The Central Nervous System

Abstract In segmented animals such as annelids and arthropods, the nervous system is organized with one ganglion or a pair of ganglia per segment, plus an enlarged ganglionic mass (brain) at the anterior end. The absence of segmentation in molluscs allows the nervous system to be organized in a variety of ways. Whereas some molluscs have no nervous ganglia at all (nerve cells are simply scattered along the nerves), one striking feature of the gastropod anatomy is nervous ganglia. The fact that the ganglia occur in a bewildering variety of numbers and positions makes it nearly impossible to discuss subtleties in a brief description.