigations. Our analyses reveal a lack of complexity within the anterior‐most neuronal condensation (herein called brain) of adult specimens. Hence, prominent tracts or glomeruli are absent, and a glial layer surrounding the brain or radial‐like glial cells are lacking. Nevertheless, the ultrastructure of the ventral nervous system is comparable to other Errantia. Therefore, our investigations hint towards a reduction of neuronal complexity in Myzostomida. Additionally, an ontogenetic simplification seems plausible, although further investigations are necessary to verify such a hypothesis. A simplification of neuronal structures due to a drastic change in lifestyle was so far mainly observed for basally branching annelid clades.