174 ZOOLOGY 



rectly at the base of the tentacles. How is it swallowed ? How 

 long does it take? What becomes of it? How long does it 

 remain in the body ? Classify the results which you have at- 

 tained, under the following heads: motion and locomotion, 

 nutrition, sensation. Devise still other experiments to test 

 special points which you desire to know. Have you dis- 

 covered anything that would lead you to think that they 

 learn from experience? 



2. General Structure. Transfer a living animal to the slide, 

 covering it with a drop or two of water. Observe with a low 

 power without cover-glass. Draw carefully in outline everything 

 discovered. 



Note body regions. 



Foot (attached end). 



Column. 



Tentacles, position, number (examine several specimens). 



Hypostome, surrounded by the tentacles. 



Mouth. 



To what extent do these regions vary in their dimensions 

 during the different stages of contraction of the hydra ? Would 

 you say there is any distinct symmetry? Which is the main 

 axis ? Is there any indication of an internal (Castro-vascular) 

 cavity ? What is its extent ? Are the tentacles solid structures ? 

 Evidences ? Are there any buds in your specimen ? Relation to 

 the parent ? To what extent do different parts of the body do 

 different work ? 



3. Microscopic Structure. Cover with a cover-glass supported by objects 

 nearly as thick as the animal. Study with a higher power. Verify the points 

 studied above. Follow the gastro-vascular cavity more fully. Is there an aboral 

 opening? 



Body wall. 



Ectoderm, or outer layer of cells. 

 Entoderm, or inner layer of cells. 



Determine the extent of each layer. Are they continued into the tentacles? 

 What differences do you find in the thickness of the layers and in the shape and 

 character of the cells of each layer in the various parts of the body? Is there 

 anything between the ectoderm and entoderm? 



In the ectoderm, especially in the knobs on the tentacles, find highly refractive 

 oval bodies, the nettle capsules. Irrigate with a drop of dilute acetic acid, and 

 watch the tentacle all the while. What changes have occurred in the nettle cells? 



