Peptidergic Messengers: A New Perspective of the Nervous System of Parasitic Platyhelminths

Abstract

Platyhelminths represent a strategic phylogenetic stage in the evolution of the nervous system. They are the most primitive animals alive today exhibiting bilateral symmetry with attendent cephalization and condensation of neurons into a central nervous system. Not surprisingly, their free-living members, the turbellarians, have been the subject of numerous behavioral, physiological, and anatomical studies from which sufficient information has been gleaned to indicate that much of the basic neuronal machinary of higher organisms is present in flatworms (Koopowitz, 1989; Gustafsson, 1990). Thus, even in lower organisms the so-called simple nervous system appears to be singularly complex, if less elaborate than that of higher organisms. But what of the nervous system in parasitic platyhelminths? Has parasitism led to a reduction or secondary simplification of the flatworm nervous system? Historically, the nervous system of parasitic platyhelminths has been difficult to evaluate. For example, it never has been an easy system to delineate or identify anatomically, using conventional staining with methylene blue or Golgi stains or with gold and silver chloride impregnations, nor does it lend itself readily to ultrastructural reconstruction by serial sectioning. In common with other organ systems in acoelomates, the flatworm nervous system is embedded in parenchymatous tissue and is therefore virtually impossible to dissect free from the rest of the body; and the small size of most parasitic organisms and the physiological problems of maintaining them in vitro present enormous technical difficulties for even the most deter-