STAR-FISH AND SEA-URCHINS. 



293 



rig. 53. Exiciiiul neivj-plexusof Edifiius. 



Fig. 54. Structure of a nerve-trunk of Eclnnus. 



r^>-~.^ 



Fig. 55. Nerve-cells lying among the muscnlir fibres at the base of a spine in 

 Echiuus. 

 20 " 



