STRUCTURE AND FUNCTION 67 



Directions. 



In a section under a simple lens, identify the dorsal 

 and ventral surfaces, the body wall, the body cavity, the 

 alimentary canal, and, if possible, the dorsal and ventral 

 blood vessels and the ventral nerve cord. 



Under a microscope identify the same structures. 

 Notice that the body wall consists of three layers of cells : 

 an outer single layer, the epidermis ; a middle layer, the 

 circular muscles ; and an inner one, the longitudinal 

 muscles. 



The nephridia show as loosely scattered fragments in 

 the body cavity, at the right and left of the alimentary 

 canal. 



If you happen to have a section which shows one or 

 more setae, identify the muscles which operate it, and the 

 group of glandular cells at its inner end, which are known 

 as setigerous (from setci) cells. 



Questions. 



1. Describe the epidermal cells. What is their probable 

 function } Among them notice larger cells, clear and 

 rounded. These are the mucous (slime) cells. 



What is the use of mucus to the worm ? 



2. Describe the muscle cells. In which direction do the 

 muscle fibers extend .■' What is their function ? Which 

 layer of muscle cells is thicker, the circular or the lon- 

 gitudinal ? Why should it be .' 



3. Notice the cells in the walls of the alimentary canal. 

 What layers do you find ? How are they arranged .i" 



4. If the section you are studying is a cross section from 

 the region back of the gizzard, the alimentary canal will 

 look horseshoe shaped, indented from the dorsal surface. 



