XXVI 



AMBULACRAL SYSTEM 



313 



The central part of the ambulacral system is a pentagonal 

 tube (Fig. 78, c ; Fig. 76, C. Amb. V) which surrounds the 

 gullet, and is called the ambulacral ring-vessel. From each 

 angle of the pentagon is given off a radial ambulacral vessel 



FIG. 78. The water vascular system of a Starfish (diagramatic). 



The ring-vessel (c} gives off five radial vessels (r), lateral off-shoots of 

 which (/) are connected with the tube-feet (/) and ampullae (a}. 



Inter-radially the ring-vessels give off Polian vesicles (<?/) and the 

 madreporic canal (m'} ending in the madreporite (in\ (From 

 Gegenbaur. ) 



(Fig. 78, r ; F'ig. 76, Rad. Amb. V) which proceeds to the 

 end of the corresponding arm, lying in the dihedral angle 

 included by the double row of ambulacral ossicles, and 

 consequently external to this portion of the skeleton (Fig. 76, 

 p). Each radial vessel sends off side branches (Fig. 78, r) 



