224 



GENERAL ZOOLOGY 



One who has not observed a living starfish gets the idea 

 from preserved specimens that the body is rigid, and that 

 the plates of the skeleton make the arms inflexible. Such is 

 not the case, for the body of the living animal is very supple. 

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

 of the angles between the arms lies a small round disk usu- 

 ally of different color from 

 the surrounding parts (Fig. 

 121, ra; Fig. 122; Fig. 123). 

 This is the sieve plate, some- 

 times known by the name 

 madreporic body. The plate 

 is perforated with fine holes, 

 through which the sea water 

 passes to the stone canal (Fig. 

 121, s; Fig. 123). Near the 

 mouth the stone canal joins 

 the ring canal (Fig. 121, r; 

 Fig. 123), which in turn joins 

 five radial canals (Fig. 121, c ; 

 Fig. 123) 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. 121, a; Fig. 122; Fig. 123). From the 

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

 because of the fact that water passes through the entire set 

 of tubes it is called the water-vascular system. 



This system of tubes provides the starfish with its means 

 of locomotion. The walls of the tube feet and ampulla? are 



iFrom Hertwig-Kingsley's Manual of Zoology. 



Fig. 121. 



Water-vascular system 

 of starfish 



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

 canal ; m, sieve plate ; n, radial nerve ; 

 p, Polian vesicle ; r, ring canal, with 

 nerve ring beneath it ; s, stone canal ; 

 t, Tiedemann's vesicle l 



