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Abstract: In recent years, there has been a growing interest in synthesizing studies on the structure of the nervous systems with phylogenetic questions (“neurophylogeny”). However, in the discussion on the phylogeny of Malacostraca, and the question of their closest relatives, little attention has been paid to brain morphology in the basal representatives of this taxon. To close this gap in our knowledge, this study sets out to analyze the architecture of the brain of Nebalia herbstii (Leptostraca, Phyllocarida, Malacostraca) in a neurophylogenetic context. Most phylogenetic studies consider the Phyllocarida to be the sister group to the Eumalacostraca, together forming the taxon Malacostraca, so that the brain organization of N. herbstii may possess many ancestral features. Classical histological techniques and 3D reconstruction as well as immunostaining and confocal laser-scan imaging revealed that besides taxon specific characteristics, the brain organization of N. herbstii exhibits a strong eumalacostracan affinity. The four nested optic neuropils are connected by two successive chiasms. The olfactory globular tract, connecting the olfactory lobes with second order integration centers in the lateral protocerebrum, features a contralateral connection. The olfactory lobes are composed of a radial array of spherical/spheroid glomeruli but serotonergic innervations of the olfactory glomeruli are absent. These and other architectural features are discussed in comparison to the central nervous system of Eumalacostraca, Remipedia, non-malacostracan Crustacea and Hexapoda, and are synthesized into a ground pattern of the malacostracan brain that might serve as a basis for neurophylogenetic discussion of the evolutionary relationship of these taxa. [Copyright &y& Elsevier]

Crustaceans provide a fascinating opportunity for studying adaptations to a terrestrial lifestyle because within this group, the conquest of land has occurred at least ten times convergently. The evolutionary transition from water to land demands various morphological and physiological adaptations of tissues and organs including the sensory and nervous system. In this review, we aim to compare the brain architecture between selected terrestrial and closely related marine representatives of the crustacean taxa Amphipoda, Isopoda, Brachyura, and Anomala with an emphasis on the elements of the olfactory pathway including receptor molecules. Our comparison of neuroanatomical structures between terrestrial members and their close aquatic relatives suggests that during the convergent evolution of terrestrial life-styles, the elements of the olfactory pathway were subject to different morphological transformations. In terrestrial anomalans (Coenobitidae), the elements of the primary olfactory pathway (antennules and olfactory lobes) are in general considerably enlarged whereas they are smaller in terrestrial brachyurans compared to their aquatic relatives. Studies on the repertoire of receptor molecules in Coenobitidae do not point to specific terrestrial adaptations but suggest that perireceptor events – processes in the receptor environment before the stimuli bind – may play an important role for aerial olfaction in this group. In terrestrial members of amphipods (Amphipoda: Talitridae) as well as of isopods (Isopoda: Oniscidea), however, the antennules and olfactory sensilla (aesthetascs) are largely reduced and miniaturized. Consequently, their primary olfactory processing centers are suggested to have been lost during the evolution of a life on land. Nevertheless, in terrestrial Peracarida, the (second) antennae as well as their associated tritocerebral processing structures are presumed to compensate for this loss or rather considerable reduction of the (deutocerebral) primary olfactory pathway. We conclude that after the evolutionary transition from water to land, it is not trivial for arthropods to establish aerial olfaction. If we consider insects as an ingroup of Crustacea, then the Coenobitidae and Insecta may be seen as the most successful crustacean representatives in this respect. • The review compares the olfactory pathway in terrestrial and marine Crustacea. • Elements of the olfactory pathway are largely reduced in most terrestrial Crustacea. • Elements of the olfactory system are highly enlarged in terrestrial hermit crabs. • Perireceptor events may be essential for establishing aerial olfaction in Crustacea. • Water-to-land transition is not trivial for arthropods to establish aerial olfaction. [ABSTRACT FROM AUTHOR]