124 ELEMENTARY BIOLOGY. [CHAP. 



ing the skin and the epithelium of the alimentary 

 canal, with which it is continuous at the buccal anil 

 cloacal orifices, may be cited as examples. There 

 are several main types of epithelium, viz. — 



a. Ciliated cpiihcliiivi. Gently scrape the mucou-; 

 membrane of the tongue or roof of the mouth of 

 a recently-killed frog and transfer that which is 

 obtained to a slide ; mount in salt solution, avoiding 

 pressure, and examine under a high power. 



a. The cells ; occurring singly or in aggregates. A 

 shimmering appearance {ciliary actioii) will be 

 seen along their free edges, produced by the 

 rapidly moving cilia. 



As the cilia die their movements slacken. 

 Watch an individual cilium as this happens ; it 

 moves in a definite direction from a point of 

 rest and does not oscillate. 



Place a freshly-killed frog upon its back and 

 open the mouth to its utmost. If a fragment of 

 pith or cork be placed on the roof of the mouth, 

 it will be carried back to the gullet by the action 

 of the cilia. 



/?. The cell protoplasm. Stain with eosin or ma- 

 genta, and examine the individual cells. Shape : 

 flattened at the free surface, rounded or elongated 

 at the base. Structure ; granular for the most 

 part (emloplasfu) ; clear and transparent peri- 

 pherally {ectoplasm), especially so at the free 

 border. iVuclcus ; usually central and rounded, 

 containing one or more nucleoli. 



/'. Columnar epithelium. Scrape gently the inner 

 surface of the mucous membrane of the intestine 



