224 ECHINODERMATA 



lantern coelom. 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 der- 

 mal branchiae. It is important in respiration. 



2. The tip of the lantern is attached to the flexible per- 

 istoma, and muscles extending from various parts of it are 

 attached to the hard parts of the surrounding test. 



In shape the lantern is a five-sided, radially symmetrical 

 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 rotulae are the spokes. 

 Each rotula has a more slender bar, forked at the free ex- 

 tremity, 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 separated and closed, and the jaws 

 protruded and retracted by means of muscles. 



3. Connecting adjacent alveoli from top to bottom are the 

 commutator 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 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 contrac- 

 tion moves these plates upon one another and thus causes 

 a rocking motion of the jaws. 



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

 across the base of an alveolus. 



