The nervous systems of Hoplonemertea (Nemertea)

We investigated the nervous system of 13 species of hoplonemerteans of different lineages using serial histological sectioning, Azan staining, immunohistology and electron microscopy. The central nervous system (cns) of hoplonemerteans consists of a circular brain and laterally located medullary cords. It is composed of a central neuropil and surrounding neurons. The neurites differ in diameter and may be arranged as spherical regions inside the brain, known as glomeruli in Annelida. Glia cells are abundant in the cns. Several sensory structures such as eyes are present and well-developed. The present study completes our survey into the nervous system of all subgroups of Nemertea. Based on these data we are able to reconstruct the putative ancestral state of the nervous system of Neonemertea and Hoplonemertea. The brain of the last common ancestor of Neonemertea was most likely composed of two crescent anterior halves connected by a dorsal and a ventral commissural tract. Each half split posteriorly into a dorsal and a ventral lobe that is continuous with the lateral medullary cord. The organism must have had eyes, which got lost in several groups of Hoplonemertea independently and a single cerebral organ duct, which is doubled in certain polystiliferan and cratenemertean taxa. Additionally, certain changes in the morphology of the cns correlate with the life style such as the reduction of the dorsal lobe of the brain in Pelagica and the loss of the cerebral organs in certain commensal, interstitial and pelagic species.