HYDRA ■ 59 



200 mm. long and approximately 15 mm. in diameter, and the 

 other one in proportion. 



7. 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 entoderm. 



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

 cells, of at least two different types. Are they arranged in 

 definite order? The short hair-like projections seen on the 

 margin of the tentacle are the cnidocils mentioned under (b) on 

 the next page. 



9. On each of the two drawing called for above, show 

 the arrangement of the nettling cells on a portion of one tentacle 

 as seen in surface view. 



III. 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 cellular 

 entoderm. Note the general appearance and the relative thick- 

 ness of these layers. The space bounded by the entoderm is the 

 coelentric cavity. This cavity serves for the digestion and dis- 

 tribution of food. 



B. High Power. Study a transverse section under high 

 power. 



(a) Examine cells of the ectoderm carefully. Note that 

 the protoplasm does not fill the entire cell but usually sur- 

 rounds a more or less conspicuous open space called a vacuole. 

 With a little practice the nuclei of these cells may be readily 

 distinguished from the nettling cells and other bodies lying in 

 the protoplasm. Because the cells which cover the surface of 

 the body of the Hydra have become partially specialized for 

 movement they are called epithelio-muscular cells. In transverse 

 sections that have been specially prepared note that each of these 

 ectoderm cells shows a row of dark dots close to the margin 



