ARBACIA OR STRONGYLOCENTROTUS 225 



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 com- 

 passes and thus enlarges the lantern coelom. 



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

 of depressor muscles. By their contraction the lantern 

 coelom is compressed. 



Understand the action of this mechanism in respiration. 

 (See Von Uexhull or the Cambridge Natural History, Echino- 

 derms, p. 527.) 



Make a drawing to illustrate the arrangement of the mus- 

 cles. 



10. Remove the lantern by cutting the peristome, clear 

 away the external tissues, and examine the construction of 

 the lantern. With a scalpel cut the interalveolar muscles so 

 the jaws may be separated. Find: 



(a) The large V-shaped alveoli (a straight suture in- 

 dicates that each is formed by the fusion of two parts). 

 Notice the roughenings on their esophageal sides. What pur- 

 pose can they serve? Why should the alveoli be so large 

 and the interalveolar (comminator) muscles be so strong? 



(6) The epiphyses, which are fused with the upper corners 

 of each alveolus and extend in to form a bar over its base, 

 thus being functionally a part of the alveolus itself. The 

 sutures between them and the alveolus proper can usually be 

 seen. 



(c) The rotulae, one of which joins the ends of each ep- 

 iphysis and extends to the position of the esophagus. The 

 five rotulae of the lantern articulate with each other around 

 the esophagus, and each rotula articulates with the epiphyses 

 of two adjacent jaws. Do you understand how the jaws move 

 on the rotulae? 



(d) The compasses, lying over the rotulae, are slender and 

 bifurcated at their outer ends. 



(e) The teeth, one enclosed in each alveolus. Examine 

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