A <xref>taxonomic</xref> classification, within <xref>phylum</xref> <xref>Mollusca</xref> - those in the classes having heads: <xref>Cephalopoda</xref>, Pteropoda and Gasteropoda
User Defined Definitions
"that part of the central nervous system that includes all the higher nervous centers; enclosed within the skull; continuous with the spinal cord"