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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 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. 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. 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. 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 deuterostomes are generally difficult to interpret, and the nervous systems of echinoderms and enteropneusts appear completely enigmatic. 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 anterior loop becomes part of the brain. 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 ciliated protostome larvae show ganglia/nerve cords that are retained as the adult central nervous system (CNS). It appears to be sensory, probably involved in metamorphosis, but knowledge is scant. Saving Earth Britannica Presents Earth’s To-Do List for the 21st Century.The apical organ of ciliated larvae of cnidarians and bilaterians is a true larval organ that disappears before or at metamorphosis.Britannica Beyond We’ve created a new place where questions are at the center of learning.100 Women Britannica celebrates the centennial of the Nineteenth Amendment, highlighting suffragists and history-making politicians.
#IN THE PROTOSTOME LINE THE BLASTOPORE BECOMES THE HOW TO#
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