1 88 Practical Zoology. 



6. Between the spines in certain areas find soft tubular pro- 

 jections, 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 pinchers. In life these 

 pinchers keep opening and shutting. 



8. Pick away the spines and other projections preparatory 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 connection with 

 the complicated apparatus known as the lantern ; now open the 

 test till the two halves are side by side and complete the dissec- 

 tion under water. 



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

 find the brown esophagus. Trace this as it passes in festoons 

 about the body walls, widening to become the stomach. Trace 

 the intestine to the anus, describing carefully its course. 



3. Pick away the digestive tube from the oral half of the test. 

 Note the five double rows of ampullae; between each of these 

 double rows runs the radial water tube, and between the water 

 tube and the test is the radial nerve. 



4. In the aboral half, note the ovaries and spermaries in the 

 loops of the intestine. Trace their ducts to the genital pores. 



5. After cleaning away the intestine, ovaries, and spermaries, 

 trace the ampullae as they converge to the ocular plate. Compare 

 the inside and outside of the test to see if the ampullae are 

 really opposite the ambulacral pores noticed in the dry test. 



6. Study the lantern, make out how it is supported, and how 

 its various parts are moved, and how they are used. 



