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 tubefeet 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. Fie. 93.—Sea-lily or crinoid. 
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 
