ECHINODERMA 3 



side branches issue, each ending in a tube-foot. At the base of 

 each tube-foot there is generally a small vesicle, the afnjyulla. 

 When this am^^uUa is compressed, the water (which fills the whole 

 watervascular system) is pressed out into the tube-foot which 

 is thus extended. When the ampulla is relaxed and the muscles 

 of the tube-foot contract, the water flows back from the tube-foot 

 which is thus shortened. A peculiar valvular structure in the am- 

 pullae serves to regulate the filling and emptying of the tube-feet. 

 On the ring canal are generally found one or more larger 



Fig. 1. 



-Diagram of the watervascular system of a Sea-star. 

 (From Danmark's Fauna.) 



A, Ampulla ; Ax, Axial organ ; M, Madreporite ; P, Polian vesicle ; R, Eing canal ; 

 Kd, Radial canal ; Sf, Tube-foot ; St, Stone canal. 



stalked out-growths, the Polian vesicles. Finally, there issues 

 from the ring canal, in one of the interradii (more rareh^ in all 

 of them), a canal that opens outwards through a larger perforated 

 plate, the madreporite ; through this canal, the stone canal, the 

 watervascular system is filled with water. It is often strongly 

 calcified, to which fact its name is due. Exceptionally (most 

 of the sea-cucumbers) it oj)ens into the body cavity, in which 

 case the fluid that fills the watervascular system is derived from 

 the body cavity. 



To the position of stone canal and madreporite in one of the 

 interradii is due the fact that the body is not perfectly radiate, 

 this single interradius thus differing from the other interradii. 

 A section through this interradius and the opjiosite radius will 



