78 ECHINODERMATA. 



pyramid. Each of the sides consists of a massive calcareous 

 structure, the alveolus, which supports an elongated tooth the 

 tip of which projects through the peristome. The base of the 

 pyramid may be compared with a wheel, in which the ten 

 epiphyses, 1 two of which are attached to each alveolus, are the 

 tire, and the five radially directed rotulce are the spokes. Each 

 rotula 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 of the teeth can thus be sepa- 

 rated and closed, and the jaws protruded and retracted by means 

 of muscles. 



3. Connecting adjacent alveoli from top to bottom are the 

 comminator 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- 

 troding the jaws. 



6. A pair of retractor muscles 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? 



7. There are also internal and external rotula muscles that 

 connect the epiphyses with the rotulse. Their contraction 

 moves these plates upon one another and thus causes a rocking 

 motion of the jaws. 



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

 and retracted by means of these muscles. 



8. The compasses are attached one to the other by the 

 elevator muscles. Their contraction elevates all of the compasses 

 and thus enlarges the lantern ccelom. 



9. Attached to the forked end of each compass is a pair of 



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

 the base of an alveolus. 



