VERMES. 31 



4. Does the pulsation always move in the same direc- 

 tion? 



H. Internal Structure. Anatomy. 



1. In a thick cross-section of the body of the earth- 



worm observe, — 

 (a) the body wall ; 



(6) the alimentary canal, the central digestive tube ; 

 (c) the body cavity, the space between the body wall 

 and the alimentary canal. 



2. The Body Wall. (Use a thin prepared cross-section.) 

 Observe three layers of tissue in the body wall. 



These are, — 



(a) the epidermis, a single outer layer of cells ; 



(b) the circular muscles, long fibers (muscle cells) 



extending circularly around the body ; 



(c) the longitudinal muscles. Notice that these fibers, 



cut crosswise, appear like small dots arranged 

 in feather-form order. 



3. The Setce. 



In the same section identify bristle-like structures 

 projecting from the body wall (setae). Notice the 

 muscle strands. 



4. The Alimentary Canal. (Use a specimen which has 



the dorsal body wall removed.) 



a. Identify the body wall, body cavity and alimen- 



tary canal. 



b. Does the alimentary canal extend the entire 



length of the body? 



c. Observe the membranes (septa) which suspend 



the alimentary canal in the body cavity. How 

 are the septa situated with reference to the 

 somites? 



