The architecture of the nervous system of Echiniscus testudo (Echiniscoidea, Heterotardigrada)

Despite the fact that there are several descriptions of the nervous system of Tardigrada there is still dispute about how to interpret the organisation of these structures. Is the structure of the brain homologous to segmental equivalents of the arthropod brain or not? The latest studies concerning these questions concentrate on specimens from the taxon Eutardigrada, but a representative of Heterotardigrada has been missing. Therefore, in this immunohistochemical study the organisation of the nervous system of Echiniscus testudo is investigated and can be described as follows. In the brain of E. testudo several distinct regions can be recognised. These are the anterior, dorsal, dorsoventral, inner and posterior clusters. Furthermore a ventral cluster can be detected that is connected to the dorsoventral clusters. The brain is followed by four ventral ganglia that are positioned slightly anterior to their corresponding pair of legs. The brain and the first ventral ganglion are connected via connectives showing tyrosinated α-tubulin and RFamide immunoreactivity. A connection between the first ventral ganglion and the ventral cluster could not be detected, at least not with our antibody set. In longitudinal direction the ventral ganglia are chained by connectives, and transversal interconnections via commissures are present. In addition to the commissures connecting the hemiganglia, three commissures could be detected lying anterior to the second, the third and the fourth ventral ganglion. In some specimens between the second and the third ganglion, bilaterally arranged neurites could be detected that run in a loop to the dorsal part of the specimens. The hereindescribed organisation is compared with previously published data on the nervous system of Tardigrada, and we conclude that the lobate organisation of the brain of E. testudo comprises an arrangement due to functional needs rather than to reflect an organisation into arthropod-like proto-, deuto- and trito-cerebrum.