STRONGYLOCENTROTUS. 79 



depressor muscles. By their contraction the lantern coelom is 

 compressed. 



Understand the action of this mechanism in respiration. 

 (See Von Uexhiill 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 its construction. With a 

 scalpel cut the inter-alveolar muscles so the jaws may be sepa- 

 rated. Find: 



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

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

 roughenings on their esophageal sides. What purpose can they 

 serve? Why should the alveoli be so large and the inter-alveolar 

 (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 be- 

 tween them and the alveolus proper can usually be seen. 



(c) The rotulcc, one of which joins the ends of each epi- 

 physis and extends to the position of the esophagus. The five 

 rotulae of the lantern articulate with each other around the eso- 

 phagus, 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 rotulse, are slender and 

 bifurcated at their outer ends. 



(e) Inclosed in each alveolus is a tooth. Examine both 

 extremities of it and determine why the inner end is soft. 



Understand thoroughly how the jaws are used and why the 

 animal needs them. Why does the sea-urchin not need large 

 hepatic caeca? 



The Nervous System. The nervous system is difficult to 

 demonstrate in dissections, but is easy to trace in sections. It 

 consists of: 



