section_path: Definition
section_name: Definition
paragraph_text: Definition
  
  The most universal features of the body structure of molluscs are a mantle with a significant body cavity used for breathing and excretion, and the organization of the nervous system. Many have a calcareous shell.
  
  Molluscs have developed such a varied range of body structures, finding synapomorphies (defining characteristics) to apply to all modern groups is difficult. The most general characteristic of molluscs is they are unsegmented and bilaterally symmetrical. The following are present in all modern molluscs:
  
  Other characteristics that commonly appear in textbooks have significant exceptions: