48 ELEMENTARY ZOOLOGY 



TECHNICAL NOTE. Place small slips of paper on the slide near 

 the Hydra, put cover-glass over the whole, and examine with the 

 low power of the microscope. 



Note that the whole animal is made up of cells closely 

 joined. Are the cells in the tentacles all alike ? Note 

 nodule-like projections above some of the cells; these are 

 stinging cells, or cnidoblasts. In some cases a small hair- 

 like process, the trigger hair or cnidocil, may be seen pro- 

 jecting above the surface of the cell. Note in some of the 

 tentacles dark-colored particles. These are food-particles 

 which have been taken through the mouth into the diges- 

 tive cavity and have passed thence into the tentacles. 

 The central digestive cavity communicates freely with the 

 cavities in the tentacles, for the tentacles are merely 

 evaginations of the body-wall. 



Make drawings of the Hydra expanded and of the same 

 individual contracted. r 



TECHNICAL NOTE. From the preparation which you have under 

 the microscope pull out the slips of paper, thus letting the cover- 

 glass drop down on the specimen. With a small pipette put a 

 drop of anilin-acetic stain (see p. 451 ) on the slide at one side 

 of the cover-glass and with a piece of filter-paper draw the water 

 through from the other side of the cover-glass. When the stain is 

 diffused press down the cover-glass gently and examine the tentacles 

 first under a low power of the microscope, then under a high one. 



Note the distortion that the animal has undergone 

 through the action of the reagent. Observe the cnido- 

 blasts of the tentacles and note that many of them have 

 thrown out long whip-like processes (fig. 7, C). On 

 what parts of the body do the cnidoblasts occur ? Care- 

 fully examine one of the cnidoblasts which has been dis- 

 charged and note a clear transparent bag-like structure 

 within, the nematocyst, to which is attached the long 

 whip-like process. In another cnidoblast cell which has 

 not been discharged note that the whip-like process is 

 coiled about inside of the bag-like structure. The whole 



