ISSN 1062-3590, Biology Bulletin, 2010, Vol. 37, No. 3, pp. 277–287. © Pleiades Publishing, Inc., 2010. Original Russian Text © N.M. Biserova, I.I. Gordeev, J.V. Korneva, M. M. Salnikova, 2010, published in Izvestiya Akademii Nauk, Seriya Biologicheskaya, 2010, No. 3, pp. 333–344. 277 Glial cells were revealed for the first time in brains of vertebrates. It was thought for a long period of time that they are absent in the nervous system of higher animals. Glial cells of vertebrates occupy up to 50% of the brain volume; they have wide structural diversity and are of various origins, from neuroectoderm, mes- enchyma, neuromesoderm, and stem cells. Glia is topographically divided into central (astrocyte, olygo- dendrocytes, and their derivatives) and peripheral (Shwann’s cells and satellites), which form myelin membranes of nerves. Morphological types of glial cells differ in structure and specific molecular mark- ers, but may also transform from one type into another. It is proved that glial cells of vertebrates and inverte- brates make a contribution to functioning of neurons and are able to perceive signals from them (Coles, Abbot, 1996). The electrophysiological and metabolic answer to the action of many transmitters and modu- lators is registered in the glial cells (Cooper, 1995). Close functional interactions between neurons and glial cells of many invertebrates are established. It is not known yet at what stage of evolutionary development structural and functional differentiation of glial cells occurred. Ultrastructural and immunocy- tochemical studies confirm the existence of glia in nemerteans and annelids. Glia of nemerteans is pre- sented by numerous small cells, which are revealed in cerebroganglion and in lateral trunks. Their cytoplasm is rich with fibrils; processes form a membrane of neu- rons and neuropile located between the fibrillar plate and neural elements (Golubev, 1982; Sotnikov et al., 1994). Glial cells of annelids are necessary compo- nents of the nervous system of all studied species. They form ganglion and neuropile membranes and differ in the presence of fibrils and filaments as the main com- ponents of cytoplasm of perikaryon and trunks (Hulsebosch, Bittner, 1981). Glia of annelid worms is divided into cortical, which forms the external mem- brane of ganglions, and medullar, which participates in isolation of neuropiles and trunks (Golubev, 1982). Structural differences between cortical and medullar cells of glia are most strongly pronounced in suctorial annelids, the medullar glia of which is composed of gigantic glial cells of neuropiles. A constant number and location and a clearly expressed zonality of the cytoplasm, which includes fascicles of intermediate filaments by their width replying glial filaments of ver- tebrates (Riehl, Schlue, 1998), are typical for them. The cortical membrane is made up of microglial cells, which are able to move (Golubev, 1970; 1982; Sotni- kov et al., 1994). Oligochaetes possess a developed sys- tem of glia-neuron interactions; myelin is revealed in them, and gigantic nerve fibers in the trunk are iso- lated by a myelin-like membrane (Zoran et al., 1988). The myelin membrane of the earthworm consists of 20–200 layers and is spirally twisted in the ends in the area of the isthmus as in vertebrates (Roots, Lane, 1983; Roots et al., 1991). The difference from verte- brates is in the unequal compact structure of myelin; sometimes layers of cytoplasm are left between mem- branes in the form of a sandwich (Gunther, 1976). There is still no answer to the question whether specialized glial cells in the nervous system of free-liv- ing and parasitic flatworms exist. A review of the liter- ature indicates that turbellarian worms, cestodes, trematodes, sturgeon cestodes, temnosephallides, and monogenes have cells that form membranes of ganglia and trunks, which participate in wrapping of axons ZOOLOGY Structure of the Glial Cells in the Nervous System of Parasitic and Free-living Flatworms N. M. Biserova a , I. I. Gordeev a , J. V. Korneva b , and M. M. Salnikova c a Moscow State University, Faculty of Biology, Leninskie Gory, Moscow, GSP-1, 119991 Russia e-mail: nbiserova@ yandex.ru b Papanin Institute for Biology of lnland Waters, Russian Academy of Sciences, Borok, Nekouz, Varoslavl oblast, 152742 Russia c Ulianov–Lenin Kazan State University, Biology-Soil Faculty, Kremlevskaya ul. 18, Kazan, 420008 Russia Received May 12, 2009 Abstract—This study is devoted to ultrastructural and immunosytochemical investigation of the nervous sys- tem in parasitic and free-living platyhelminthes to learn if glial cells exist in the nervous system of flatworms. We described the ultrastructure of different types of glial cells and the peculiarities of myelinization of gigantic axons; immunoreactivity to the S100b protein is revealed. Comparative analysis of the glia structure of anne- lids and platods is given; structural, functional, and evolutionary aspects of myelinization of gigantic axons, which are revealed in cestodes, are discussed. DOI: 10.1134/S106235901003009X