72 ECHINODERMATA. 



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 which contains the 

 lantern ccelom. 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 

 branchae. It is important in respiration. 



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 rotulae. 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 

 depressor muscles. By their contraction the lantern ccelom 

 is compressed. 



Understand the action of this mechanism in respiration. 

 (See Cambridge Natural History, Echinoderms, p. 527.) 

 Make a drawing to illustrate the arrangement of the muscles. 



10. Remove the lantern by cutting the peristome, clear away 

 the external tissues, and examine its construction. With a 



