The apical organ of ciliated larvae of cnidarians and bilaterians is a true larval organ that disappears before or at metamorphosis. It appears to be sensory, probably involved in metamorphosis, but knowledge is scant. The ciliated protostome larvae show ganglia/nerve cords that are retained as the adult central nervous system (CNS). Two structures can be recognized, viz. a pair of cerebral ganglia, which form the major part of the adult brain, and a blastoporal (circumblastoporal) nerve cord, which becomes differentiated into a perioral loop, paired or secondarily fused ventral nerve cords and a small perianal loop. The anterior loop becomes part of the brain. This has been well documented through cell-lineage studies in a number of spiralians, and homologies with similar structures in the ecdysozoans are strongly indicated. The deuterostomes are generally difficult to interpret, and the nervous systems of echinoderms and enteropneusts appear completely enigmatic. The ontogeny of the chordate CNS can perhaps be interpreted as a variation of the ontogeny of the blastoporal nerve cord of the protostomes, and this is strongly supported by patterns of gene expression. The presence of 'deuterostomian' blastopore fates both in an annelid and in a mollusk, which are both placed in families with the 'normal' spiralian gastrulation type, and in the chaetognaths demonstrates that the chordate type of gastrulation could easily have evolved from the spiralian type. This indicates that the latest common ancestor of the deuterostomes was very similar to the latest common pelago-benthic ancestor of the protostomes as described by the trochaea theory, and that the neural tube of the chordates is morphologically ventral.
KEY WORDS: Deuterostomy, Metamorphosis, Nervous systems, Dorsal-ventral orientation
IntroductionStudies on ontogeny are very important for our understanding of nervous system evolution and diversification, and new methods have made important contributions to knowledge of the structure and development of larval nervous systems and their relationships to adult nervous systems.The nervous system has for a long time been considered one of the most conservative animal organ systems and has therefore been given high importance in studies of animal evolution. More than a century ago, Hatschek (Hatschek, 1888) introduced the names Zygoneura (now Protostomia), Ambulacraria and Chordonia (now Chordata) for three major groups of the Bilateria, based on the structure and position of the central nervous system (CNS). The Zygoneura were characterized by a paired longitudinal ventral nerve cord [a cluster of nervous cells including their cell bodies, as opposed to a nerve, which is a bundle of axons (Richter et al., 2010); the ventral nerve cord may
REVIEWThe Natural History Museum of Denmark, University of Copenhagen, Universitetsparken 15, DK-2100 Copenhagen, Denmark.*Author for correspondence (cnielsen@snm.ku.dk) be specialized into rows of ganglia connected by connectives] and the Chordonia by an unpaired dorsal n...