The brain of Echiniscus viridissimus Peterfi, 1956 (Heterotardigrada): a key to understanding the phylogenetic position of tardigrades and the evolution of the arthropod head

The brain of Echiniscus viridissimus , Peterfi, 1956 is composed of a series of orthogonally arranged neuropils. The most anterior neuropils are rireumbuccal, positioned dorso‐and ventrolateral to the buccal tube and are associated with ganglia for sensory receptors of the mouth cone. Posterior to these are neuropils and ganglia for the (1) internal cirri and (2) cephalic papillae, external cirri, cirri A and clavae. They are joined by two pairs of vertical tracts to neuropils lateral to the buccal tube. A model based on the postcephalic organization of the tardigrade nervous system is used to propose a transformation of segmental ganglia that gives an arrangement congruent with the pattern of neuropils in the brain. The analysis suggests that the brain is derived from nervous elements of four segments with the fourth segment having contributed paired dorsal ganglia and their connecting vertical tracts to the first trunk ganglia of the ventral chain. The organization of the head of tardigrades is compared with that of other lobopods and arthropods and several possible key evolutionary innovations are offered. In addition homologous characters for the heads of tardigrades and other lobopods and arthropods are proposed and the nomenclature for the tardigrade cephalic nervous system is discussed.