ARBACIA. 65 



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 five epiphyses are the tire and the five radially directed rotu- 

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

 forked at the free extremity, the 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 of the 

 teeth can thus be separated and closed, and the jaws protruded 

 and retracted by means of muscles. 



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

 the peripharyngeal or lantern membrane. 



3. Connecting adjacent alveoli from top to bottom are the 

 inter-alveolar muscles, that by their combined action close the 

 jaws. 



4. To each of the arms of the radius fork a muscle is attached. 

 Where is it attached at the other end? 



5. A pair of protractor muscles pass down from each epi- 

 physis. To what are they attached? They are used in pro- 

 truding the jaws? 



6. A pair of retractors is attached to the tip of each alveolus. 

 They can be used in opening the jaws or in retracting the jaws. 

 Do you see how? 



Understand how the jaws may be protruded, opened, closed, 

 and retracted by means of these muscles. 



Make a drawing to illustrate the arrangement of the muscles. 

 5 



