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Structure and Evolution of Invertebrate Nervous Systems$
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Andreas Schmidt-Rhaesa, Steffen Harzsch, and Günter Purschke

Print publication date: 2015

Print ISBN-13: 9780199682201

Published to Oxford Scholarship Online: March 2016

DOI: 10.1093/acprof:oso/9780199682201.001.0001

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Annelida: Myzostomida

Annelida: Myzostomida

Chapter:
(p.241) 22 Annelida: Myzostomida
Source:
Structure and Evolution of Invertebrate Nervous Systems
Author(s):

Conrad Helm

Christoph Bleidorn

Publisher:
Oxford University Press
DOI:10.1093/acprof:oso/9780199682201.003.0022

Myzostomida comprise a taxon of mainly soft-bodied, flattened, oval, and disc-like marine protostomes. They are usually associated with echinoderms. Most myzostomids exhibit five pairs of uniramous parapodia comprising aciculae and hooked setae, but a distinct head, pygidium, or signs of external segmentation are lacking. In Myzostoma cirriferum, development comprises four free-swimming trochophora-like larval stages and juvenile development, including segment formation, takes place on its echinoderm host. The position of the group within Metazoa is under discussion, but recent phylogenomic data strongly support a close relationship with annelids or even a grouping of Myzostomida within the errant Annelida. The nervous system of myzostomids exhibits a circum-oral ring, nerves within the axial sucking pharynx, a prominent ventral nerve mass, and peripheral nerves leading towards the parapodia and lateral organs. The myzostomid brain is located dorso-laterally within the pharynx (introvert). The stomatogastric nerves originate in the region of the terminal nerve ring, located apically in the pharynx and run towards the posterior into the pharynx. The ventral nerve mass represents a ventrally oriented ring-shaped neuropil each consisting of two longitudinal nerve cords. The neuroanatomical data available for Myzostomida indicate a segmented ancestry of Myzostomida, and it supports recent molecular and morphological analyses that reveal myzostomids as part of the Annelida, such as the annelid-like nervous system comprising a dorsal anterior ganglion, presence of distinct circumoesophageal connectives, longitudinal nerves connected via twelve commissures, serial clusters of somata, and paired lateral nerves that run from the longitudinal nerves towards the body margin.

Keywords:   Parapodia, lateral organs, sucking pharynx, ventral nerve mass, commissure, lateral nerve

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