HYDRA. 21 



Make two drawings, one showing the animal expanded and 

 the other contracted. 



Place your specimen on a slide under a cover-glass that is 

 supported by the edge of another cover-glass, so it can be exam- 

 ined with a high power. Be careful not to crush it. Notice: 



4. The outer layer, ectoderm. What is its color? Is it 

 continuous over the whole outer surface ? Does it vary in thick- 

 ness? 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 composed apparently 

 all alike? Do they differ in appearances from those of the ecto- 

 derm other than in color? If the specimen is not deeply colored, 

 look for flagella moving in the internal cavity. 



6. Examine the ectoderm of the tentacles carefully and 

 notice that each of the large, rounded, clear cells, the nematocysts, 

 shows a rather indefinite streak 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 saffranin, and see if any of the 

 nematocysts are discharged when the saffranin touches them. 

 Examine with a high power and notice the appearance 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 arrangement 

 as you see it. 



