Branch Echinodermata 39 



under side of the ray. These structures are called 

 ampullae (a single one is called an ampulla), and are 

 directly connected with the tube feet. In fact, an 

 ampulla is an enlarged extension of a tube foot. At 

 the junction of the ampulla and the tube foot, there is 

 a short tube connecting them with a long one which 

 passes close to the nerve trunk along the roof of the 

 ambulacral groove. 



This long tube passes the entire length of the ray, 

 and connects with a pentagonal, or rather circular, tube 

 around the peristome. Carefully push aside the parts 

 lying beneath the madreporite, and you will see a long, 

 hard, white tube extending downward. This is called 

 the stone canal. Does it connect with the circular 

 tube just mentioned .■■ The madrepgrite is very finely 

 perforated, and through these openings water passes 

 down the stone canal, filling the circular tube. From 

 the circular tube the water passes along the radial 

 tubes in the arms, and from them into the short branch 

 tubes leading to the ampullae and tube feet. The 

 whole system is called the water-vascular system. A 

 patient study is required to understand well the ar- 

 rangements of parts in this system. 

 XVIII. — Learn how locomotion is performed by the water- 

 vascular system. As has been said, the system is filled 

 with water through the pores of the madreporite. 

 Water is forced from the ampullae into the tube feet, 

 throwing them forward. The contracting walls of the 

 tube feet then force the water back into the ampullae, 

 and suction makes the feet stick to the surface upon 

 which they were lying. The body may be then drawn 

 forward and the operation of extending the feet re- 

 peated. From what you know of the animal's mode 



