150 AN INTRODUCTION TO THE STUDY OF FOSSILS 



of the water vascular system which is especially characteristic 

 of the echinoderms. This system consists of a series of tube- 

 like passages filled with sea water. The water is admitted 

 through a finely perforated plate, the madreporite, situated on 

 the dorsal side near the junction of two of the arms. From the 



Fig. 58. — A four-months-old starfish, Aster ias forbesi, from Narragansett Bay, 

 Mass. Natural size, mad., madreporite. (From Mead.) 



madreporite an S-shaped tube, the madreporic or stone canal, 

 descends to the ventral side of the animal and connects with a 

 five-sided ring-like canal surrounding the mouth. From this 

 ring-vessel radiate the five straight ambulacral vessels to the 

 extremities of the five arms. Along each of these radial tubes 

 are given off the four rows of tube-feet. Each of these tube-feet 

 is a small muscular tube, closed at one end and expanded at the 

 other into a bladder-like ampulla. The ampullae are inside the 



