TYPE ECHINODEKMA. 



537 



the left enferoccel, partly or wholly separated off from the 

 rest of that cavity. This ampulla, as already mentioned, 

 communicates with the exterior through, the madreporite. In 

 the radii tubes (re) arise from the ring which extend out to 

 the aboral extremity of the body in the elongated and spheri- 

 cal forms, and to the ends of the rays in the brachiate forms, 

 terminating frequently in tentacular structures (t) which pro- 

 trude to the exterior, pushing the ectoderm before them. 

 Along the course of these tubes lateral branches are given off 



FIG. 248. DIAGRAM TO SHOW THE ARRANGEMENT OF THE HYDROCCEL OF 



AN ECHINOUERM. 



a = ampulla. p = Poliau vesicle. 



as axial siuus. re = radial canal. 



cc = circular canal. sc stone-canal. 



M = madreporite. t = terminal tentacle. 



if tube-foot. 



which terminate either in tentacular structures, or else in 

 tubes terminating in a sucker, which, since they play an im- 

 portant role in locomotion, are termed tube-feet (tf). In many 

 forms a globular reservoir or ampulla (a) is attached to each 

 tube-foot, and valves are found at the junction of the branch 

 passing to the foot with the radial canal, so that the foot can 

 be extended to a considerable distance by the contraction of 

 the muscles in the walls of the ampulla and the consequent 

 forcing of water into it. By means of the sucker they may 

 then adhere to foreign objects, and their contraction then 

 produces a movement of the body towards the point of fixa- 

 tion. 



