THE EARTHWORM 116 



collarlike circumpharyngeal connectives. Determine the number and 

 place of junction of the nerves joining the nerve cord in the somites 

 just back of the connectives and also in the somites toward the pos- 

 terior end of the body. Cut across the nerve cord in the mid-body 

 region and tear out a piece of it with a quick pull of the forceps. This 

 will help you to determine the number of nerves in each somite. Make 

 a drawing of the nervous system from dorsal view (X 5). Indicate 

 the relation of the nerves and ganglia to the somites. 



m. STRUCTURE AS SHOWN BY SECTIONS 

 Exercise 8. — The Gross Structure. 



(a) Take a piece of an undissected specimen about 3 cm. in length, 

 and cut, with sharp scissors or a scalpel, transverse sections about one 

 somite in thickness from the mid-body region. Sections previously cut 

 and cleared in oil of wintergreen show structures better than freshly 

 cut sections. It is important to have a specimen in which the stomach- 

 intestine is not unduly distended. Study with the handlens, and iden- 

 tify the position of the stomach-intestine, ccelom, nephridia, septa, 

 nerve cord, and blood vessels. Can you distinguish the different layers 

 in the body and gut walls? Notice the typhlosole, a fold of the dorsal 

 wall of the stomach-intestine. How may it be of importance in diges- 

 tion and absorption? Make a drawing about 8 cm. across, which will 

 show the structures appearing in a region midway between septa. 



Exercise 9. — The Cellular Structure. 



(b) Study thin sections of the mid-body region as shown in pre- 

 pared slides. Examine first with handlens and then with the low- 

 power objective of the microscope and recognize all the parts. Examine 

 carefully the cellular structure of each part with the high-power 

 objective. 



(c) The body wall is made up of four layers: (1) The outermost 

 layer, or epidermis, is composed of columnar epithelial cells, some of 

 which are gland cells in various stages of activity. Covering the outer 

 surface of these cells is a thin non-cellular membrane, the cuticle, 

 which is a secretion of the epidermis. Do you find perforations in it? 



(2) The circular muscle layer is composed of non-striated muscle 

 cells which encircle the body and are, therefore, cut lengthwise in this 

 section. Considerable fibrous connective tissue and numerous blood 

 vessels can be seen. What is the function of these blood vessels? 



(3) The longitudinal muscle layer is the next and thickest layer of 

 the body wall. It is made up of non-striated muscle cells running in a 

 longitudinal direction and so arranged and held together by connective 



