240 GENEPxAL ZOOLOGY 



because of the fact that water passes through the entire set 

 of tubes it is called the water-vascular system. 



This set of organs is a mechanism which enables the star- 

 fish to move. The stone-canal being inclosed in a sheath of 

 calcareous substance has a constant diameter, and so have the 

 ring-canal and the radial canals ; but the ampullse and the 

 tube-feet are not inclosed by an unyielding cover, and hence 

 they can increase and decrease the volume of their contin- 

 uous cavity. Preparation for locomotion is made when the 

 ampulla by contracting causes the water to close a valve at 

 the branch from the radial canal, and then forces the contained 

 .vater into the cavity of the tube-foot. Simultaneously with 

 the act of forcing water into the tube-feet, circular muscle- 

 libers in those organs contract and cause the water to extend 

 them to their greatest length. Next, the sucking-disks fasten 

 to some object, it may be even a smooth surface like the glass 

 side of an aquarium tank. The longitudinal muscle-fibers 

 in the tube-feet then contract and drive the contained water 

 back to the ampullae. When the hundreds of tube-feet in the 

 advancing arms of a starfish are contracting to pull the body 

 along and extending to renew their hold, the arms themselves, 

 turned up a little on the tips, remain in a more or less set 

 attitude, although they may bend now and then to pass an ob- 

 struction. When it is necessary the animal can bend its arms 

 and central disk, and pass through seemingly impossible holes. 



The Digestive System. The mouth (Fig. 121, 1) is a circular 

 opening surrounded by a thin, circular membrane, the lip. 

 There is ho oesophagus, for the stomach (Fig. 121, 2) begins 

 at the mouth and occupies the greater portion of the central 

 disk, besides extending two short pouches (Fig. 119, l ; Fig. 

 121, 3) into every arm. At the aboral end of the stomach five 

 tubes branch off (Fig. 119, 2; Fig. 121, 5) and are divided 

 again into ten pyloric cceca (Fig. 119, 3; Fig. 121, 6). The 

 cavities of these organs are continuous with the cavity of the 



