The Starfish. 31 



The Water-vascular System should be studied in a 

 specimen that has been artificially injected. The ambulacral 

 suckers were noted above as a series of tube-like organs 

 lying along the ambulacral furrows. Each sucker passes 

 upward, aborally, through a pore, the ambulacral pore, and 

 expands within the cavity of the arm, into an ambulacral 

 vesicle or ampulla. The ampullae of each arm are in com- 

 munication with a common radiating water-tube, which lies 

 in the median line of each ray beneath the ridge of calcare- 

 ous ambulacral ossicles. Follow the radial tube centripe- 

 tally. It will be found to arise from a circum-oral ring. 

 The Polian vesicles, ten in number, are enlarged ampullae, 

 arranged in a circle around the mouth. The racemose vesi- 

 cles, nine in number, have their axes lying at right angles to 

 the Polian vesicles and extend horizontally into the cavity 

 surrounding the oesophagus. At the place where the tenth 

 vesicle might occur a rigid tube, the stone-canal, takes its 

 origin. The stone-canal extends to the lower side of the 

 madreporic plate. 



Make a drawing of the water-vascular system. 



The Circulatory System. A delicate sac, the "pericar- 

 dium" will be found lying immediately posterior to and be- 

 neath the stone-canal. Careful injecting will show that from 

 this sac certain minute blood-vessels pass to varying parts of 

 the body, following, in the main, the course already taken by 

 the water-vascular system. 



The Nervous System. - - Part the ambulacral suckers 

 from the median line of each furrow, and note the deeply 

 lying nerve-cord, the " radiating nerve." It extends from a 

 circum-oral nerve-ring to a red eye-spot. The latter occu- 

 pies a position at the tip of the arm. 



