ASTERIAS TUBE-FEET 



443 



appendage, called the median tentacle, at the base of which is 

 the eye. 



The manner in which this complicated system acts is as 

 follows : When the tube-foot is to be stretched out the ampulla 

 contracts and drives the fluid downwards. The contraction of 

 the ampulla is brought about by muscles running circularly 

 around it. The tube-foot is thus distended and its broad flattened 

 end is brought in contact with the surface of the stone over 

 which it is moving and is pressed close against it. The muscles of 

 the tube-foot itself, which are arranged longitudinally, now com- 

 mence to act, and the pressure of the water preventing the tearing 



Fig. 191. Diagrammatic cross- 

 section of the arm of a 

 Starfish, adamb, Adamtra- 

 lacral ossicle ; amb, ambu- 

 lacral ossicle ; amp, ampulla 

 of tube - foot ; brunch, 

 papula ; car, carina! plate ; 

 d.tat, dorso-lateral plate ; 

 inf.marg, infero - marginal 

 plate ; p.br, peribronchial 

 space ; ped, pedicellaria ; 

 s.marg, supero - marginal 

 plate. The nervous ridge 

 between the bases of the 

 tube-feet and the two peri- 

 haemal canals above this 

 ridge are shown in the figure 

 but not lettered. 



away of the sucker from the object to which it adheres, the 

 Starfish is slowly drawn forward, whilst the fluid in the tube-foot 

 flows back into the ampulla. 



If each tube-foot were practically water-tight, then each would 

 be entirely independent of all the rest, and it would not be easy 

 to suggest a reason for the presence of the complicated system 

 of radial canals and stone-canal. Just at the spot, however, 

 where the transverse canal leading from the radial canal enters 

 the tube- foot there is a pair of valves which open inwards and 

 allow fluid to pass from the radial canal into the tube-foot but 

 prevent any passing outwards in the reverse direction. The 

 presence of these valves renders it probable that the tube-foot is 

 not quite water-tight ; that when it is distended under the 

 pressure produced by the contraction of the muscles of the 

 ampulla, some fluid escapes through the permeable walls ; and 



