THE SEA-URCHIN. 163 



see its range of motion. Remove it and make out 

 how it is articulated to the test. The fleshy tube en- 

 sheathing the base is muscular tissue, by the contrac- 

 tion of which the spine is moved. Clean the spine 

 and make a drawing of it. 



4. Note any variations in size and shape of the spines in 

 various regions. 



5. Study carefully the arrangement of the spines, using 

 the cleaned test for comparison. 



6. Between the spines in certain areas find soft tubular 

 projections, the tube feet or ambulacra. In life they 

 may be extended a considerable distance beyond the 

 spines, being used for locomotion as in the starfish; 

 carefully examine the tips of the tube feet to find 

 what is therein contained. 



7. Find also among the spines and on the peristome, 

 slender flexible stalks, bearing three-pronged pinch- 

 ers. In life these pinchers keep opening and shutting. 



8. Pick away the spines and other projections prepara- 

 tory to dissection, 



DISSECTION OF THE SEA-URCHIN. 



After removing the spines, cut, or better, saw with the 

 blade of a metal saw, through the equator of the test; 

 place under water and carefully raise the aboral portion 

 at one side. 



1. Press on the tips of the teeth to show their connec- 

 tion with the complicated apparatus known as the 

 lantern; now open the test till the two halves are side 

 by side and complete the dissection under water. 



2. Arising from the middle 1 of the inner surface of the 

 lantern find the brown esophagus. Trace this as it 



