CHAPTER III 



BRANCH C(ELENTERATA : THE HYDRAS AND THE SEA- 

 ANEMONES 



THE FRESH-WATER HYDRA 



Either the green or the brown Hydra may be used for 

 this study. Before the specimen is placed upon the slide 

 beneath the microscope it is desirable that the student 

 examine the specimens in the culture vessel. Notice their 

 color, general shape, and mode of attachment. With a 

 camel's-hair brush or small glass tube, remove the Hydra 

 to a watch glass or a glass slide with a hollow cell, holding 

 a little water, and study it with the low power of the 

 microscope. 



I. — Observe the general form of the Hydra, noticing the 

 shape of the body and of the projections, called tentacles, 

 at its upper end. See that the most noticeable move- 

 ments are the gentle lashings of these tentacles. 



II. — Study and describe the contraction and expansion of 

 the body and of the tentacles. These movements are 

 caused by the contractile power of muscle processes 

 attached to the inner ends of those cells which form 

 the outer layer of the body wall. 



III. — Sometimes the Hydra moves from place to place by 

 bending the body into a loop and using each end as a 

 foot. This is not usually to be seen, however. 



IV. — Watch again the slow sweeping movements of the 

 tentacles. Possibly you may see one tentacle lay hold 

 upon a small water-flea or other particle of food. If so, 



