HYDRA 1 



1. Examine, with the dissecting microscope, a living Hydra in a watch 

 glass containing water. Observe the animal both when expanded and 

 when contracted. Touch the expanded animal with a dissecting needle 

 and observe the rapidity of contraction. Note that it is almost spherical 

 in shape when fully contracted. 



2. Examine the animal in the watch glass as before, with the low power 

 of the compound microscope. Note : (a) the body, which resembles an 

 elastic tube and is attached at one end by (b) the foot, (c) At the 

 opposite end (hypostome) of the body there is (d) a central opening 

 (mouth) which is surrounded by (e) a circlet of tentacles. Note the 

 number of tentacles your specimen possesses and compare with others at 

 your table. 



3. Focus on the cellular body wall and note that it is composed of (a) an 

 outer layer (ectoderm) and (6) an inner layer (endoderm) which sur- 

 rounds (c) a large central cavity (enteric cavity) into which the mouth 

 opens. Focus on one of the tentacles and note that it is also composed of 

 ectoderm and endoderm, and that the enteric cavity continues throughout 

 its length. Note the batteries of stinging cells (nematocysts) embedded 

 in the ectoderm of the tentacles. 



4. Hydra commonly reproduces asexually by budding. Examine your 

 specimen and see if any buds are present. At certain seasons of the year it 

 also reproduces sexually. At such times the male gonads (testes), which 

 produce the sperm, develop as swellings in the body wall just below the 

 tentacles and the female gonads (ovaries), which produce the eggs, 

 develop nearer the foot. 



5. Make (a) a detailed drawing of an expanded animal, showing all the 

 parts observed above, (b) an outline drawing of a contracted animal. 



6. Examine, with the low and with the high power, a prepared transverse 

 section through the body of Hydra. Note : (a) the outer layer of ecto- 

 derm and (6) the inner layer of endoderm, both of which are composed of 

 a great number of cells, and are separated from each other by (c) a thin, 

 noncellular layer (mesogloea). These layers surround (d) the central 

 enteric cavity. Draw the section as observed. 



i B. pp. 58-67. 

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