94 ELEMENTARY BIOLOGY. [CHAP. 



c. Structure. 



a. The thin, transparent, homogeneous external 

 layer (cuticle). 



/?. The granular layer (cortical layer) inside the 

 cuticle. 

 [Its fine transverse striation.] 



y. The central more fluid part, not sharply 

 marked off from /?. 



The various clear spaces (alimentary va- 

 cuoles) in it, containing foreign (swallowed) 

 bodies (Diatoms, Protococcus, &c.). 



8. The contractile vesicle; its position, in the 

 cortical layer just beneath the disc; its systole 

 and diastole. 



e. The nucleus; an elongated curved body in 

 the cortical layer; sometimes nearly homo- 

 geneous, sometimes more distinctly granular. 

 The nucleus is usually indistinguishable 

 until after treatment with iodine (4). 



. Thegu&f; sometimes seen in optical trans- 

 verse section as a clear round space; some- 

 times seen sidewise as a canal opening above 

 on the disc, and ending abruptly below in the 

 body-substance. 



b. The stalk. 



a. Its length and diameter (measure). 



p. Its structure; the external homogeneous layer 

 (sheatli) continuous with the cuticle ; the highly 

 refractive centre (axis) generally surrounded 

 with granules, and continuous with the cor- 

 tical layer of the bell. 



