STRONGYLOCENTROTUS. 71 



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 

 V 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 tract, to the ocular plates. They can be 

 easily seen along the si<jjes of the test, but are difficult to see 

 before they leave the lantern. 



3. Along the course of each radial canal, the ampulla, 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. In shape the lantern is a five-sided pyramid, having the 

 "teeth" at its apex projecting through the peristome. The 

 base of the pyramid may be compared with a wheel, in which 

 the ten epiphyses 1 are the tire and the five radially directed rot- 

 uloe are the spokes, each one of which has a more slender bar, 

 forked at the free extremity, the compass or radius lying over it. 

 Each of the five segments represents a jaw that is articulated 

 to its neighbors at its base, near the esophagus. The points 



1 In Arbacia the epiphyses form small hooks that do not unite across 

 the base of an alveolus. 



