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Recent gene expression data suggest that the region on which the onychophoran antenna is situated corresponds to the anteriormost, apparently appendage-less region of the arthropod head. The fate of the onychophoran antenna (or any appendage-like precursor), also called the primary antenna, has been discussed intensively, and there are conflicting suggestions that this anteriormost non-segmental appendage gave rise either to the arthropod labrum or, alternatively, to the so-called frontal filaments found in certain crustaceans. Our data on early axogenesis in anostracan crustaceans show that even in the earliest embryos, before the antennula and antennal nerves are developed, the circumoral anlagen of the brain display very prominent nerves which run into the frontal filament organ (also known as the cavity receptor organ). This situation resembles the development of the antennal nerves in onychophorans, which leads us to conclude that the frontal filaments are indeed homologous to the primary antenna. Frontal filaments also appear to be more common in crustaceans than previously thought, removing the need for a complicated scenario of transformation from a primary antenna into the labrum.

Using immunohistochemical labeling against acetylated a-tubulin and serotonin in combination with confocal laser scanning microscopy and 3D-reconstruction, we investigated the temporary freshwater pond inhabitant Branchinella sp. (Crustacea: Branchiopoda: Anostraca) for the first time to provide detailed data on the development of the anostracan nervous system. Protocerebral sense organs such as the nauplius eye and frontal filament organs are present as early as the hatching stage L0. In the postnaupliar region, two terminal pioneer neurons grow from posterior to anterior to connect the mandibular neuromeres. The first protocerebral neuropil to emerge is not part of the central complex but represents the median neuropil, and begins to develop from L0+ onwards. In stage L3, the first evidence of developing compound eyes is visible. This is followed by the formation of the visual neuropils and the neuropils of the central complex in the protocerebrum. From the deutocerebral lobes, the projecting neuron tract proceeds to both sides of the lateral protocerebrum, forming a chiasma just behind the central body. In the postnaupliar region, the peripheral nervous system, commissures and connectives develop along an anterior-posterior gradient after the fasciculation of the terminal pioneer neurons with the mandibular neuromere. The peripheral nervous system in the thoracic segments consists of two longitudinal neurite bundles on each side which connect the intersegmental nerves, together with the ventral nervous system forming an orthogon-like network. Here, we discuss, among other things, the evidence of a fourth nauplius eye nerve and decussating projecting neuron tract found in Branchinella sp., and provide arguments to support our view that the crustacean frontal filament (organ) and onychophoran primary antenna are homologous. J. Morphol. 277:1423-1446, 2016. © 2016 Wiley Periodicals, Inc.

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