1. Numerous sense organs are closely associated with the nerve rings . 2. Nervous system consists of a nerve ring and many longitudinal nerve cords. 3. The nervous system is reasonably simple, consisting of a ganglionated nerve ring surrounding the pharynx. 4. Below this lies a second nerve ring , giving off two brachial nerves into each arm. 5. It consists of a central nerve ring surrounding the mouth, and radial nerves branching into the arms. 6. The nervous system usually consists of a distributed net of cells, although some species possess more organised nerve rings . 7. Two nerve rings lie close to the margin of the bell, and send fibres into the muscles and tentacles. 8. In species lacking nerve rings , the nerve cells are instead concentrated into small structures called " rhopalia ". 9. It appears quite structureless'in vivo', but histological staining makes its primitive gut, nerve ring and muscles apparent. 10. This arrangement means the brain, sub-pharyngeal ganglia and the circum-pharyngeal connectives form a nerve ring around the pharynx.
