ECHINODERMA TA 



65 



hard calcified walls, the stone canal (Fig. 31, sc). The stone 

 canal ends in the little perforated plate, the madreporite, 

 which will have already been observed in the external 

 examination of the starfish. Through the madreporite, water 

 enters and fills the whole system of tubes and ampullae ; the 

 latter can be contracted at will, forcing the water from them 

 into the tube-feet and so extending them ; then again by a 

 contraction of the muscles of the tube-feet, the water is 

 driven back into the ampullae and the feet consequently 

 contracted. In this way the tube-feet are brought into use in 

 locomotion as explained previously. 



It is obvious from the foregoing account that 

 the radial structure which is so marked externally, 

 extends to nearly all the internal organs, the stone canal being 

 the only important exception. This fact helps to explain the 

 curious power of regeneration possessed by a starfish. It is 

 well known that any arm detached from its body can grow 

 into a fresh five-rayed creature once more, for it contains all 

 the vital organs necessary 

 for life and growth. 



Other Asteroidea. 



Regeneration. 



Sun-stars. 



Besides the 

 common five- 

 rayed starfish, we find 

 on our coasts, living be- 

 tween tide marks, the 

 beautiful rosy Sun-stars 

 (Solaster papposus), each 

 having twelve or more 

 short rays. In both Star- 

 fish and Sun-stars and in 

 all members of the class of 

 the Echinoderms known 

 as Asteroidea, the digestive organ is prolonged into the arms. 



FIG. 33. The Sun-star (Solaster papposus). 



Class II.: OPHIUROIDEA 



The Brittle-stars or Ophiuroids differ from the 

 'Asteroids in their thinner longer arms, which are 

 sharply separated from the disc, and which contain no 

 VOL. i F 



Brittle-stars. 



