English: Neuroanatomy and serotonin-immunoreactivity in Remipedia and ground patterns of serotonin-immunoreactive neurons in Euarthropoda. (A) The cell cortex is colored in blue, neuronal tissue revealed by acetylated α-tubulin in red. The segmental nerves (ASN and PSN), the intersegmental nerve (ISN), the anterior and posterior commissure (AC and PC), the unpaired longitudinal medial neurite bundle (MNB) and the connectives (CON) are indicated for three trunk segments. (B) The 5HT-ir neurons in Remipedia can be distinguished in three groups: one pair of anterior 5HT-ir cells (ASC; blue), three pairs of medial 5HT-ir cells (MSC; red) and two pairs of posterior 5HT-ir cells (PSC; green). The fine homogenous network of 5HT-IR in the neuropil and projections into the CONs are indicated in gray. Two 5HT-ir projections (black) with unknown origin and target region could be observed in the ASN. (C) Ground patterns of 5HT-ir neurons in the ventral nerve cord of major euarthropod taxa (modified after [34]). Blue, green and red neurons indicate the anterior, posterior and medial cells, respectively. For cells drawn in unsaturated hues, a potential homology can only be surmised based on soma location. Black neurons represent cells for which no clear homology has been assumed to pancrustacean cells. Abbreviations: AC: anterior commissure; ASC: anterior 5HT-ir cell; ASN: anterior segmental nerve; CON: connective; ISN: intersegmental nerve; MNB: unpaired longitudinal median neurite bundle; MSC: medial 5HT-ir cell; PC: posterior commissure; PSC: posterior 5HT-ir cell; PSN: posterior segmental nerve.