STRONGYLOCENTROTUS. 77 



5. The rectum, running from the end of the intestine to the 

 anus. 



6. The mesenteries which hold the various organs in place. 

 Make a drawing to show the reproductive and digestive organs. 

 Water-vascular System. 1. The stone canal leads from the 



madreporite to the circular canal, which encircles the esophagus 

 at a point just above the lantern. 



2. From the circular canal radial tubes pass over the top and 

 down the sides of the lantern, to pass through the auricles and 

 up the ambulacral tracts, to the ocular plates. They can easily 

 be seen along the sides of the test, but are difficult to see be- 

 fore they leave the lantern. 



3. Along the course of each radial canal, the ampullce, which 

 supply the tube feet, are to be seen. The relations of the tube 

 feet and radial canals are practically the same as in the starfish 

 except that the removal of the radial tubes to the inner sides 

 of the ambulacral plates causes two perforations for each foot 

 here, while the starfish has only one. One of these perforations 

 is for the connection between the ampulla and the foot, the other 

 is for the connecting tube between the radial canal and the foot. 

 The connecting tube joins the foot outside of the plate (as in 

 the starfish), while it joins the radial canal inside of the plates 

 (different from the starfish). 



Remove the intestine and study the lantern and its attach- 

 ments. 



1. The whole lantern is inclosed in a delicate membrane, 

 the peripharyngeal or lantern membrane which contains the 

 lantern coelom. This space communicates with the five radial 

 perihemal canals, which run along the ambulacral areas be- 

 tween the radial canals and radial nerves, and with the dermal 

 branchse. It is important in respiration. 



2. The tip of the lantern is attached to the flexible peristoma, 

 and muscles extending from various parts of it are attached to 

 the hard parts of the surrounding test. 



In shape the lantern is a five-sided radially symmetrical 



