44 COELENTERATA 



continuous over the whole outer surface? Does it vary in 

 thickness? Are the cells of which it is composed apparently 

 all alike? 



5. The inner layer, endoderm. What is its color? If 

 color is present, is it evenly diffused or is it collected in 

 special bodies? Are the cells of which the endoderm is com- 

 posed apparently all alike? Do they differ in appearance 

 from those of the ectoderm other than in color? If the speci- 

 men is not deeply colored, look for flagella moving in the in- 

 ternal cavity. 



6. Examine the ectoderm of the tentacles carefully and 

 notice that each of the large, rounded, clear bodies, the ne- 

 matocysts, shows a rather indefinite streak (the stinging 

 thread) running from its outer end, back into the interior. 

 See if you can find the trigger {cnidocil) on any of these cells. 



Draw a portion of a tentacle showing the distribution of 

 the nematocysts. 



7. Place your specimen under the low power of the micro- 

 scope, carefully run in a drop of safranin, and see if any of 

 the nematocysts are discharged when the safranin touches 

 them. Examine with a high power and notice the appear- 

 ance of the thread. Notice the change in the shape of the 

 nematocysts that have discharged. See if you can find two 

 kinds. 



Make an enlarged drawing of an exploded nematocyst. 



8. Examine prepared transverse sections of Hydra. Notice 

 that the body is composed of two layers of cells, between 

 which is an almost structureless thin layer. Do the cells of 

 the two layers differ in size, shape, and structure? Do you 

 find more than one kind of cell in each or either of these 

 layers? Where are they? What are they? 



Make a careful drawing of the section showing the ar- 

 rangement as you see it. 



Examine longitudinal sections, for differences in the char- 

 acter of the ectoderm and endoderm in different parts of the 

 body. 



