164 PRACTICAL ZOOLOGY. 



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 alimentary canal from the oral half of 

 the test. Note the five double rows of ampullae ; be- 

 tween 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 reproductive bodies in the 

 loops of the intestine. Trace their ducts to the geni- 

 tal pores. 



5. After cleaning away the intestine and reproductive 

 bodies, 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 am- 

 bulacra! 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. 



Place in water the pieces of tests left after dissec- 

 tion and macerate till the spines are readily detached. 

 Then clean and keep them for the next class. They 

 will be useful for pulling to pieces to make out the 

 structure of the test. The sea-urchin and the star- 

 fish may be taken as convenient types of the branch 

 Echinodermata. To this group also belong the Brit- 

 tle Stars, Holothurians, and Crinoids. 



THE DEVELOPMENT OF THE SEA-URCHIN. 



The reproductive bodies are very much alike in the 

 two sexes, distinguishable only by color or by microscopic 

 examination. 



