ARBACIA OR STRONGYLOCENTROTUS 223 



2. The esophagus, passing between the jaws, and bending 

 over to one side to join the intestine. 



3. The intestine. Notice its size and its shape. Do its 

 loops have any relation to the positions of the gonads? 



4. The intestinal siphon, lying along the intestine and 

 attached to it at both ends. 



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 before they leave the lantern. 



3. Along the course of each radial canal, the ampullae, 

 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 

 an»d 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 plates (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 at- 

 tachments. 



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

 the peripharyngeal or lantern membrane which contains the 



