Cephalocarida (horseshoe shrimp) are a fascinating group of marine crustaceans that is unique in its ancient external morphology. Here the morphology and development of the cephalocarid nervous system was re-investigated in the species Hutchinsoniella macracantha, based for the first time on a combination of modern and traditional methods (semi-thin sectioning, immunolabeling, confocal microscopy, computer-aided 3D-reconstruction).
The study allowed for an unprecedented comparison to neuroanatomical data on other arthropods and improved our understanding of the evolution of the nervous system. The cephalocarid olfactory system, for instance, exhibits a complex secondary associative center termed the multi-lobed complex, which does not occur in any other crustacean but shows homologies to spiders, insects, and centipedes, suggesting that it dates back to the ancestor of all arthropods. Another cephalocarid peculiarity in comparison to other crustaceans is found in the ventral nerve cord: There, ganglia occur not only in the limb-bearing, but also in limb-less segments of the trunk. During the embryonic and larval phase, trunk ganglia develop extremely early, yet before the limb, unlike in any other known arthropod.
Since all studied neuroanatomical features in Cephalocarida are here interpreted as either plesiomorphic (“primitive”) with respect to the last common ancestor of crustaceans or autapomorphic (modern), they cannot be used to reconstruct the yet unresolved phylogeny of crustaceans and insects.