ECHINODERMS 



147 



tube-feet, which, as we have seen, are slender tubular or- 

 gans, many in number, filling the grooves on the ventral 

 surface of each arm. This entire system of tubes and 

 reservoirs is full of water, taken in, it is said, through the 

 perforated plate, and, when the starfish wishes to advance, 

 many of the little reservoirs con- 

 tract, forcing water into the cav- 

 ity of the feet, with which they 

 are in communication, thus ex- 

 tending the extremity of the tubes 

 a considerable distance. The 

 terminal sucker of each foot, act- 

 ing upon the same principle as 

 those on the cuttlefish, attaches 

 firmly to some foreign object, 

 whereupon the muscles of the 

 foot contract, drawing the body 

 toward the point of attachment. 

 This latter movement is similar 

 to that of a boatman pulling him- 

 self to land by means of a rope 

 fastened to the shore. When the 

 shortening of the tube-feet has 

 ceased, the sucking disks release 

 their attachment, project them- 

 selves again, and this process is 

 repeated over and over. At all 

 times some of the feet are con- 

 tracting, and a steady advance of 

 the body is the result. 



This method of locomotion 



also obtains in the sea-urchins and cucumbers, but in the 

 serpent-stars the tube-feet have become modified into feel- 

 ers, and the animal moves, often rapidly, by means of twist- 

 ing movements of the arms. The feet have this character 

 also in the crinoids, where the animal is generally without 



PIG. 93. Sea-lily or crinoid. 



