STARFISHES. 69 



rays, which, as we have seen, are each moment in- 

 creasing their surface of attachment to the rock, are 

 finally fully fixed thereto. Finally the body and other 

 rays are twisted or swept over, and the star finds itself 

 at last in its natural position mouth downwards. 



These tube-feet are unique as regards the starfish 

 class. You may note on the upper surface of the 

 star, at the angle formed by the juncture of two of 

 the rays with the body, a round plate. This is the 

 " madreporiform plate," and, like the lid of a pepper- 

 box, it is perforated with holes. It acts as a strainer, 

 and keeps solid matters from getting into the tube to 

 which it serves as an entrance. This tube leads 

 downwards to another and circular tube, inside the 

 starfish body and surrounding its mouth. From this 

 circular tube pass five other tubes, one for each ray. 



Now, if we dissected our starfish, you would find 

 that the tube-feet are given off from the main tube 01 

 each ray or arm, and each tube-foot has at its attached 

 end, or base, a little bag or sac. The whole apparatus 

 I have described is one whereby, through the admis- 

 sion of water to the system of tubes, the tube-feet 

 can be distended or the reverse, and the movements 

 of the animal be duly provided for. If the starfish 

 wishes to walk, water is sent from the little sacs at- 

 the bases of the feet into the tubes. These latter are 

 thus rendered tense. Their suckers are applied to the 

 rock-surface, with the result that part of the water is 

 forced back into the sacs, the sucker is left firmly 

 adherent by the pressure of the sea-water outside. 

 Then, by forcing the water from the sac into the tube- 

 feet, the suckers are set free, and, by this alternate 

 contraction and expansion of the tube- feet, the ad- 

 hesion of these organs or their freedom is secured. 



