THE STARFISH. 159 



motion allowed by the joint. Observe the muscles 

 connecting the ambulacral plates of the opposite sides, 

 just inside of the nerve. 



25. In the angle formed by the ambulacral plates, find the 

 cut-off end of the water-tube of the ray. Insert in 

 the end of this, the point of a drawn-out glass tube, 

 and inflate. When the ampullae are distended, press 

 on them with the finger and note the effect on the 

 tube feet ; with a lens examine the distended ampullae. 

 In fresh specimens the ampullae may be injected with 

 a colored liquid or with gelatine to be kept as perma- 

 nent preparations. In such preparations and in a 

 microscopic section of a properly prepared ray, it may 

 be seen that the water-tube of the ray sends off side 

 branches to the tube feet, and also that the cavities of 

 the tube feet and ampullae are continuous. By the 

 contraction of the ampullae the tube feet are extended, 

 and by the muscles in their walls they are moved from 

 side to side and applied to the surfaces on which the 

 starfish rests. The end is fixed by means of the sucker- 

 like disk at the tip of the foot to some foreign object ; 

 then by the contraction of the tube feet, the starfish 

 pulls its body along. 



The water finds its way through the madreporic 

 body into the stone canal, thence to the water-ring 

 around the mouth, and from this to the radial canals. 

 The water thus taken in probably serves for respira- 

 tion as well as for locomotion. 



26. Make a drawing of a cross section of a ray, showing as 

 many as possible of the above noted points of structure. 

 A slide with a series of very small starfishes shows well 

 how the rays are formed' as outgrowths of the diak. 



