70 HANDBOOK OF INVERTEBRATE ZOOLOGY. 



water-tube running around the mouth, just inside the 

 mouth-pentagon. Trace one of the radiating tubes to the 

 point of union with the circum-oral tube. 



li. Examining the inside of the specimen, notice that 

 the stone-canal also joins the circum-oral tube. 



^. The racemose vesicles; nine small sacculated diver- 

 ticula, which project inwards from the circum-oral water- 

 tube opposite all the ambulacral areas except the one 

 nearest the stone-canal. 



j. Make a diagram of the water vascular system. 



k. Cut off the top of a ray of a living specimen, and 

 placing the animal in a tub of fresh sea-water, notice, after 

 it has recovered from the operation, the manner in which 

 the ambulacral vesicles inside the ray contract and cause 

 the protrusion of the corresponding ambulacra, by dis- 

 tending them with the fluid which is thus forced into 

 them. 



/. Cut off the tip of a ray from a specimen. which has 

 not been opened, and introducing into the radiating water- 

 tube the point of a small injecting syringe, fill the water 

 system with a colored fluid, and notice that the ampulte, 

 the ambulacra, the radiating and circum-oral water-tubes 

 are all filled. Water Avhich has been colored with a few 

 drops of carmine dissolved in ammonia may be used in 

 making the injection ; and if no small injecting-syringe 

 can be procured, the fluid may be blown into the speci- 

 men throuo^h a orlass tube which has been drawn out to a 

 fine point. 



VT. The Nervous System. 



Examine the lower or outer surface of the circum-oral 

 water-tube of an alcoholic specimen with a lens, and notice 

 a thickened rido-e along: its outer surface runninof around 

 the mouth. This ring is the circum-oral nerve-ring. 



