STRUCTURE OF THE STARFISH. 69 



The various organs which compose this system are now 

 exposed. They are : — 



a. The madreporic body (Fig. 30, c) ; situated upon 

 the ab-oral surface of the inter-radius of the bivium. 



b. The stone canal (Fig. 31 , c) ; a calcareous tube 

 which passes along the free central edge of the inter-radial 

 partition from the stone canal to a point upon the oral 

 surface within the mouth-pentagon. The course of the 

 canal is like the letter S. 



c. The ambulacral vesicles, or ampullce (Fig. 30, J) ; 

 a double row of small white globular vesicles, with mus- 

 cular walls, on the inner surface of the ambulacral area, on 

 each side of the vertebral ridge. The lower side of each 

 vesicle gives rise to a tul)e which passes into one of the 

 ambulacral pores, between the ambulacral ossicles. 



d. The Polian vesicles; ten muscular sacculi, some- 

 what larger than the ordinary ambulacral vesicles, and 

 situated upon the ten ambulacral plates which form the 

 sides of the mouth-pentagon. 



e. The ambulacra, or feet, which are arranged on the 

 lower surfiice in four rows in the ambulacral furrow, along 

 the oral surface of each ray. If one of the ambulacra 

 be pulled off and carefully examined, its upper end will 

 be found to be prolonged to form a small tube which 

 passes through one of the ambulacral pores to connect 

 with an ambulacral vesicle. 



f. Pull off the feet from a portion of one of the rays 

 with a pair of line-pointed forceps, and notice the radial 

 water-tube, a small longitudinal vessel, which, runs along 

 the middle of the lower surface of each ray at the top of 

 the ambulacral furrow. 



g. Carefully remove the spines which project from the 

 angles of the mouth-pentagon, and notice the circum-oral 



