THE STARFISH AND SOME ALLIES 



239 



plates setting together like the rafters of a frame house. 

 Between the plates four rows of slender, flexible tube-feet 

 (Fig. 120, ab) extend to the outside. The groove is called 

 the ambulacral groove, because it protects the ambulacral 

 organs (Lat. ambulare, to walk). A few of the ambulacral 

 organs (tube-feet) near the tips of the arms are sense-organs 

 for "smelling" food. The remainder are the organs of loco- 

 motion. At the very end of each arm there is a small red 

 eye protected by a circle of spines. 



The Water-Vascular System. On the aboral surface at one 

 of the angles between the arms lies a lens-shaped structure 

 (Fig. 11 9, 8; Fig.l20,w; Fig. 

 121, 10). This is the sieve- 

 plate, sometimes known by 

 the name madreporic body. 

 The plate is perforated with 

 fine holes through which sea- 

 water passes to the stone-canal 

 (Fig. 120, s ; Fig. 121, 11). 

 Near the mouth the stone- 

 canal joins the ring-canal 

 (Fig. 120, r; Fig. 121, 12), 

 which in turn joins five 

 radial canals (Fig. 120, c\ Fig. 

 121, 13) that extend through 

 the middle of the roof of the 

 ambulacral groove. Many 

 short tubes branch off in pairs 

 from the radial canals and 

 join the tube-feet. At the 

 exposed end of a tube-foot is 

 a sucking-disk ; at the inner end is a bulb-shaped expansion, 

 the ampulla (Fig. 119, 9 ; Fig. 120, a ; Fig. 121, 14). From the 

 sieve-plate to the tube-feet there is a continuous cavity, and 



FIG. 120. Ambulacral System of 

 Starfish 



a, ampullae; ab, tube-feet; c, radial 

 canal ; m, sieve-plate ; ?^, radial nerve ; 

 p, Poliau vesicle ; r, ring-canal, with 

 nerve-ring beneath ; ,s, stone-canal ; 

 t, Tiedemann's vesicle 



(From Hertwig-Kingsley's Manual of 

 Zoology) 



