314 THE STARFISH less. 



which communicate with the hollow tube-feet (Fig. 78,/; 

 Fig. 76, T. F.), and each tube-foot is connected by a narrow 

 canal passing through an ambulacral pore (p. 310) with a 

 bladder-like body, the ampulla (Fig, 78, a ; Fig. 76, Amp) 

 lying in the coelome. The ampull?e consequently form a 

 double row of bladders along the ventral region of the 

 interior of the arm. 



The ring-vessel also gives off inter-radially, i.e., in the 

 intervals between the arms, bladder-like bodies, the Polian 

 vesicles ( Fig. 78, ap\ one or two in each inter-radius. In one 

 of the inter-radii there also goes off from the ring-vessel a tube, 

 called the stone-canal (Fig. 78, m ; Fig. 76, St. c) from the 

 fact that its walls are calcified, which passes directly upwards 

 and becomes connected with the madreporite (Fig, 78, tn ; 

 Fig. 76, A, Mdpr). The latter is perforated by minute 

 apertures which are in communication with the cavity of the 

 stone-canal, and in this way the ambulacral system is placed 

 in direct communication with the surrounding water. 



The whole ambulacral system contains a watery fluid, and 

 its walls consist of a lining of epithelium and an outer 

 muscular layer particularly well developed in the ampullae 

 and tube-feet. Contraction of the muscles of the ampullae 

 forces water into the tube-feet, and causes protrusion of 

 these organs : their withdrawal is brought about by the con- 

 traction of the longitudinal muscles in their walls, by which 

 the fluid is forced back into the ampullae. 



Thus the whole ambulacral system forms an elaborate 

 locomotory apparatus worked by water-power. It is quite 

 confined to Echinoderms. In all the other higher animals 

 movements are effected by the direct, and not, as in this 

 case, by the indirect action of muscles. 



A second system of vessels constitutes the so-called 

 Iflood-system. Surrounding the gullet below the ambulacral 



