STARFISH 77 



with the perforated calcareous madreporite situated on the aboral 

 surface of the central disc. From the madreporite a tube, the 

 stone canal, runs orally toward the mouth, in which region it 

 connects with the ring canal, encircling the mouth. From the 

 ring canal, five radial canals lead off — one following the roof of 

 the ambulacral groove to the tip of each arm. From the inner 

 surface of the ring canal nine tiny sacs (tiedemann's bodies) 

 project toward the mouth. In them, amoeboid cells are formed 

 which probably function in excretion. Throughout the length of 

 each radial canal, many minute paired branches arise to which the 

 tube feet are attached. 



Each of the hollow, muscular tube feet is provided with the 

 bulb-like portion, the ampulla, which lies above the roof of the 

 ambulacral groove in the body cavity. The body of each tube 

 foot proper extends orally and projects to the exterior through 

 a tiny opening between the ambulacral ossicles, where it ends in a 

 circular suction disc. The latter is a very important feature 

 because it can be pressed against almost any solid object, and the 

 contact maintained by suction. It is by such action that food is 

 secured and locomotion performed. 



We may now consider certain important features connected with 

 the functioning of this extraordinary system. Water is drawn 

 into the water vascular system through the madreporite, it is 

 believed, by the action of ciliated cells lying within the madreporite, 

 and it fills the various canals and the ampullae of the tube feet. 

 When it is necessary to extend the tube feet, the muscular walls of 

 the ampullae contract and thus force the water down into the body 

 of each tube foot, and this results in its enlargement and extension. 

 A tube foot thus forced into contact with a solid object adheres 

 through the action of the suction disc noted above. Release of the 

 tube feet is brought about by a reverse flow of water into the am- 

 pulla. When a considerable number of tube feet thus become 

 attached to an immovable object, a slow creeping movement of 

 the entire organism can be effected by a coordinated contraction 

 and shortening of the tube feet alternating with their release and 

 a subsequent reextension and attachment in an advanced position. 



In feeding, the tube feet of an arm may be applied and attached 

 to a piece of food and the latter gradually moved to the mouth by 

 the action of the tube feet alone or by the coordinated bending 

 action of the arm. 



