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AGAIN ON THE PHYLOGENY OF WATER BEARS AND VELVET WORMS
and THEIR RELATIONSHIP TO ARTHROPODS

PAIR THIS POST with Scientific Illustration’s recent post …

Data reveal three major correspondences between the segmental ganglia of tardigrades and arthropods, including (i) contralateral projections and commissures in each ganglion, (ii) segmentally repeated sets of immunoreactive neurons, and (iii) an anteriorly shifted (parasegmental) position of ganglia.

These correspondences support the homology of segmental ganglia in tardigrades and arthropods, suggesting that these structures were either lost in Onychophora or, alternatively, evolved in the tardigrade/arthropod lineage.

[1] Alternative hypotheses on the phylogenetic position of the Tardigrada within the Ecdysozoa and the evolution of segmental ganglia.

  • (A) Tree illustrating controversial views on the phylogenetic position of tardigrades as either the sister group of arthropods, onychophorans, onychophorans plus arthropods, or one of the cycloneuralian taxa.
  • (B–H) Alternative scenarios on the evolution of segmental ganglia (dark-blue dots) in panarthropods, depending on the phylogenetic position of tardigrades favoured. More

[2] Schema comparing the ventral nervous systems of Onychophora, Tardigrada and Arthropoda.

Hatched lines indicate the position of legs. Abbreviations: al, anterior leg nerve; cf, central fibre mass; cn, somata-free connective; co, commissure; ga, ganglion; ip, interpedal commissure; lv, leg nerve; nc, nerve cord; ne, neuropil; pl, posterior leg nerve. More

[3] Organisation of the ventral nervous system and alignment of leg nerves in the onychophoran Euperipatoides rowelli.Light micrographs of a dissected specimen with removed digestive and genital tracts. Anterior is up. Arrows point to nephridial ducts

[4] General anatomy and organisation of the ventral nervous system and position of trunk ganglia in tardigrades.  Follow this link to see the organisation of the tardigrade nervous system. 

Read the article: Insights into the evolution of segmental ganglia in panarthropods.

Selective neuronal staining in tardigrades and onychophorans provides insights into the evolution of segmental ganglia in panarthropods  ||  by Georg Mayer, Christine Martin, Jan Rüdiger, Susann Kauschke, Paul A Stevenson, Izabela Poprawa, Karin Hohberg, Ralph O Schill, Hans-Joachim Pflüger and Martin Schlegel. 

BMC Evolutionary Biology 2013, 13:230
Open Access article: doi:10.1186/1471-2148-13-230

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