292 



E CHINODERMS. 



tube-feet; but sometimes they end in a point, and cannot assist in locomotion, 

 though they may help respiration, when they are sometimes called tentacles. 

 If a single foot be touched, it immediately shrinks up, and if the touch be 

 vigorous, the adjacent tube-feet probably follow its example. Tube-feet torn from 

 the animal sometimes continue their waving motion, showing that this is, partly 

 at least, due to muscular action. Their movements are also caused by the squeezing 

 of a fluid into them ; for each foot is like an indiarubber tube closed at the end, 

 and passing through the test (as the shell of the sea-urchin is termed) to join with 

 one main tube, which runs along under the ambulacrum in a radial direction ; and 

 before it joins this radial canal, each tube-foot gives off a small swelling likewise 

 filled with fluid, so that when this swelling is contracted all the fluid is squeezed 



up into the foot, and pushes 

 it out like the linger of a 

 glove when blown into. The 

 radial canals pass along under 

 the ambulacra till they join 

 in a ring -canal surrounding 

 the mouth. Eventually this 

 circular canal is connected 

 with the surrounding water 

 by a canal passing right 

 across the body - cavity to 

 the other side of the animal, 

 near the vent, where it opens 

 to the exterior through a 

 plate pierced with a number 

 of pores. This plate is called 

 the madreporite,and the canal 

 leading to it — owing to the 

 limy deposits formed in its 

 walls — the stone-canal. This 

 whole system of fluid-filled 

 canals is termed the water- vascular system. The foregoing description refers to 

 its arrangement in a star-fish, or regular sea-urchin ; but the system occurs, with 

 various modifications, in all Echinoderms, and is one of the features that separate 

 the group from other animals. 



The Echinoderms are also peculiar in the possession of three, or perhaps four, 

 different systems of nerves, of which three, or at least two, are present at the same 

 time. One system supplies the skin, the tube-feet, and the intestine ; its chief parts 

 being a ring round the mouth, and radial nerves radiating therefrom. The second 

 system has a similar arrangement, but lies deeper, and supplies the internal 

 muscles of the body-wall. The third system, which is most fully developed in 

 crinoids, starts from the other side of the body, opposite to the mouth, and supplies 

 the muscles that work the arms and stem. If the arm of a star-fish be opened 

 from the back, there will be seen a pair of pleated extensions from the stomach. 

 If these be removed, there will be exposed a pair of orange-coloured tubes, some- 



DIAGRAM OF AMBULACRAL SYSTEM OP A STARFISH. 



/, Small swellings connected with the tube-feet ; k, The radial canal with 

 which they unite ; e, Ring-canal into which the radial canals open ; 

 c, d, Membranous sacs that serve as reservoirs for water from radial 

 canals ; a, Stone-canal, leading from ring-canal to the madreporite, 

 n ; m, Mouth. 



