ECHINODERMATA. 



211 



canal, radial canals pass out into each arm (Fig. 65, r. t). The radial 

 canals in their turn become distributing centers, passing the water 

 sidewise into bulbs (ampullae, Fig. 65, a) which are the inner end 

 of the ambulacral feet which were seen hi the external studies of the 

 starfish, in the grooves on the oral surface. The feet are constructed 

 in such a way as to have a sucking disc at the free end, and it is by 

 the attachment of hundreds of these that the animal holds to solid 

 objects and crawls in its slow fashion. Its arms are also flexible, 

 and if placed on its back, it can turn the end of the arm so that a few 



-f 



FIG. 65. Diagram of a portion of the water-vascular system of a starfish, taken 

 alone, a, ampulla; /, ambulacral feet; m, madreporic body; p, Polian vesicles; r.c., 

 ring canal, with the upper portion removed at the right; r.t., radial water tubes; s., 

 stone canal. 



Questions on the Figure. Where does the water enter the system ? 

 Write the parts enumerated above in their proper order, commencing 

 with the point of entrance. At what points is the system really 

 of use to the animal? In what various ways is the water- vascular 

 system believed to be of use in the different classes? 



