54 LABORATORY DIRECTIONS 



The hypostome, part included between the tentacles. Form? 

 (5) Position of mouth? Is the body hollow? Draw when well 

 extended and when contracted. The extended drawing should be 

 at least 200 mm. long, and the other in proportion. 



With a pipette transfer the Hydra to a drop of water on a 

 slide. Place two pieces of thread in the water so that when the 

 coverglass is added they will be at two opposite sides of the 

 coverglass for support. Study ectoderm and entodenn. 



Study tentacles. Note small round bodies, the nettling cells. 

 How many sizes? Are they arranged in any definite order? 

 The short hair-like projections seen on the margin of the ten- 

 tacle are the cnidocils mentioned under (b) on the next page. 

 Make outline drawings to show differences in tentacles when ex- 

 tended and when contracted, being sure to indicate correct loca- 

 tion of nettling cells. 



3. EXPERIMENT. Run a little methylen blue under the cover- 

 glass. This acts as a chemical stimulus, causing the nematocysts 

 to be shot out and at the same time stains them blue. If threads 

 are not shot out from the surface of the body upon contact 

 with the stain, tap lightly with a dissecting needle upon the 

 coverglass immediately above the Hydra. Make out at least two 

 kinds of nematocysts. Draw an 'exploded' specimen of each. 

 The sac of the larger should be 10 mm. in diameter. In the ex- 

 ploded condition the nettling cells have been completely separ- 

 ated from the body so that they may be seen to consist of a long 

 thread-like tube, one end of which is attached to a small sac-like 

 structure, the nematocyst. Often some of the nematocysts which 

 are discharged from the body are more deeply stained and more 

 irregular in outline than the others. Such a nematocyst is sur- 

 rounded by its cnidoblast, the cell which forms the nematocyst 

 (see next page, section b). 



4. TRANSVERSE SECTION. A. Low POWER. Examine trans- 

 verse sections and see the outer, cellular ectoderm, the very thin 

 middle non-cellular layer or mesoglea, and the inner rellular en- 

 todenn. Note the general appearance and the relative thickness 



