GENERA L CHA RA C TERIS TICS 



3259 



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 India rubber tube closed at the end, and passing 



through the test (as the shell of the sea urchin is termed) to join with one 



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



before it joins the 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 finger 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 y^ - y-^onv^c ^\\ </ 



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 DIAGRAM OP AMBTTLACRAI, SYSTEM OF A STARFISH. 



. ,. , f. Small swellings connected with the tube feet; k. The radial canal 



leading tO It OWing tO ttie with W hich 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. 



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 starfish, 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 starfish 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-colored tubes, somewhat branched 



