ANATOJir AND PHYSIOLOGY OF ECHlNODEliMti 241 



rounding the oesophagus, and from this canal a tube runs out and 

 along each of the radii. From this radial canal in turn a series 

 of tube-feet are given off on the right and left ; these are the 

 locomotor organs (Fig. 232). 

 Thus the locomotor organs are 

 tubes closed at the outer end 

 and full of water. At its inner 

 end each tube-foot is swollen 

 into a large bladder. The way 

 a tube-foot acts is as follows: 

 The circular muscles in the wall 

 of the bladder and of the foot 

 itself contract, thus forcing 

 water out into the tube-foot, 

 which in consequence elongates 

 enormously and may reach out 

 beyond the tip of the spines. 

 Upon touching a solfd object 

 the sucker on the end of the 

 tube catches hold. Then the 

 muscles that run lengthwise of 

 the foot contract and shorten 

 the foot, its fluid flowing in- 

 to the bladder. If the solid 

 object is immovable, then the 

 starfish or sea-urchin moves toward it ; but if the object is 

 easily moved, it may be brought toward the echinoderm. 



Organs of Nutrition. — A complete foot-canal, beginning 

 with a mouth and ending at an anus, is found in echinoderms 

 in general, excepting the serpent-star and a few other starfish. 

 The mouth is guarded by hard parts which in sea-urchins 



Fig. 2.31. — Spiculaj from the skin of 

 sea-cucumbers. 1, anchor and an- 

 chor plate; 2, "stool" of Cucu- 

 maria; (Fig. 229) .3, " cross " from 

 a Cucumaria ; 4. rod from tube-feet ; 



5, supporting plate from tube-feet ; 



6, another form of " stool " ; 7, rod 

 from tube-feet ; 8, latticed hemi- 

 sphere ; 9, "wheel." All much 

 magnified. From Lang. 



