122 HOW A STAR-FISH WALKS. 



disk, which are so evidently the star-fish's means of locomotion that you 

 at once call them feet. This is true enough so far as their function is 

 concerned — for Five -fingered Jack certainly does walk by means of 

 them — but entirely wrong anatomically; no Radiate has "feet," prop- 

 erly speaking. In order to see how the little animal makes use of these 

 hundreds of walking appendages we must dissect him. 



It then appears that through the sieve-like madreporic bodj r , on the 

 back of the disk, filters a constant current of pure sea -water. This is 

 received into a system of circular canals, which ramify on each side of 

 every ray, and through minute openings in the broad plates on the lower 

 side of the arms send out fibres, which, when swelled full of water, ap- 

 pear as the rows of feet-tubes already mentioned. These feet-tubes are 

 called ambulacrce ; the grooves along each side of the arm, where they 

 spring, and where they are supplied with water from the main canal un- 

 derneath, the arribulacral grooves ; while the plates themselves, and the 

 whole concave under-surface between the spiny processes bordering the 

 rays, form the arribulacral tract. 



Now the star-fish's body is always full of water; besides the large 

 stream flowing in through the madreporic body, a constant inflow seems 

 to take place by absorption through the thousand minute water-tubes that 

 wreathe about each spine, notwithstanding that the microscope is unable 

 to detect any opening in them. This insures that the ambulacrae shall 

 always be full of water ; but the creature can control this matter. 



When he wishes to take a step he stretches one, a dozen, or a score 

 of feet-tubes forward, and draws a slight amount of water from each, 

 which causes a contraction of their sucker-disks, and gives them a firm 

 hold. By a reverse process he lets go with his other feet, and by main 

 strength drags his body up as far as he can. This operation, frequently 

 repeated, would give a continuous movement to his body which is not un- 

 graceful, as he dips down into a hollow or bends himself slowly over some 

 obstacle. His movements are very deliberate, and his progress hardly as 

 fast as the second-hand of a watch. It is to the fulness of this water- 

 system that the animal owes its plump appearance. Take him out of the 

 sea, and the water will pour out all over him in a fierce perspiration, 

 which soon leaves him flat and thin on your palm. I may as well say 

 here that any one can handle star-fishes without fear; the old idea that 

 they were poisonous was a worthless superstition. 



In addition to their water-system for locomotion, star-fishes have a heart 

 and system of blood-vessels. This consists of two circular vessels, one 

 around the intestine, and one around the gullet, or heart, intervening be- 



