692 CIRCULATION. 



connecting branch leading to the circular ring of the water system, is well 

 shown in Echinarachnius [PL XXXI. /'. ..'). No heart has been found in 

 Spatangoids proper. In Clypeastroids we have what we may perhaps con- 

 sider a heart, — an expansion of the vessel running along the inner side of 

 the oesophagus. 



