HYDRA 55 



of these layers. Make a diagram showing the arrangement of 

 the body layers, 60 mm. in diameter. Cell boundaries should be 

 shown in about one-fifth the circumference. Details of struc- 

 ture will be shown in another drawing called for later. 



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 vacuolc. 

 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 which 

 lies next to the mesoglea. These are muscle threads cut in cross 

 section. 



(b) The nettling cells or cnidoblasts are peculiar in that 

 they are included within the protoplasm of the epithelio-muscu- 

 lar cells. The nematocyst may be distinguished as a clear blad- 

 der-like structure which frequently encloses a solid elongated 

 body. This last named structure is the series of barbs which be- 

 come evident on one type of nettling cells when they are dis- 

 charged. A small nucleus is often present just outside the wall 

 of the nematocyst. This nucleus with the protoplasm immedi- 

 ately around the nematocyst constitutes the cnidoblast, or cell 

 which produces the nematocyst. In a nettling cell which lies at 

 the outermost surface of the ectoderm a small pointed projection 

 called the cnidocil is frequently observable extending beyond the 

 general surface of the body. The cnidocil used to be called the 

 "trigger" on the supposition that it controlled the explosion of 

 the nematocyst. 



(c) Small wedge-shaped cells called interstitial ceils are 

 frequently found between the bases of the epithelio-muscular 



